Abstract:
A demodulating circuit for use in contactless integrated circuit cards and systems, the demodulating circuit receiving an input signal, biasing the input signal by a preselected voltage, amplifying the signal such that is becomes half-wave rectified, and removing a modulation signal component from the signal, thereby demodulating the signal in a simple and efficient manner.

Description:
This is a divisional of application Ser. No. 09/327,757, filed Jun. 7, 1999 now U.S. Pat. No. 6,198,361. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention generally relates to a contactless IC card system. More specifically, the present invention is directed to an amplifying circuit, a modulating circuit, a demodulating circuit, a transmitter apparatus, and a receiver apparatus, applicable to a contactless IC card capable of reading/writing various sorts of data in a contactless manner, and is also applicable to an IC card reader/writer capable of communicating data with this contactless IC card. 
     2. Description of the Related Art 
     Conventionally, in IC card systems which employ IC cards, these IC card systems are applied to ticket inspection systems used in transportation facilities, and person entrance/exit management systems for rooms. Such conventional IC card systems utilize IC cards carried by users and IC card readers/writers capable of transmitting/receiving various sorts of data between the IC cards and the readers/writers. These data may be transmitted/received in the contactless manner between the IC cards and the IC card readers/writers. 
     In other words, in this sort of IC card system, an IC card reader/writer modulates a carrier wave having a preselected frequency by using a desired data stream so as to produce a transmission signal, and then transmits this produced transmission signal to the IC card. 
     The IC card receives this transmission signal via an antenna, and then demodulates this transmission signal to decode the data sent from the IC card reader/writer. Furthermore, the IC card modulates internally saved data, such as personal data, by using a preselected carrier wave in response to this received data, and then sends out the modulated data to the IC card reader/writer. 
     Then, the IC card reader/writer receives the data sent from this IC card. Based upon this received data, a door of a ticket inspection machine is opened/closed. Also, any person is allowed to enter into a room, and/or to come out from this room. 
     In such a conventional IC card system, these data are modulated by the ASK (Amplitude Shift Keying) modulating method, and then, the ASK-modulated data is transmitted/received between the IC card and the card reader/writer. Conventionally, as such a modulating means for an ASK modulation signal, a modulating circuit with using a variable gain amplifying circuit, and a multiplying circuit is employed. Also, as such a demodulating means for the ASK modulation signal, a demodulating circuit with using an envelope detecting circuit constructed of a diode, and using a synchronization detecting circuit is employed. 
     FIG. 1 is a schematic block diagram for showing one conventional modulating circuit constructed of this variable gain amplifying circuit. In this modulating circuit  1 , the gain of the variable gain amplifying circuit  2  is switched in response to a logic level of a data stream D to be sent. Also, a carrier signal SC is amplified by this variable gain amplifying circuit  2 . As a result, this modulating circuit  1  modulates the amplitude of the carrier signal SC outputted from the variable gain amplifying circuit  2  in response to the logic level of the data stream D so as to produce an ASK modulation signal SM. 
     Also, FIG. 2 is a schematic block diagram for indicating another conventional modulating circuit arranged by a balanced modulating circuit with employment of a multiplying circuit. In this modulating circuit  3 , the carrier signal SC is multiplied by the data stream D in the multiplying circuit  4 , and while the amplitude of this carrier signal SC is varied in response to the logic level of the data stream D, the ASK modulation signal SM is produced. 
     In contrast, FIG. 3 is a schematic block diagram for showing one conventional demodulating circuit arranged by an envelope detecting circuit with employment of a diode. In this demodulating circuit  6 , the ASK modulation signal SM is rectified by employing the diode D. Furthermore, this rectified ASK modulation signal SM is entered into a smoothing circuit having a predetermined time constant defined by a resistor R and a capacitor C. As a result, the envelope-detected output of the ASK modulation signal is outputted as the demodulation signal SD. 
     FIG. 4 is a schematic block diagram for representing another conventional demodulating circuit arranged by a synchronization detecting circuit. In this demodulating circuit  8 , a carrier signal component SCC is extracted from the ASK modulation signal SM by employing a phase-synchronization system circuit  9  arranged by, for example, a filter circuit arrangement and a PLL circuit arrangement. Both this carrier signal component SCC is multiplied by the ASK modulation signal SM by a multiplying circuit  10 . In the demodulating circuit  8 , a baseband component is extracted from the multiplied result of this multiplying circuit  10  by a low-pass filter (LPF)  12  to thereby be outputted as the demodulation signal SD. 
     On the other hand, the following demands are made in these conventional IC card systems. That is, these modulating circuits and demodulating circuits can be simply and readily manufactured in the IC form in combination with other circuit blocks. Moreover, these modulating/demodulating circuits can be operated in high efficiencies. 
     Moreover, these modulating circuits and demodulating circuits with employment of the conventional circuit arrangements can hardly satisfy the necessary items for the IC card systems. 
     In further detail, in the modulating circuit arranged by the variable gain amplifying circuit, the voltage range which can be effectively utilized by the variable gain amplifying circuit is limited. This voltage range limitation causes the lower power efficiency of the conventional modulating circuit. Also, as to the modulating circuit with employment of the multiplying circuit, there are such drawbacks that the circuit arrangement becomes complex, and this complex modulating circuit cannot be simply and readily manufactured in the IC form. 
     For instance, also in the power amplifying circuit for the ASK modulation signal, which is similarly required to be manufactured in the IC form similar to such modulating circuits, the ASK modulation signal must be amplified while saving a change contained in the amplitudes of this ASK modulation signal. After all, this power amplifying circuit must be operated in a better linearity region. This causes the power efficiency to be lowered also in the power amplifying circuit. Also, this power amplifying circuit has a drawback in that in order to transmit sufficiently high power, the active elements capable of satisfying the necessary allowable current and the allowable loss must be used instead of commercially available general-purpose electronic components. 
     In contrast, the demodulating circuit arranged by the envelope detecting circuit with employment of the diode owns such a drawback that a leakage current is produced in the diode when this demodulating circuit is manufactured in the IC form, and therefore, the detection efficiency of the ASK modulation signal is considerably lowered. In other words, as shown in FIG. 5, when an envelope detecting circuit with employment of a diode is manufactured in the IC form, both polarities must be set to floating potentials at the diode D. As a result, a stray transistor is necessarily produced. Accordingly, as represented in FIG. 6, a leakage current is produced. 
     On the other hand, in the conventional demodulating circuit arranged by the synchronization detecting circuit, there is another drawback in that the circuit arrangement of the phase synchronization system circuit  9  becomes complex. 
     SUMMARY OF THE INVENTION 
     The present invention has been made to solve the above-explained drawbacks of the prior art, and therefore, has an object to provide a modulating circuit, a demodulating circuit, an amplifying circuit used in this modulating circuit, and furthermore, a transmitter apparatus as well as a receiver apparatus with employment of these modulating circuit and demodulating circuit, which can be simply and easily manufactured in an IC form in combination with other circuit blocks, and also which can be operated with high efficiencies. 
     Another object of the present invention is to provide the following modulating circuit, amplifying circuit applicable to this modulating circuit, and also a transmitter apparatus with using this modulating circuit. That is, first and second output signals having predetermined phases with respect to an input signal are added to each other, and the added result is outputted. At least, the second output signal is gated in response to the input data. Also, on the output side of a power amplifying circuit, the power-amplified results are attenuated in accordance with the input data so as to produce an amplitude-modulated signal. This circuit arrangement can be simply and readily manufactured in the IC form together with other circuit blocks. 
     Also, since amplitude-modulated signals are biased to be amplified, or clamped, a demodulating circuit and a receiver apparatus with using this demodulating circuit can be simply and easily manufactured in the IC form in combination with other circuit blocks. 
     Furthermore, amplitude-modulated signals are clamped, polarities of these amplitude-modulated signals are judged to multiply the polarity judgement result by the amplitude-modulated signals, and furthermore, the amplitude-modulated signals are selectively outputted based upon the polarity judgement results of the amplitude-modulated signals. As a result, another demodulating circuit and another receiver apparatus with using this demodulating circuit can be simply and readily manufactured in the IC form together with other circuit blocks. 
     To achieve the above-described objects, a modulating circuit, according to a first aspect of the present invention, includes: 
     first signal output means for outputting a first output signal having a predetermined phase with respect to that of an input signal; 
     a second signal output means for outputting a second output signal having a predetermined phase with respect to that of the input signal; 
     gate means for gating at least the second output signal; 
     calculation means for adding, or subtracting the first output signal and the second output signal; and 
     control means for controlling the operation of the gate means in response to a logic level of input data. 
     Also, a modulating circuit as recited in the first aspect, according to a second aspect, is configured so that: 
     the first signal output means outputs the first output signal having the same phase as that of the input signal; and 
     the second signal output means outputs the second output signal having the phase opposite to that of the input signal. 
     Another aspect of the above-described modulating circuit as recited in the first aspect, according to a third aspect, is configured so that: 
     the first signal output means power-amplifies the first output signal to output the power-amplified first output signal; and 
     the second signal output means power-amplifies the second output signal to output the power-amplified second output signal. 
     A modulating circuit as recited in the first aspect, according to a fourth aspect, includes features wherein 
     the input signal is constituted by a sine wave signal having a single frequency. 
     A modulating circuit as recited in the first aspect, according to a fifth aspect, includes features wherein 
     the input signal is constituted by a rectangular wave signal having a single frequency. 
     A modulating circuit as recited in the fifth aspect, according to a sixth aspect, includes features wherein 
     the second signal output means outputs the second output signal having the phase opposite to that of the input signal by inverting the logic level of the input signal. 
     A modulating circuit as recited in the third aspect, according to a seventh aspect, includes features wherein 
     the gate means controls to stop the power amplifying process operation by the second signal output means so as to gate the second output signal; and 
     the second signal output means maintains an impedance of an output terminal at a high impedance for a time period during which the power amplifying process operation is stopped. 
     A modulating circuit as recited in the third aspect, according to an eighth aspect, includes features wherein 
     both the first output signal means and the second output signal means are arranged by a switching circuit for switching operations in response to the input signal. 
     A modulating circuit as recited in the third aspect, according to a ninth aspect, includes features wherein 
     at least the second signal output means and the gate means are tri-state buffer circuits. 
     A modulating circuit as recited in the first aspect, according to a tenth aspect, includes features wherein 
     the first signal output means outputs the first output signal from a first antenna; 
     the second signal output means outputs the second output signal from a second antenna; and 
     the calculation means is formed by way of an electromagnetic coupling between the first antenna and the second antenna. 
     Also, to achieve the above-described objects, an amplifying circuit, according to an 11th aspect of the present invention, includes an amplifying circuit in which an operation of a field-effect transistor is switched in response to an input signal so as to output a power-amplified signal of the input signal from the field-effect transistor, comprising: 
     a drive circuit for switching the operation of the field-effect transistor by applying a voltage to a gate of the field-effect transistor, the voltage being higher than, or equal to a source-to-drain voltage of the field-effect transistor. 
     An amplifying circuit as recited in the 11th aspect, according to a 12th aspect, includes features wherein 
     an output terminal is constituted in such a manner that the output terminal can be set to a high impedance. 
     Further, to achieve the above-explained objects, an amplifying circuit, according to a 13th aspect of the present invention, includes 
     first variable resistor means, one end of which is held at a first potential, and the resistance value of which is varied in response to a first control signal; 
     second variable resistor means, one end of which is connected to the other end of the first variable resistor means, the other end of which is held at a second potential different from the first potential, and the resistance value of which is varied in response to a second control signal; and 
     control means for switching a signal level of the first control signal and a signal level of the second control signal so as to switch a potential at a connection center point between the first variable resistor means and the second variable resistor means to another potential corresponding to the first and second potentials, and also so as to switch an impedance of the connection center point to a high impedance in response to both an input signal and a control signal. 
     An amplifying circuit as recited in the 13th aspect according to a 14th aspect, includes features wherein 
     the first variable resistor means and the second variable resistor means are field-effect transistors. 
     An amplifying circuit as recited in the 14th aspect according to a 15th aspect, includes features wherein 
     the control means switches the signal level of the the control signal and the signal level of the second control signal to a voltage higher than, or equal to a source-to-drain voltage of the field-effect transistor. 
     To achieve these objects, a transmitter apparatus, according to a 16th aspect of the present invention, includes a transmitter apparatus for amplitude-modulating input data by using a modulating circuit to thereby transmit the amplitude-modulated input data, wherein: 
     the modulating circuit is comprised of: 
     first signal output means for outputting a first output signal having a predetermined phase with respect to that of an input signal; 
     a second signal output means for outputting a second output signal having a predetermined phase with respect to that of the input signal; 
     gate means for gating at least the second output signal; 
     calculation means for adding, or subtracting the first output signal and the second output signal; and 
     control means for controlling the operation of the gate means in response to a logic level of input data. 
     A transmitter apparatus as recited in the 16th aspect, according to a 17th aspect, includes features wherein 
     the first signal output means outputs the first output signal from a first antenna; 
     the second signal output means outputs the second output signal from a second antenna; and 
     the calculation means is formed by way of an electromagnetic coupling between the first antenna and the second antenna. 
     Also, to achieve the objects, a transmitter apparatus according to an 18th aspect of the present invention, includes a transmitter apparatus for amplitude-modulating input data to thereby transmit the amplitude-modulated input data, includes: 
     a first modulating circuit for producing a first amplitude-modulated signal in response to the input data; and 
     a second modulating circuit for producing a second amplitude-modulated signal made of a carrier wave having a phase opposite to that of the first amplitude-modulated signal; wherein: 
     each of the first modulating circuit and the second modulating circuit is comprised of: 
     first signal output means for outputting a first output signal having a predetermined phase with respect to that of an input signal; 
     a second signal output means for outputting a second output signal having a predetermined phase with respect to that of the input signal; 
     gate means for gating at least the second output signal; 
     calculation means for adding, or subtracting the first output signal and the second output signal; and 
     control means for controlling the operation of the gate means in response to a logic level of input data. 
     To achieve these objects, a modulating circuit, according to a 19th aspect of the present invention, includes 
     a variable attenuator provided at an output terminal of a power amplifying circuit, for attenuating a power-amplified result of the power amplifying circuit in response to an input signal. 
     A modulating circuit as recited in the 19th aspect, according to a 20th aspect, includes features wherein 
     the signal amplified by the power amplifying circuit is constituted by a sine wave signal having a single frequency. 
     A modulating circuit as recited in the 19th aspect, according to a 21st aspect, includes features wherein 
     the signal amplified by the power amplifying circuit is constituted by a rectangular wave signal having a single frequency. 
     Further, to achieve the above objects, a transmitter apparatus, according to a 22nd aspect of the present invention, is featured by such a transmitter apparatus comprising a modulating circuit by way of an amplitude modulation, wherein: 
     the modulating circuit includes 
     a variable attenuator provided at an output terminal of a power amplifying circuit, for attenuating a power-amplified result of the power amplifying circuit in response to an input signal. 
     Also, to achieve these objects, a transmitter apparatus, according to a 23rd aspect of the present invention, is featured by such a transmitter apparatus for amplitude-modulating an input signal to thereby transmit the amplitude-modulated input signal, includes 
     a first modulating circuit for producing a first amplitude-modulated signal in response to the input signal; and 
     a second modulating circuit for producing a second amplitude-modulated signal made of a carrier wave having a phase opposite to that of the first amplitude-modulated signal in response to the input signal; wherein: 
     each of the first modulating circuit and the second modulating circuit includes 
     a variable attenuator provided at an output terminal of a power amplifying circuit, for attenuating a power-amplified result of the power amplifying circuit in response to an input signal. 
     Also, to achieve the objects, a demodulating circuit, according to a 24th aspect of the present invention, includes 
     amplifying means for amplifying an input signal; 
     bias means for biasing the input signal; and 
     band limiting means for removing a component of the input signal from the output signal derived from the amplifying means. 
     A demodulating circuit as recited in the 24th aspect, according to a 25th aspect, includes features wherein 
     the amplifying means corresponds to any one of an amplifying circuit with employment of a transistor, another amplifying circuit with employment of a field-effect transistor, and a differential amplifier circuit. 
     A demodulating circuit as recited in the 24th aspect, according to a 26th aspect, includes features wherein 
     the band limiting means corresponds to any one of a low-pass filter, a band-pass filter, and a trap filter. 
     also, to achieve the objects, a receiver apparatus, according to a 27th aspect of the present invention, is featured by such a receiver apparatus for demodulating sequentially-entered amplitude-modulated signals by using a demodulating circuit, wherein: 
     the demodulating circuit is comprised of: 
     amplifying means for amplifying the amplitude-modulated signal; 
     bias means for biasing the amplitude-modulated signal; and 
     band limiting means for removing a component of the amplitude-modulating signal from the output signal derived from the amplifying means. 
     Also, to achieve these objects, a demodulating circuit, according to a 28th aspect of the present invention, includes 
     a limiter for limiting an amplitude of an input signal; and 
     band limiting means for removing a component of the input signal from the output signal derived from the limiter. 
     A demodulating circuit as recited in the 28th aspect, according to a 29th aspect, includes features wherein 
     the limiter is constituted by a series circuit formed by connecting a diode in series to a constant voltage power source. 
     A demodulating circuit as recited in the 28th aspect, according to a 30th aspect, includes features wherein 
     the band limiting means corresponds to any one of a low-pass filter, a band-pass filter, and a trap filter. 
     Also, to achieve the objects, a receiver apparatus, according to a 31st aspect of the present invention, includes features for demodulating sequentially-entered amplitude-modulated signals by using a demodulating circuit, wherein: 
     the demodulating circuit includes: 
     a limiter for limiting the amplitude of the amplitude-modulated signal; and 
     band limiting means for removing a component of the amplitude-modulated signal from the output signal of the limiter. 
     Further, a demodulating circuit, according to a 32nd aspect of the present invention, includes 
     clamping means for clamping an input signal; and 
     band limiting means for removing a component of the input signal from the output signal of the clamping means. 
     A demodulating circuit as recited in the 32nd aspect, according to a 33rd aspect, includes features wherein 
     the clamping means is constituted by a grounded type diode. 
     A demodulating circuit as recited in the 32nd aspect, according to a 34th aspect, includes features wherein 
     the band limiting means corresponds to any one of a low-pass filter, a band-pass filter, and a trap filter. 
     Also, to achieve the objects, a receiver apparatus, according to a 35th aspect of the present invention, includes features for demodulating sequentially-entered amplitude-modulated signals by using a demodulating circuit, wherein: 
     the demodulating circuit includes: 
     clamping means for clamping the amplitude-modulated signal; and 
     band limiting means for removing a component of the amplitude-modulated signal from the output signal of the clamping means. 
     Also, to achieve the object, a demodulating circuit, according to a 36th aspect of the present invention, includes 
     signal processing means for producing first and second input signals having phases different from a phase of an input signal by approximately 180 degrees; 
     first clamping circuit for clamping the first input signal; 
     second clamping circuit for clamping the second input signal; 
     first band limiting means for removing a component of the first input signal from the output signal of the first clamping circuit; 
     second band limiting means for removing a component of the second input signal from the output signal of the first clamping means; and 
     calculating means for adding, or averaging the output signal of the first band limiting means and the output signal of the second band limiting means. 
     A demodulating circuit as recited in the 36th aspect, according to a 37th aspect, includes features wherein 
     the first and second clamping means are constituted by a grounded type diode. 
     A demodulating circuit as recited in the 36th aspect, according to a 38th aspect, includes features wherein 
     the band limiting means corresponds to any one of a low-pass filter, a band-pass filter, and a trap filter. 
     To achieve the objects, a receiver apparatus, according to a 39th aspect of the present invention, features a receiver apparatus for demodulating sequentially-entered amplitude-modulated signals by using a demodulating circuit, wherein: 
     the demodulating circuit includes 
     signal processing means for producing first and second amplitude-modulated signals having phases different from a phase of the amplitude-modulated signal by approximately 180 degrees; 
     first clamping circuit for clamping the first amplitude-modulated signal; 
     second clamping circuit for clamping the second amplitude-modulated signal; 
     first band limiting means for removing a component of the first amplitude-modulated signal from the output signal of the first clamping circuit; 
     second band limiting means for removing a component of the second amplitude-modulated signal from the output signal of the first clamping means; and 
     calculating means for adding, or averaging the output signal of the first band limiting means and the output signal of the second band limiting means. 
     Further, to achieve the objects, a demodulating circuit, according to a 40th aspect of the present invention, includes 
     signal processing means for producing first and second input signals having phases different from a phase of an input signal by approximately 180 degrees; 
     first clamping circuit for clamping the first input signal; 
     second clamping circuit for clamping the second input signal; 
     calculating means for adding, or averaging the output signal of the first band limiting means and the output signal of the second band limiting means; and 
     band limiting means for removing a component of the input signal from the output signal of the calculating means. 
     A demodulating circuit as recited in the 40th aspect, according to a 41st aspect, includes features wherein 
     the first and second clamping means are constituted by a grounded type diode. 
     A demodulating circuit as recited in the 40th aspect, according to a 42nd aspect, includes features wherein 
     the band limiting means corresponds to any one of a low-pass filter, a band-pass filter, and a trap filter. 
     To achieve the objects, a receiver apparatus, according to a 43rd aspect of the present invention, includes features for demodulating sequentially entered amplitude-modulated signals by using a demodulating circuit, wherein: 
     the demodulating circuit includes 
     signal processing means for producing first and second amplitude-modulated signals having phases different from a phase of the amplitude-modulated signal by approximately 180 degrees; 
     first clamping circuit for clamping the first amplitude-modulated signal; 
     second clamping circuit for clamping the second amplitude-modulated signal; 
     calculating means for adding, or averaging the output signal of the first band limiting means and the output signal of the second band limiting means; and 
     band limiting means for removing a component of the amplitude-modulated signal from the output signal of the calculating means. 
     To achieve the objects, a demodulating apparatus, according to a 44th aspect of the present invention, a demodulating apparatus includes 
     polarity judging means for judging a polarity of an amplitude-modulated signal to thereby output a polarity judgement result; 
     multiplying means for multiplying the polarity judgement result by the amplitude-modulated signal to thereby output a multiplication result; and 
     band limiting means for removing a component of the amplitude-modulated signal from the multiplication result. 
     A demodulating circuit as recited in the 44th aspect, according to a 45th aspect, includes features wherein 
     the polarity judging means is constituted by a limiter for limiting the amplitude of the amplitude-modulated signal on a positive side and on a negative side. 
     A demodulating circuit as recited in the 44th aspect, according to a 46th aspect, includes features wherein 
     the multiplying means is constructed of a double balanced mixer. 
     A demodulating circuit as recited in the 44th aspect, according to a 47th aspect, includes features wherein 
     the band limiting means corresponds to any one of a low-pass filter, a band-pass filter, and a trap filter. 
     Furthermore, to achieve the objects, a receiver apparatus, according to a 48th aspect of the present invention, is featured by such a receiver apparatus for demodulating sequentially-entered amplitude-modulated signals by using a demodulating circuit, wherein: 
     the demodulating circuit includes: 
     polarity judging means for judging a polarity of an amplitude-modulated signal to thereby output a polarity judgement result; 
     multiplying means for multiplying the polarity judgement result by the amplitude-modulated signal to thereby output a multiplication result; and 
     band limiting means for removing a component of the amplitude-modulated signal from the multiplication result. 
     Moreover, to achieve the objects, a demodulating circuit, according to a 49th aspect of the present invention, includes 
     signal producing means for producing first and second amplitude-modulated signals having phases inverted from each other from an amplitude-modulated signal; 
     polarity judging means for judging a polarity of one of the amplitude-modulated signal, the first amplitude-modulated signal, and the second amplitude-modulated signal to thereby output a polarity judgement result; 
     selecting/outputting means for selectively outputting the first amplitude-modulated signal and the second amplitude-modulated signal based upon the polarity judgement result; and 
     band limiting means for removing a component of the amplitude-modulated signal from the output signal of the selecting/outputting means. 
     A demodulating circuit as recited in the 49th aspect, according to a 50th aspect, includes features wherein 
     the band limiting means corresponds to any one of a low-pass filter, a band-pass filter, and a trap filter. 
     Also, to achieve the objects, a receiver apparatus, according to a 51st aspect of the present invention, includes a receiver apparatus for demodulating sequentially-entered amplitude-modulated signals by using a demodulating circuit, wherein: 
     the demodulating circuit is comprised of: 
     signal producing means for producing first and second amplitude-modulated signals having phases inverted from each other from an amplitude-modulated signal; 
     polarity judging means for judging a polarity of one of the amplitude-modulated signal, the first amplitude-modulated signal, and the second amplitude-modulated signal to thereby output a polarity judgement result; 
     selecting/outputting means for selectively outputting the first amplitude-modulated signal and the second amplitude-modulated signal based upon the polarity judgement result; and 
     band limiting means for removing a component of the amplitude-modulated signal from the output signal of the selecting/outputting means. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     For a better understanding of the present invention, reference is made of a detailed description to be read in conjunction with the accompanying drawings, in which: 
     FIG. 1 is a schematic block diagram for showing a conventional modulating circuit with using a variable gain amplifying circuit; 
     FIG. 2 is a schematic block diagram for indicating another conventional modulating circuit with using a multiplying circuit; 
     FIG. 3 is a schematic block diagram for representing a conventional demodulating circuit with employment of a diode by way of an envelope detection; 
     FIG. 4 is a schematic block diagram for indicating another conventional demodulating circuit by way of a phase synchronization detection; 
     FIG. 5 schematically shows an integrated circuit arrangement of the conventional demodulating circuit shown in FIG. 3; 
     FIG. 6 is a connection diagram for schematically indicating an equivalent circuit of the integrated demodulating circuit shown in FIG. 5; 
     FIG. 7 is a schematic block diagram for representing an overall arrangement of an IC card system according to a first embodiment of the present invention; 
     FIG. 8 is a schematic block diagram for showing a modulating/demodulating circuit of the IC card system according to the first embodiment; 
     FIG. 9 is a schematic block diagram for indicating a basic structure of a modulating circuit applied to an IC card reader/writer according to a first embodiment of the present invention; 
     FIG. 10 is a table for explaining operation of the modulating circuit shown in FIG. 9; 
     FIGS.  11 (A)- 11 (D)are time charts for describing the operation of the modulating circuit indicated in FIG. 9; 
     FIG. 12 is a schematic block diagram for indicating a concrete circuit arrangement of the modulating circuit shown in FIG. 9; 
     FIG. 13 is a connection diagram for representing a power amplifying circuit shown in FIG. 12; 
     FIG. 14 is a table for explaining operation of a control logic of the power amplifying circuit shown in FIG. 12; 
     FIG. 15 is a schematic block diagram for representing a modulating circuit applied to an IC card reader/writer, according to a second embodiment of the present invention; 
     FIG. 16 is a schematic block diagram for representing a basic structure of a modulating circuit applied to an IC card reader/writer, according to a third embodiment of the present invention; 
     FIG. 17 is a connection diagram for explaining a variable attenuator shown in FIG. 16; 
     FIG. 18 is a connection diagram for explaining a grounded type variable attenuator as to the variable attenuator of FIG. 16; 
     FIG. 19 is a connection diagram for explaining a combined structure between the arrangement shown in FIG.  17  and the arrangement shown in FIG. 18 as to the variable attenuator of FIG. 16; 
     FIG. 20 is a schematic block diagram for showing a concrete circuit arrangement of the modulating circuit shown in FIG. 16; 
     FIG. 21 is a schematic block diagram for indicating a modulating circuit applied to an IC card reader/writer, according to a fourth embodiment of the present invention; 
     FIG. 22 is a schematic block diagram for showing a basic structure of a demodulating circuit applied to an IC card reader/writer, according to a fifth embodiment of the present invention; 
     FIG. 23 is a characteristic curve diagram for explaining a bias of a modulation signal used in FIG. 22; 
     FIG. 24 is a schematic block diagram for showing a circuit arrangement constituted by replacing the bias of the demodulating circuit shown in FIG. 22 by a limiter; 
     FIG. 25 is a characteristic curve diagram for explaining an amplitude limiter in the circuit arrangement of FIG. 24; 
     FIG. 26 is a schematic block diagram for representing a concrete arrangement of the demodulating circuit shown in FIG. 22; 
     FIG. 27 is a schematic block diagram for indicating a modulating circuit applied to an IC card reader/writer, according to a sixth embodiment of the present invention; 
     FIG. 28 is a schematic block diagram for showing a demodulating circuit applied to an IC card reader/writer, according to a seventh embodiment of the present invention; 
     FIG. 29 is a schematic block diagram for showing a basic structure of a demodulating circuit applied to an IC card reader/writer, according to an eighth embodiment of the present invention; 
     FIG. 30 is a schematic block diagram for representing a concrete arrangement of the demodulating circuit shown in FIG. 29; 
     FIG. 31 is a schematic block diagram for showing a demodulating circuit applied to an IC card reader/writer, according to a ninth embodiment of the present invention; 
     FIG. 32 is a schematic block diagram for representing a concrete arrangement of the demodulating circuit shown in FIG. 31; 
     FIG. 33 is a schematic block diagram for showing a demodulating circuit applied to an IC card reader/writer, according to a tenth embodiment of the present invention; 
     FIG. 34 is a schematic block diagram for showing a basic structure of a demodulating circuit applied to an IC card reader/writer, according to an eleventh embodiment of the present invention; 
     FIG. 35 is a schematic block diagram for representing a concrete arrangement of the demodulating circuit shown in FIG. 34; 
     FIG. 36 is a schematic block diagram for showing a basic arrangement of a demodulating circuit applied to an IC card reader/writer, according to a twelfth embodiment of the present invention; 
     FIG. 37 is a schematic block diagram for indicating a concrete arrangement of the demodulating circuit shown in FIG. 36; and 
     FIG. 38 is a connection diagram for representing a switch circuit of FIG.  37 . 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Referring now to drawings, various preferred embodiments of the present invention will be described. 
     Circuit Arrangement of First Embodiment 
     FIG. 7 is a schematic block diagram for showing an IC (Integrated Circuit) card system  21  according to a first embodiment of the present invention. This first IC card system  21  is applied to, for example, a ticket inspection system of transportation facilities. In this IC card system  21 , data is communicated between an IC card  22  and an IC card reader/writer  23 . 
     In this case, the IC card  22  is formed in a card shape in such a manner that a (circuit) board on which an integrated circuit is mounted is stacked with a protection sheet. In this IC card  22 , a loop antenna  24  is formed by a wiring pattern formed on this board. Also, a modulating/demodulating circuit  25  and a signal processing circuit  26  are constituted by the integrated circuit mounted on this board. 
     In this embodiment, the loop antenna  24  is coupled to another loop antenna  28  of the IC card reader/writer  23  so as to receive a transmission signal transmitted from this loop antenna  28 , and also to radiate a response (acknowledge) signal produced by the modulating/demodulating circuit  25 . 
     The modulating/demodulating circuit  25  produces electric power, a clock signal, and the like, which are required for the operations of this IC card  22  by using the transmit signal received by the loop antenna  24 . Furthermore, the modulating/demodulating circuit  25  is operated by using this electric power and the clock signal to demodulate a data stream “D(R→C)” transmitted from the reader/writer  23  (will be referred to as a “transmit data stream” hereinafter), and then outputs the demodulated data stream to the signal processing circuit  26 . In response to another data stream “D(C→R)” entered from the signal processing circuit  26  and requested by the transmit data stream “D(R→C)”, this modulating/demodulating circuit  25  produces a response signal based upon an ASK modulation signal. In response of this response signal, the modulating/demodulating circuit  25  drives the loop antenna  24  so as to radiate a response signal. This response data stream will be referred to as a “response data stream D(C→R)” hereinafter. 
     The signal processing circuit  26  is operated by the electric power and the clock generated from the modulating/demodulating circuit  25  so as to analyze the transmit data stream D(R→C), and to output the response data stream D(C→R) saved in a non-volatile memory build in this IC card to the modulating/demodulating circuit  25 , if required. 
     In the reader/writer  23 , the modulating/demodulating circuit  29  produces a transmit signal by the ASK modulation signal based upon a transmit data stream D(R→C) entered from an SPU (signal process unit)  30 , and drives the loop antenna  28  by this transmit signal. Also, the modulating/demodulating circuit  29  performs a signal process operation of the response signal received by this loop antenna  28  so as to demodulate the response data stream “D(C→R)” transmitted from the IC card  22 , and then outputs this demodulated response data stream D(C→R) to the SPU  30 . 
     The SPU  30  is arranged by a calculation processing unit for executing a relatively simple process sequence. This SPU  30  sends out the transmit data stream “ID(R→C)” to the modulating/demodulating circuit  29 , and also processes the response data stream “D(C→R)” entered from this modulating/demodulating circuit  29 . This transmit data stream D(R→C) is to be transmitted to the IC card  22 . In this process operation, the SPU  30  causes a display unit  31  to display thereon a process history and a process result, if required. In response to a command supplied from an input unit  32 , the operations of the SPU  30  are switched so as to input/output data about the process sequence in/from an external apparatus  33  or the like, if necessary. 
     FIG. 8 is a schematic block diagram for partially indicating a circuit structure of the modulating/demodulating circuit  29  employed in the reader/writer  23 . As shown in this drawing, the modulating/demodulating circuit  29  is arranged by a transmission-sided block  45  and a reception-sided block  46 . 
     In this embodiment, the transmission-sided block  45  modulates the transmit data stream “D(→C)” outputted from the SPU  30  in response to a control signal “RFoff” outputted from the SPU  30  to thereby send out the modulated transmit data stream D(R→C) from the loop antenna  28 . As a result, the transmission-sided block  45  produces a carrier signal SC having a frequency of 13.56 [MHz] by operating an oscillator circuit built in this transmission-sided block  45 . Also, this transmission-sided block  45  decodes the transmit data stream “D(R→C)” so as to be converted into a transmit data stream by the Manchester code. 
     In an ASK modulating circuit  47  of the transmission-sided block  45 , the carrier signal SC is ASK-modulated by using the transmit data stream “TX(D(R→C))” which is coded in this manner to thereby produce an ASK modulation signal “SM”. Furthermore, this ASK modulation signal “SM” is amplified by a power amplifying circuit  48  to thereby drive the loop antenna  28  by this amplified ASK modulation signal “SM”. 
     The reception-sided block  46  processes the response signal SM obtained via the loop antenna  28  so as to demodulate the response data stream “D(C→R)”. In other words, the reception-sided block  46  detects the response signal SM obtained via the loop antenna  28  so as to produce a detecting signal “SD” in the ASK (Amplitude Shift Keying) detecting circuit  49 . The signal level of this detection signal SD is changed in response to the logic level of the response data stream “D(C→R)”. The reception-sided block  46  limits the pass band of this detection signal SD by a low-pass filter (LPF)  50  subsequent to the ASK detecting circuit  49 , and thereafter, amplifies the filtered detection signal by a predetermined gain using an amplifying circuit  51  and furthermore, digitalizes the amplified detection signal. Furthermore, the reception-sided block  46  decodes this binary-coded data, and thus, reproduces and outputs the response data stream “D(C→R)” based on this binary-coded data. 
     Basic Circuit Arrangement of Transmitting Circuit 
     FIG. 9 is a schematic block diagram for showing a basic circuit arrangement of the above-described ASK modulating circuit  47  and power amplifying circuit  48  (will be referred to as a “transmitting circuit”  55  hereinafter). This transmitting circuit  55  contains two signal paths (systems) of processing circuits  55 A and  55 B, which each have a power amplifying circuit. 
     In this case, the first processing unit  55 A receives the carrier signal SC via a buffer amplifying circuit  56 , and then enters the output signal of the buffer amplifying circuit  56  into a second selection input terminal of a selecting circuit  57 . This output signal from the buffer amplifying circuit  56  has the same phase as that of the carrier signal SC. In this case, this selecting circuit  57  enters a signal having a phase opposite to that of the carrier signal SC into a first selection input terminal, and grounds the remaining third selection input terminal. This signal having the opposite phase is derived from the second processing circuit  55 B. The selecting circuit  57  switches contacts under control of a control circuit  58 , and a power amplifying circuit  59  provided subsequent to this selecting circuit  57  amplifies the output signal derived from the selecting circuit  57 . 
     As a result, the first processing circuit  55 A is arranged in such a manner that the selecting circuit  57  switches the first contact and the second contact, so that the signal having the phase opposite to that of the carrier signal SC can be gated. Similarly, this first processing circuit  55 A is so arranged that the selecting circuit  57  switches the second contact and the third contact, so that the signal having the same phase as that of the carrier signal SC can be gated. 
     In contrast, the second processing circuit  55 B enters the carrier signal SC into a buffer amplifying circuit  60  having the same amplification factor as that of the buffer amplifying circuit  56 , which is arranged by an inverting amplifying circuit. As a result, the second processing circuit  55 B produces a signal having a phase opposite to that of the buffer amplifying circuit  60 , and inputs this signal having the opposite phase into a first selecting input terminal of a selecting circuit  61 . In this case, this selecting circuit  61  enters a signal having a phase opposite to that of the carrier signal SC into a second selection input terminal, and grounds the remaining third selection input terminal. This signal having the opposite phase is derived from the first processing circuit  55 A. The selecting circuit  61  switches contacts under control of the control circuit  58 , and a power amplifying circuit  62  provided subsequent to this selecting circuit  61  amplifies the output signal derived from the selecting circuit  61 . 
     As a result, the second processing circuit  55 B is similarly arranged in such a manner that the selecting circuit  61  switches the first contact and the third contact, so that the signal having the phase opposite to that of the carrier signal SC can be gated. Similarly, this second processing circuit  55 B is so arranged that the selecting circuit  61  switches the second contact and the third contact, so that the signal having the same phase as that of the carrier signal SC can be gated. 
     The transmitting circuit  55  outputs the output-signals derived from the first and second processing circuits  55 A and  55 B by connecting these processing circuits to the antenna by way of a wiring line. As a result, as indicated in FIG. 10, the transmitting circuit  55  is arranged as follows. Assuming now that the power outputs of the power amplifying circuits  59  and  62  are set to “P 1 ” and “P 2 ”, when the contacts of either the selecting circuit  61  or the selecting circuit  57  are switched under such a condition that the contacts of either the selecting circuit  57  or the selecting circuit  61  are positioned on the ground side (namely, condition indicated by “OFF” in FIG.  10 ), transmit outputs by the power outputs P 1  and P 2  can be obtained by the in-phase and the reverse phase. 
     Also, the transmitting circuit is so arranged as follows. When the remaining contacts of either the selecting circuit  61  or the selecting circuit  57  are switched to thereby gate the input signal of either the power amplifying circuit  62  or the power amplifying circuit  59  under such a condition that the contacts of either the selecting circuit  57  or the selecting circuit  61  are set to either the in-phase side or the opposite side, the power amplified results of the power amplifying circuits  59  and  62  are added to each other. Therefore, the transmit outputs outputted from the antenna can be switched. Accordingly, the transmitting circuit  55  can obtain the power-amplified ASK modulation signals SM from the output-terminals of the power amplifying circuits  59  and  62 . 
     The control circuit  58  switches the contacts of the selecting circuits  57  and  61  in response to the transmit data stream TX, so that the ASK modulation signal SM is produced from this transmit data stream TX. In other words, as shown in FIG. 11, under such a condition that the power output from the buffer amplifying circuit  56  is continuously selected on the side of the selecting circuit  61  (see FIG.  11 (C)), the control circuit  58  switches the contacts of the selecting circuit  57  in response to the logic level of the transmit data stream TX so as to gate the input signal (see FIG.  11 (A) and FIG.  11 (B)). As a result, the power-amplified ASK modulation signal SM is produced (see FIG.  11 (D)). It should be noted that although FIG. 11 represents such a case that the contacts of the selecting circuit  57  are switched between the output of the buffer amplifying circuit  56  and the ground, the ASK modulation signal may be similarly produced by switching the contacts between other combinations. Also, even when the selecting operations of the selecting circuits  57  and  61  are switched, or even when the contacts of the selecting circuits  57  and  61  are switched at the same time, the ASK modulation signal may be similarly produced. 
     Concrete Circuit Arrangements of ASK Modulating Circuit/Power Amplifying Circuit 
     FIG. 12 is a schematic block diagram for showing concrete circuit arrangements of the ASK modulating circuit  47  and the power amplifying circuit  48  according to the first embodiment. This transmitting circuit  65  contains two signal paths of a transmitting circuit  65 A and another transmitting circuit  65 B, which may drive both terminals of the loop antenna  28 . The above-described two signal paths of processing circuits  55 A and  55 B are equivalently arranged in the respective transmitting circuits  65 A and  65 B. 
     That is, the transmitting circuit  65 A enters thereinto the carried signal SC having the in-phase via the buffer amplifying circuit  66 , and then this carrier signal SC is entered into a power amplifying circuit  67  and another power amplifying circuit  68 . In this case, as represented in FIG. 13, the power amplifying circuit  67 , or  68  is constituted by a P-channel MOS field-effect transistor T 1 , an N-channel MOS field-effect transistor T 2 , and a control logic  70 . 
     Among these circuit elements, the P-channel MOS field-effect transistor T 1  is series-connected to the N-channel MOS field-effect transistor T 2 , and this series-connected transistors are arranged between the power supply and the ground, and also constitute a switching circuit for switching a potential at an output terminal arranged by a joint point in response to a gate voltage set by the control logic  70 . 
     As indicated in a truth table of FIG. 14, when a control terminal input “OEI” of the control logic  70  is set to as H(high) level, this control logic  70  sets the gate terminals of the respective field-effect transistors T 1  and T 2  to an H level and an L(low) level. In this case, the control logic  70  sets the logic levels of the gate terminals by varying the voltages at the gate terminals, higher than the drain-to-source voltage of each of the field-effect transistors T 1  and T 2 . As a consequence, the control logic  70  stops the power amplifying process operation, and further holds the output terminal at the high impedance. 
     Also, when the control logic  70  sets the control terminal input OEI to an L level, this control logic  70  switches the gate terminals of the field-effect transistors T 1  and T 2  in response to a logic level appearing at the input terminal “in” thereof. As a result, the control logic  70  controls to stop the processing operation of the power amplification in response to the logic level of the input terminal “in”, so that the output signal of the power amplifying circuit can be gated in response to the logic level of the input terminal “in”. 
     The power amplifying circuit  67  can stop the power amplifying process operation by switching a control signal “RFoff”, if required, while this control signal RFoff is entered to the control terminal input OEI of this control logic  70 . Since the power amplifying process operation can be stopped, the resultant power consumption can be reduced. 
     While the logical OR output of the control signal “RFoff” obtained via an OR gate  75  and the transmit data stream TX are entered into the control terminal input OEI of the control logic  70 , the power amplifying circuit  68  can stop the power amplifying operation in conjunction with the above-described power amplifying circuit  67 . Also, while the power amplifying circuit  67  executes the power amplifying process operation, the power amplifying circuit  68  stops the power amplifying process operation in response to the logic level of the transmit data stream TX, and also gates the power amplified output having the same phase as that of the power amplified output by the power amplifying circuit  67 . In the case that the power amplifying process operation is stopped in response to the logic level of this transmit data stream TX, since the output terminals of the field-effect transistors T 1  and T 2  are maintained as high impedances, these field-effect transistors T 1  and T 2  do not give the load to the power amplifying circuit  67 . 
     As a consequence, both the power amplifying circuits  67  and  68  may each constitute a tri-state buffer circuit. 
     The first transmitting circuit  65 A supplies the power amplified output of the power amplifying circuit  68  to one terminal of the loop antenna  28 , and also supplies via a resistor  69 , the power amplified output of the power amplifying circuit  67  to one terminal of this loop antenna  28 . As a result, the transmitting circuit  65 A adds the power amplified output of the power amplifying circuit  67  to the power amplified output of the power amplifying circuit  68  via this resistor  69  so as to produce an ASK modulation signal SMA. It should also be noted in the transmitting circuit  65 A that a monitor terminal TMA is arranged at the output terminal of the power amplifying circuit  67 . 
     In contrast to the above arrangement, the second transmitting circuit  65 B is constituted in the same manner to that of the first transmitting circuit  65 A except that a buffer amplifying circuit  71  constructed of an inverting amplifier circuit arrangement is arranged instead of the buffer amplifying circuit  66  employed in the first transmitting circuit  65 A, and furthermore, except that the power-amplified output is supplied to the other terminal of the loop antenna  28 . As a result, the second transmitting circuit  65 B produces another ASK modulation signal “SMB” whose phase is inverted in conjunction with the first transmitting circuit  65 A. This second transmitting circuit  65 B drives the loop antenna  28  by using this ASK modulation signal SMB. 
     Operations of First Embodiment 
     With employment of the above-described circuit arrangements, in the IC card system  21  (see FIG.  7  and FIG.  8 ), the transmit data stream “D(R→C)” which is sent from the IC card reader/writer  23  to the IC card  22  is ASK-modulated by the modulating/demodulating circuit  29 , and then, the ASK-modulated transmit data stream is transmitted via the loop antenna  28 . 
     As a result, when the IC card  22  is approached to the reader/writer  23 , the transmission signal SM is induced by this ASK modulation signal in the loop antenna  24  of this IC card  22 . A portion of this induced transmission signal SM is converted into electric power used in the IC card  22 . This converted electric power may drive the modulating/demodulating circuit  25  and the signal processing circuit  26  of the IC card  22 . 
     Furthermore, as to the transmission signal SM obtained from this loop antenna  24 , the transmit data stream “D(R→C)” is demodulated by the modulating/demodulating circuit  25 , and this transmit data stream “D(R→C)” is analyzed by the signal processing circuit  26  so as to produce the response data stream “D(C→R)” which is transmitted to the reader/writer  23 . In the IC card  22 , this response data stream “D(C→R)” is ASK-modulated by the modulating/demodulating circuit  25 , and as a result, this resulting ASK modulation signal SM is transmitted as a response signal from the loop antenna  24 . 
     As a consequence, the response data stream “D(C→R)” is transmitted from the IC card  22  to the reader/writer  23 . The response signal SM which has been transmitted in this manner is received by the reader/writer  23  by way of the loop antenna  28  which is coupled to the loop antenna  24 . Then, the response data stream “D(C→R)” is demodulated by the modulating/demodulating circuit  29 . 
     The transmit data stream “D(R→C)” which are transmitted/received in this manner is ASK-modulated by the ASK modulating circuit  47 , and thereafter, the ASK-modulated transmit data stream is power-amplified by the power amplifying circuit  48 . Then, the amplified data stream is transmitted via the loop antenna  28 . 
     In accordance with this first embodiment, when the transmit data stream “D(R→C)” is ASK-modulated by this ASK modulating circuit  47  and then, the ASK-modulated transmit data stream is power-amplified by the power amplifying circuit  48  (see FIG.  9 ), with respect to the power-amplified output of the power amplifying circuit  59  for power-amplifying the carrier signal SC by a predetermined phase, the power-amplified output of the power amplifying circuit  62  for similarly power-amplifying the carrier signal SC by a preselected phase is gated to be added in response to the logic level of he transmit data stream “D(R→C)”. As a consequence, the power-amplified ASK modulation signal is produced. 
     Accordingly, the respective power amplifying circuits  59  and  62  can be designed by mainly considering the power efficiency without considering the linearity thereof to some extent. Therefore, the power efficiencies of these power amplifying circuit  59  and  62  can be increased, as compared with those of the conventional power amplifying circuits. Also, these power amplifying circuits  59  and  62  may be constituted by employing the commercially available electronic components. 
     To the contrary, as to the ASK modulation, the output signals are merely gated in the power amplifying circuits  59  and  62 , so that the ASK modulation signal can be produced. Therefore, the output signals can be ASK-modulated by a simple circuit arrangement suitably manufactured as an integrated circuit. Also, the power efficiency can be increased. 
     Concretely speaking, with respect to both the power amplifying circuit  67  and the power amplifying circuit  68 , which are arranged by the tri-state buffer circuits, and amplify the carrier signal SC by the same phase (see FIG.  12 ), in one power amplifying circuit  67 , when the transmit data stream “D(R→C)” is sent out, the carrier signal SC is continuously power-amplified. In contrast thereto, in the other power amplifying circuit  68 , since the carrier signal SC is power-amplified in response to the logic level of the transmit data stream “D(R→C)”, this power-amplified result is gated based on the logic level of the transmit data stream “D(R→C)”, and then, these power-amplified results of the power amplifier circuits  67  and  68  are added to each other via the resistor  69 , so that the loop antenna  28  is driven by this added signal. As a result, in both the terminals of the loop antenna  28 , the amplitude of the power-amplified carrier signal SC is varied in response to the logic level of the transmit data stream “D(R→C)”, so that the loop antenna  28  is driven by both the ASK modulation signals SMA and SMB. 
     Accordingly, the respective power amplifying circuits  67  and  68  can be designed by mainly considering the power efficiency without considering the linearity thereof to some extent. Therefore, the power efficiencies of these power amplifying circuits  67  and  68  can be increased, as compared with those of the conventional power amplifying circuits. Also, these power amplifying circuits  67  and  68  may be constituted by employing the commercially available electronic components. 
     Also, as to the ASK modulation, the power amplifying process operation executed by the power amplifying circuit  68  is simply and intermittently controlled to be stopped and the output signal is merely gated, so that the ASK modulation signal can be produced. As a result, the ASK modulation can be carried out by using a simplified circuit arrangement suitable for manufacturing this circuit arrangement as the IC. Also, the power efficiency can be increased. 
     As previously explained, the power amplifying circuits  67  and  68  for power-amplifying the carrier signal are constituted by series-connecting the P-channel field-effect transistor T 1  to the N-channel field-effect transistor T 2  (see FIG.  13 ). The power amplifying process operation may be stop-controlled by such a manner that while the gate voltages of these P-channel/N-channel field-effect transistors T 1 /T 2  are varied higher than, or equal to the source-to-drain voltage, these gate voltages are kept in the H level and the L level, respectively. 
     As a consequence, when the transmit data stream “D(R→C)” is transmitted, in contrast to the power amplifier circuit  67  for continuously power-amplifying the carrier signal SC, in such a power amplifying circuit  68  for intermittently power-amplifying the carrier signal in response to the logic level of the transmit data stream “D(R→C)”, the output terminal thereof is maintained under high impedance state within a time period during which the power amplifying process operation is stopped. The power-amplified output of the power amplifying circuit  67  is not consumed by the power amplifying circuit  68  whose amplifying operation is stopped. As a result, this control operation can increase the power efficiency. Also, since substantially no source currents of these field-effect transistors T 1  and T 2  will flow, this may increase the power efficiency. 
     Advantages of First Embodiment 
     In accordance with the above-described circuit arrangement of the first embodiment, with respect to the power-amplified output of the power amplifying circuit  59  for power-amplifying the carrier signal SC by a predetermined phase, the power-amplified output of the power amplifying circuit  59  for similarly power-amplifying the carrier signal SC by a preselected phase is gated to be added in response to the logic level of the transmit data stream “D(R→C)”. As a consequence, the power-amplified ASK modulation signal can be produced by employing the power amplifying circuits which are designed by mainly considering the power efficiency without considering the linearity thereof to some extent. Therefore, the power efficiencies of these power amplifying circuit can be increased, as compared with those of the conventional power amplifying circuits. Also, these power amplifying circuits may be constituted by employing the commercially available electronic components. Also, these power amplifying circuits can be simply and readily manufactured in the form of ICs in connection with other circuit blocks. Both the modulating circuit and the power amplifying circuits, the efficiencies of which are further increased, can be obtained. 
     Also, since the power amplifying circuit is constituted by employing the tri-state buffer circuit type amplifying circuit, the power efficiency can be improved by the simple control operation. This tri-state buffer circuit switches the output terminal into the high impedance when the operation is stopped. As a consequence, these power amplifying circuits can be simply and readily manufactured in the form of ICs in connection with other circuit blocks. The power amplifying circuits, the efficiencies of which are further increased, can be obtained. 
     ASK Modulating Circuit of Second Embodiment 
     As shown in FIG. 15 in contrast with FIG. 9, there is indicated a basic idea of an ASK modulating circuit applied to a second embodiment of the present invention. In this ASK modulating circuit  79 , the power-amplified results obtained from the power amplifying circuits  59  and  62  are supplied to two signal paths of antennas  28 A and  28 B, respectively. These two power-amplified results are added to each other in electromagnetic fields transmitted from the antennas  28 A and  28 B. 
     As represented in FIG. 15, even when the ASK modulating circuit is so arranged that these two power-amplified results are added to each other in the electromagnetic fields, it is possible to achieve a similar effect to that of the first embodiment. 
     ASK Modulating Circuit of Third Embodiment 
     FIG. 16 is a block diagram for representing a basic arrangement of an ASK modulating circuit applied to a third embodiment of the present invention. In this third embodiment, this ASK modulating circuit  80  is applied instead of the ASK modulating circuit  47  and the power amplifying circuit  48  (see FIG. 8) of the reader/writer  23 . 
     In other words, in the ASK modulating circuit  80 , after the carrier signal is amplified by the power amplifying circuit  81 , the amplified carrier signal is outputted via a variable attenuator  82 . In response to the transmit data stream TX(D(R→C)), the variable attenuator  82  attenuates this carrier signal to thereby output the attenuated carrier signal. As a result, this ASK modulating circuit  80  outputs such an ASK modulation signal SM, the amplitude of which is varied in response to the transmit data stream TX(D(R→C)). 
     As a typical variable attenuator type, for example, the variable attenuator  82  as shown in FIG. 17 is arranged by which a transfer path is terminated; the variable attenuator  82  as shown in FIG. 18 is arranged which is inserted in series of a transfer path; and also, as represented in FIG. 19, two sets of the above-explained variable attenuators are combined with each other. 
     In further detail, in the ASK modulating circuit according to this third embodiment, such an ASK modulating circuit as indicated in FIG. 20 is applied. That is, in the ASK modulating circuit  90 , the carrier signal SC is entered into the buffer amplifying circuit  91 , and then, the output signal of this buffer amplifying circuit  91  is amplified by the power amplifying circuit  92 . The power amplifying circuit  92  supplies the power-amplified result via a resistor  93  to one terminal of the loop antenna  28 . This one terminal of the loop antenna  28  is connected through another resistor  94  and a field-effect transistor T 3  to the ground. As a result, in the ASK modulating circuit  90 , the variable attenuator may be constituted in such a way that the attenuation amount is switched by ON/OFF-controlling the field-effect transistor T 3 , and the power-amplified result outputted from the power amplifying circuit  92  is ASK-modulated by this variable attenuator. 
     Also, in the ASK modulating circuit  90 , the carrier signal SC is inputted to another buffer amplifying circuit  95  arranged as an inverting amplifier circuit, and then, the output signal of this buffer amplifying circuit  95  is amplified by the power amplifying circuit  96 . The power amplifying circuit  96  supplies the power-amplified result via another resistor  97  to the other terminal of the loop antenna  28 . The other terminal of the loop antenna  28  is connected through another resistor  98  and a field-effect transistor T 4  to the ground. As a result, in the ASK modulating circuit  90 , the variable attenuator may be constituted in such a way that the attenuation amount is switched by ON/OFF-controlling the field-effect transistor T 4 , and the power-amplified result outputted from the power amplifying circuit  96  is ASK-modulated by this variable attenuator. 
     In accordance with the circuit arrangement of the third embodiment, on the output side of the power amplifying circuit, the power-amplified result is attenuated in response to the transmit data stream TX so as to produce the ASK modulation signal SM. As a consequence, in the power amplifying circuit, the carrier signal is amplified without saving the amplitude component, so that the ASK modulation signal can be produced. Accordingly, the power amplifying circuit can be operated in the saturation region, and the power amplifying circuit can be arranged by employing such general-purpose logic ICs as CMOS standard logic ICs and TTL circuits. As a result, the power efficiency can be increased as compared with the conventional power amplifying circuit. The ASK modulating circuit can be arranged by employing the commercially available general-purpose electronic components. The ASK modulating circuit of the third embodiment can be simply and easily manufactured together with other circuit blocks in the IC form. Moreover, it is possible to arrange the modulating circuit and the power amplifying circuit, which can be operated in higher efficiencies. 
     ASK Modulating Circuit of Fourth Embodiment 
     As shown in FIG. 21 in contrast with FIG. 16, there is indicated a basic idea of an ASK modulating circuit applied to a fourth embodiment of the present invention. In this ASK modulating circuit  100 , the power-amplified result obtained from the power amplifying circuit  81  is supplied to the first antenna  28 A, and the variable attenuator  82  is connected to the second antenna  28 B electromagnetically coupled to this first antenna  28 A. As a result, in this ASK modulating circuit  100 , an electromagnetic field radiated from the first antenna  28 A is varied in response to the transmit data stream TX via the second antenna  28 B, so that an ASK modulation signal is radiated toward the IC card. 
     In accordance with the circuit arrangement shown in FIG. 21, even when the power-amplified result is attenuated to produce the ASK modulation signal SM in the electromagnetic field produced on the output side of the power amplifying circuit, it is possible to achieve a similar effect to that of the third embodiment. 
     ASK Demodulating Circuit of Fifth Embodiment 
     FIG. 22 is a block diagram for representing a basic arrangement of an ASK demodulating circuit in accordance with a fifth embodiment of the present invention. In this fifth embodiment, this ASK demodulating circuit  110  is utilized instead of the ASK detecting circuit  49  and the low-pass filter  50  (see FIG.  8 ). 
     The ASK demodulating circuit  110  receives the output signal SM of the loop antenna  28  via a coupling capacitor  111 . A resistor  112  and a DC power supply  113  bias the output signal SM by a preselected voltage VB. As shown in FIG. 23, the bias voltage VB is set such that only a half wave of an input signal is amplified in a post-staged amplifying circuit  114 . As a result, in the ASK demodulating circuit  110 , the response signal SM obtained via the loop antenna  28  is half-wave-rectified by the amplifying circuit  114 , and then, as to the ASK modulation signal corresponding to the response signal, a baseband converted component of a side-band wave signal made by multiplying a carrier wave by a side-band wave signal is produced. As a result, this baseband converted component may constitute a demodulation signal SD of the ASK modulation signal. 
     Alternatively, a limiter is arranged on the output side of the amplifying circuit  114  as shown in FIG. 24, and then, only the half wave of the response signal SM is amplified as represented in FIG.  25 . As a result, the baseband converted component of the side-band wave signal may be produced. 
     A carrier removing circuit  118  removes an ASK modulation signal component from such an output signal of the amplifying circuit  114 , into which the demodulation signal SD produced in this manner has been mixed, and then outputs the removed ASK modulation signal component. It should be understood that the ASK modulation signal component is removed from a limiter output of a limiter  116  in the circuit arrangement of FIG.  24 . 
     Also, it should be understood that with this type of amplifying circuit  114 , an amplifier circuit which employs of a transistor and a field-effect transistor, and a differential amplifying circuit may be utilized. Also, as the carrier removing circuit  118 , a low-pass filter, a band-pass filter, and a trap filter may be applied in accordance with the frequency band of the demodulation signal SD. 
     In further detail such a circuit arrangement as indicated in FIG. 26 is applied to the ASK demodulating circuit according to this fifth embodiment. That is to say, in the ASK demodulating circuit  120 , the output signal SM of the loop antenna  28  is inputted via a coupling capacitor  121  to a field-effect transistor T 5 . 
     In this case, this field-effect transistor T 5  may constitute a source-grounded type amplifying circuit having a drain resistor  124 , to which a gate voltage is biased by a diode-connected field-effect transistor T 6 , and resistors  122  and  123 . As a result, the ASK modulation signal SM input to the gate of the field-effect transistor T 5  is biased only by a predetermined voltage, so that this field-effect transistor T 5  amplifies only the input signal wave along a positive direction. 
     In the fifth embodiment, since only the half wave of the input signal is amplified by setting the bias voltage so as to remove the carrier component, the ASK modulation signal can be demodulated by such a simple circuit arrangement. As a consequence, this ASK demodulating circuit can be simply and easily manufactured in the IC form in combination with other circuit blocks. Furthermore, it is possible to achieve the demodulating circuit operable in the high efficiency. 
     ASK Demodulating Circuit of Sixth Embodiment 
     As shown in FIG. 27 in contrast with FIG. 26, there is indicated schematic block diagram of an ASK demodulating circuit according to a sixth embodiment of the present invention. In this ASK demodulating circuit  130 , transistors T 7  and T 8  are used instead of the field-effect transistors T 5  and T 6 . 
     In accordance with the circuit arrangement shown in FIG. 27, even when transistors T 7  and T 8  are used instead of the field-effect transistors T 5  and T 6 , it is possible to achieve a similar effect to that of the fifth embodiment. 
     ASK Demodulating Circuit of Seventh Embodiment 
     FIG. 28 is a schematic block diagram of an ASK demodulating circuit according to a seventh embodiment of the present invention, as shown in contrast with FIG.  26 . In this ASK demodulating circuit  140  of the seventh embodiment, the ASK modulation signal SM entered via the coupling capacitor  121  is amplified by an emitter-grounded type amplifying circuit constructed of resistors  131  to  133 , and a transistor T 9 , and thereafter, the amplified ASK modulation signal SM is entered into the carrier removing circuit  118 . 
     Furthermore, the amplitude of the signal outputted from this emitter-grounded type amplifying circuit is limited by a limiter constituted by a diode  134  and a constant voltage power supply  135 , which are arranged at the input terminal of this carrier removing circuit  118 . As a result, only a half wave of the amplified signal result is input into the carrier removing circuit  118  so as to convert a side-band wave component into a baseband component. 
     In accordance with the circuit arrangement shown in FIG. 28, even when only the half wave of the modulation signal is amplitude-limited on the output side of the amplifying circuit, it is possible to achieve a similar effect to that of the above-described fifth embodiment. 
     ASK Demodulating Circuit of Eighth Embodiment 
     FIG. 29 is a block diagram for representing a basic arrangement of an ASK demodulating circuit applied to an eighth embodiment of the present invention. In this eighth embodiment, this ASK demodulating circuit  150  is utilized instead of the ASK detecting circuit  49  and the low-pass filter  50  (see FIG.  8 ). 
     This ASK demodulating circuit  150  receives the output signal SM of the loop antenna  28  via the coupling capacitor  121 . This output signal SM is entered into a clamping circuit  151 . In this embodiment, this clamping circuit  151  clamps the entered ASK modulation signal SM so as to apply a waveform distortion to the ASK modulation signal SM. As a result, the ASK demodulating circuit  150  converts the side-band wave component of the ASK modulation signal SM into a baseband component. 
     In further detail, in the ASK demodulating circuit according to this eighth embodiment, a circuit arrangement shown in FIG. 30 is utilized. That is, in this ASK demodulating circuit  150 , the output terminal of the coupling capacitor  121  is grounded by using a diode D having a reverse polarity in order to clamp the ASK modulation signal. 
     In accordance with the eighth embodiment, since the amplitude of the ASK modulation signal is limited and the circuit arrangement is suitably manufactured in the IC form by the grounded type diode, this amplitude limiting means can be formed. As a consequence, this ASK demodulating circuit can be simply and easily manufactured in the IC form in combination with other circuit blocks. Furthermore, it is possible to achieve the demodulating circuit operable in the high efficiency. 
     ASK Demodulating Circuit of Ninth Embodiment 
     FIG. 31 is a block diagram for representing a basic arrangement of an ASK demodulating circuit according to a ninth embodiment of the present invention. In this ninth embodiment, this ASK demodulating circuit  160  is applied instead of the ASK detecting circuit  49  and the low-pass filter  50  (see FIG.  8 ). 
     In this ASK demodulating circuit  160 , the ASK modulation signal SM is entered into an amplifying circuit  161  so as to produce an ASK modulation signal SMA having an in-phase and another ASK modulation signal SMB having a reverse phase with respect to the phase of the first-mentioned ASK modulation signal SM. Then, the ASK demodulating circuit  160  inputs these ASK modulation signals SMA and SMB via the coupling capacitor  121  to the clamping circuits  162 . In this case, both the ASK modulation signals SMA and SMB are clamped by the clamping circuit  162  based on the substantially same signal level. Waveform distortion is applied to these ASK modulation signals SMA and SMB at a substantially same degree. 
     The ASK demodulating circuit  160  inputs to the carrier removing circuit  118 , such ASK modulation signals SMA and SMB to which the waveform distortion is applied and also which contain the demodulation signal SD, so that signal components of these ASK modulation signals SMA and SMB are removed. Thereafter, this ASK demodulating circuit  160  enters the output signals of the carrier removing circuit  118  into an adding circuit  163 . In this adding circuit  163 , since these output signals are added to each other, the cross modulation components which are contained in the demodulation signal SD by applying the waveform distortion are removed by cancellation. Alternatively, the demodulation signal may be extracted with high efficiency by employing the averaging process operation instead of this adding process operation. 
     In further detail, in the ASK demodulating circuit  160  according to this ninth embodiment, a circuit arrangement shown in FIG. 32 may be utilized. In other words, in the ASK demodulating circuit  160 , the ASK modulation signal SMA having the in-phase and the ASK modulation signal SMB having the reverse phase are clamped by the diode D, respectively. 
     Furthermore, in the ASK demodulating circuit  160 , the ASK modulation signals SMA and SMB, whose amplitudes have been limited in the above-described manner, are added to each other by an adding circuit arranged as a non-inverting amplifier circuit made of resistors  164  to  166 , a bias power supply  167 , and an operational amplifier circuit  168 . As a result, the cross modulation component is canceled together with the ASK modulation signal component, and the demodulation signal components commonly contained in the ASK modulation signals SMA and SMB to which the waveform distortion is given. Thereafter, this ASK demodulating circuit  160  limits the frequency band by using the carrier removing circuit  118  to thereby output the demodulating signal SD. 
     In accordance with the ninth embodiment, the ASK modulation signal having the reverse phase is clamped and the waveform distortion is applied thereto. Thereafter, these ASK modulation signals are added to each other so as to cancel the unwanted signal component, so that the ASK modulation signal can be demodulated by the simple circuit arrangement. As a result, this ASK demodulating circuit can be simply and readily manufactured in the IC form together with other circuit blocks, and furthermore, such a demodulating circuit operable in a high efficiency can be obtained. 
     ASK Demodulating Circuit of Tenth Embodiment 
     FIG. 33 is a block diagram for representing a basic arrangement of an ASK demodulating circuit according to a tenth embodiment of the present invention. In this tenth embodiment, this ASK demodulating circuit  170  is applied instead of the ASK detecting circuit  49  and the low-pass filter  50  (see FIG.  8 ). 
     This ASK demodulating circuit  170  is arranged in such a manner that an ASK modulation signal is balance-outputted which is detected via a balance type transformer  171  by the loop antenna  28 . As a result, the ASK modulation signals SMA and SMB having the phases opposite to each other can be produced without employing the above-described amplifying circuit  161  in FIG.  32 . 
     In the ASK demodulating circuit  170 , one ASK modulation signal SMA is clamped by a clamping circuit constructed of diodes, the clamping potentials of which are set to a positive polarity side and a negative polarity side, respectively. Similarly, the other ASK modulation signal SMB is clamped by another clamping circuit constructed of diodes, the clamping potentials of which are set to the positive polarity side and the negative polarity side, respectively. 
     Furthermore, with respect to the ASK demodulating circuit  170 , in a low-pass filter circuit arranged by resistors  172 ,  173 , and a capacitor  174 , the ASK modulation signals SMA and SMB which are clamped at the clamping potentials on the positive potential side are added to each other by way of the resistors. The added result is entered into a non-inverting input terminal of a differential amplifying circuit  175 . Similarly, in another low-pass filter circuit arranged by resistors  176 ,  177 , and a capacitor  178 , the ASK modulation signals SMA and SMB which are clamped at the clamping potentials on the negative potential side are added to each other by way of the resistors. The added result is entered into an inverting input terminal of the differential amplifying circuit  175 . 
     As a result, the ASK demodulating circuit  170  may cancel the ASK modulation signal component, and also may extract the demodulation signal component which is clamped to be produced. 
     The frequency bands of both the non-inverting output and the inverting output derived from this differential amplifying circuit  175  are limited by the filters  179  and  180 . As a result, after the ASK demodulating circuit  170  removes the ASK modulation signal component from the band-limited signal outputs, the resultant signals are added to each other by the differential amplifying circuit  181  and then this ASK demodulating circuit  170  outputs the added signal. 
     In accordance with this tenth embodiment, the balanced outputs are obtained from the antenna and then are clamped so as to demodulate the ASK modulation signal. Thus, the ASK modulation signal can be demodulated by employing the simpler circuit arrangement than that of the ninth embodiment. 
     Also, since the respective balanced outputs are clamped by the clamping circuits, the clamping potentials of which are set to the positive polarity side and the negative polarity side to be further processed, the ASK modulation signal can be processed with a higher efficiency than that of the ninth embodiment. Also, the S/N ratio of the demodulation result can be improved. 
     ASK Demodulating Circuit of Eleventh Embodiment 
     FIG. 34 is a block diagram for representing a basic arrangement of an ASK demodulating circuit according to an eleventh embodiment of the present invention. In this eleventh embodiment, this ASK demodulating circuit  190  is applied instead of the ASK detecting circuit  49  and the low-pass filter  50  (see FIG.  8 ). 
     This ASK demodulating circuit  190  executes a similar process operation, namely the ASK modulation signal SM is rectified by the full wave rectification by such that a polarity of the ASK modulation signal SM is judged in a polarity judging circuit  191 , and then, the ASK modulation signal SM is multiplied by this polarity judgement result in a multiplying circuit  192 . As a result, the demodulating circuit  190  demodulates the ASK modulation signal SM to thereby output a demodulation signal SD. 
     In further detail, in the ASK demodulating circuit  190  according to this eleventh embodiment, a circuit arrangement shown in FIG. 35 is utilized. In other words, in this ASK demodulating circuit  190 , the ASK modulation signal SM is inputted via the coupling capacitor  121  to an inverting amplifying circuit  194 . In this case, an input terminal of the inverting amplifying circuit  194  is connected via diodes D to the ground and the power supply line. As a result, the inverting amplifying circuit  194  outputs such a polarity signal, by which the signal level is switched to both the-positive side and the negative side in response to the polarity of the ASK modulation signal. As a consequence, the polarity judging circuit  191  is arranged by a limiter for amplitude-limiting the ASK modulation signal SM on the positive side and the negative side. 
     The ASK demodulating circuit  190  outputs the demodulation signal SD in such a manner that this polarity signal is multiplied by the ASK modulation signal SM in the multiplying circuit  192 , and then, the multiplication result is band-limited by the carrier removing circuit  118 . It should be noted in this embodiment that the multiplying circuit  192  is arranged by a double balanced mixer made of the Gilbert multiplier. 
     In accordance with the eleventh embodiment, since the polarity of the ASK modulation signal is judged by the polarity judging circuit and then the judgement result is multiplied by the ASK modulation signal, the polarity judging circuit can be made simple, and the ASK modulation signal can be demodulated. As a result, this ASK demodulating circuit can be simply and readily manufactured in the IC form together with other circuit blocks. Furthermore, such a demodulating circuit operable in a higher efficiency can be obtained. 
     ASK Demodulating Circuit of Twelfth Embodiment 
     FIG. 36 is a block diagram for representing a basic arrangement of an ASK demodulating circuit according to an twelfth embodiment of the present invention. In this twelfth embodiment, this ASK demodulating circuit  200  is applied instead of the ASK detecting circuit  49  and the low-pass filter  50  (see FIG.  8 ). 
     This ASK demodulating circuit  200  outputs a similar demodulation result, namely the ASK modulation signal is rectified by the full wave rectification by such that a polarity of the ASK modulation signal SM is judged in the polarity judging circuit  191 , and the ASK modulation signal SM is selectively outputted based upon this judgement result. 
     In other words, the demodulating circuit  200  produces an ASK modulation signal SMA having the same phase as that of the ASK modulation signal SM, and also another ASK modulation signal SMB having a phase opposite to that of the ASK modulation signal SM in an amplifying circuit  201 . The demodulating circuit  200  intermittently outputs the ASK modulation signal SMB having the opposite phase via a switch circuit  203  based upon the judgement result of the polarity judging circuit  191 . Furthermore, this demodulating circuit  200  intermittently outputs the ASK modulation signal SMA having the same phase via another switch circuit  202  based upon a reverse polarity signal of a judgement result obtained via an inverting amplifying circuit  204 . As a result, this demodulating circuit alternately outputs the ASK modulation signal SMA and SMB in response to the polarity judgement result so as to full-wave-rectify the ASK modulation signal SM. 
     In further detail, in the ASK demodulating circuit  20  according to this twelfth embodiment, as represented in FIG. 37, since the polarity signal is produced by the polarity judging circuit similar to the above-described polarity judging circuit of FIG. 35, the demodulation signal SD is outputted. 
     It should be understood that, as indicated in FIG. 38, this sort of switch circuit  202  and  203  may be realized in such a manner that a P-channel field-effect transistor T 10  is connected in parallel to an N-channel field-effect transistor T 11 , and gate voltages of these transistors T 10  and T 11  are varied by an inverting amplifying circuit  206  in a complementary manner. 
     In accordance with the twelfth embodiment, since the polarity of the ASK modulation signal is judged by the polarity judging circuit and then, the ASK modulation signals are selectively outputted based on this judgement result, the polarity judging circuit can be made simple, and the ASK modulation signal can be demodulated. As a result, this ASK demodulating circuit can be simply and readily manufactured in the IC form together with other circuit blocks. Furthermore, such a demodulating circuit operable in a higher efficiency can be obtained. 
     Other Embodiments 
     It should be understood that the above embodiments have described such a case where the modulation signal is produced by employing the carrier signal having the same frequency. The present invention is not limited thereto. Alternatively, a rectangular wave signal having a single frequency may be employed instead of the carrier signal. Also, both a phase modulation signal and a frequency modulation signal may be used so as to produce a modulation signal by way of a so-called “dual modulation”. It should also be noted that when such a rectangular wave signal is employed so as to produce the modulation signal, for example, the inverting amplifier circuits  60  and  71  as explained in the first embodiment may be constituted by an inverter. 
     Also, in the specific arrangement of the first embodiment, the operation of the power amplifying circuit is intermittently stopped so as to gate the carrier signal. The present invention is not limited thereto. That is, as previously explained in the basic arrangement, the carrier signal may be gated by the switch circuit provided on the input side of the power amplifying circuit. In this alternative case, the switch circuit as previously explained with reference to FIG. 38 may be employed. Furthermore, a multiplexer arranged by a digital circuit arrangement may be used. 
     In addition, the output stage of the power amplifying circuit as the concrete arrangement of the first embodiment is arranged by the field-effect transistor. The present invention is not limited thereto. Alternatively, the output stage of the power amplifying circuit may be arranged by a bipolar type transistor. 
     Also, as the above-described specific arrangement of the first embodiment, one power-amplified result having the same phase is gated, and then this gated power-amplified result is added to the other power-amplified result having the same phase. The present invention is not limited thereto, for example, while one power-amplified result may be gated, this gated power-amplified result may be subtracted from the other power-amplified result. Also, while one power-amplified result having the reverse phase may be gated, the other power-amplified result may be added to this gated power-amplified result. Furthermore, while one power-amplified result having the reverse phase is gated, this gated power-amplified result may be subtracted from the other power-amplified result. 
     Also, in the second embodiment, the variable attenuator is arranged on the output side of the power amplifying circuit. The present invention is not limited thereto. Alternatively, for example, in a circuit arrangement operable by electric power received by an antenna, since an impedance of a power supply circuit is switched, a load is equivalently switched on the output side of the power amplifying circuit to constitute an attenuator. 
     Also, as the to the specific arrangement of the second embodiment, the field-effect transistor is used as the switching element, the power-amplified results are attenuated. The present invention is not limited thereto. Alternatively, this switching element may be constituted by a bipolar type transistor, and a PIN diode. 
     Furthermore, in the above-explained embodiments, the IC card is operated by the electric power of the transmission signal. The present invention is not limited thereto. Alternatively, this IC may be operated by a battery and the like. 
     Also, in the above-described embodiment, the transmit data is modulated and/or demodulated by employing the Manchester code. The present invention is not limited thereto. Alternatively, the inventive idea of the present invention may be widely applied to a case that various sorts of codes other than the Manchester code are modulated/demodulated, another case being that a data stream having multi values is amplitude-modulated, or amplitude-demodulated, and a further case being that an analog signal such as an audio signal is amplitude-modulated, or amplitude-demodulated. 
     In the above embodiments, the inventive idea of the present invention is applied to the IC card and the IC card reader/writer. The present invention is not limited thereto. Alternatively, this inventive idea may be widely applied to various transmitter apparatuses and various receiver apparatuses. 
     As previously described in detail, the first and second output signals having predetermined phases with respect to the input signal are added to each other, and the added result is outputted. At least, the second output signal is gated in response to the input data. Also, on the output side of the power amplifying circuit, the power-amplified results are attenuated in accordance with the input data so as to produce the amplitude-modulated signal. This circuit arrangement can be simply and readily manufactured in the IC form together with other circuit blocks. Furthermore, it is also possible to obtain the modulating circuit operable in the higher efficiency, the amplifying circuit applicable to this modulating circuit, and also the transmitter apparatus with using this modulating circuit. 
     Also, since the amplitude-modulated signals are biased to be amplified, or clipped, the demodulating circuit and the receiver apparatus with using this demodulating circuit can be simply and easily manufactured in the IC form in combination with other circuit blocks. 
     Furthermore, the amplitude-modulated signals are clamped, the polarities of these amplitude-modulated signals are judged to multiply the polarity judgement result by the amplitude-modulated signals, and furthermore, the amplitude-modulated signals are selectively outputted based upon the polarity judgement results of the amplitude-modulated signals. As a result, the demodulating circuit and the receiver apparatus with using this demodulating circuit can be simply and readily manufactured in the IC form together with other circuit blocks.