Abstract:
A processing unit with balanced outputs transfers a received digital signal to an amplification unit with balanced inputs and outputs. A control unit enables or disables the processing and amplification units in response to a power up/power down signal. To prevent disturbances due to power up/power down transients from appearing in a speaker connected between the outputs of the amplification unit, switches are provided between the outputs of the processing unit and the inputs of the amplification unit. A delay circuit generates according to a predetermined timing program enabling/disabling control signals for the processing and amplification units, and generates control signals for the switches.

Description:
FIELD OF THE INVENTION 
     The present invention relates to the field of transceivers, and, more particularly, to a receiver section of a telephone. 
     BACKGROUND OF THE INVENTION 
     A signal received by a telephone from a digital telephone network is demodulated, processed in digital form, converted to analog form, and is amplified before being applied to an electroacoustic transducer. The receiver section of a telephone can be represented schematically as shown in FIG.  1 . 
     A demodulated digital signal RX-IN produced by a receiving and demodulating unit  10  is applied to the input of a processing unit  11 . The processing unit  11  converts the demodulated digital signal to analog form, filters and amplifies the analog signal. The analog signal leaving the processing unit  11  is applied to the input of a power amplifier  12  for transferring the signal to an electroacoustic transducer  13 , such as an electromagnetic earphone or speaker, with sufficient electrical power for its operation. 
     The gain of the power amplifier  12  is determined by the ratio between the resistance of a feedback resistor R 2  connected between the output and the inverting input of the amplifier, and by the resistance of a resistor R 1  connected between the output of the processing unit  11  and the inverting input of the amplifier. 
     To prevent the dissipation of power when the telephone is not in use, circuit means are provided both in the processing unit  11  and in the amplifier  12  to respond to an external enabling or disabling command. This function is initiated via a terminal connection of the processing unit  11  and via a terminal connection of the amplifier  12  in response to a digital power down PD signal. The power down PD signal may be a single bit signal. The receiver section is put into a state of zero current absorption when PD=1 (power down), and is enabled to absorb power supply current when PD=0 (power up). 
     When a change is made from the power down state to the power up state, or vice versa, both the processing unit  11  and the power amplifier  12  are subjected to abrupt voltage and current transients before returning to a normal operating condition. During these transients, electrical disturbances are produced by components having frequencies in the acoustic band, which results in an audible noise in the speaker  13  that may be annoying. 
     Various arrangements requiring the addition of active or passive components in series and/or in parallel with the speaker  13  for filtering undesired electrical disturbances have been adopted to overcome this problem. This approach, however, requires relatively bulky components outside the power amplifier which may normally be formed as an integrated circuit. 
     SUMMARY OF THE INVENTION 
     An object of the present invention is to provide a receiving section for a telephone which is not subject to the problem described above and which can easily be integrated. 
     This and other objects of the present invention are provided by a receiver section of a telephone comprising a demodulator unit for receiving and demodulating an input signal, and a processor unit for processing the demodulated input signal. The processor unit includes balanced outputs. An amplifier unit including balanced inputs is coupled to the balanced outputs of the processor unit, and the amplifier unit includes balanced outputs coupled to a speaker. 
     A switch circuit is coupled between the balanced outputs of the processor unit and the balanced inputs of the amplifier unit. A control unit generates a closing control signal for closing the switch circuit according to a predetermined delay with respect to a power up signal for preventing disturbances in the processor unit due to transients of the power up signal from appearing on a processed input signal being applied to the amplifier unit. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The invention will be more clearly understood from the following detailed description of various embodiments, provided by way of examples, and therefore, without any restrictions, with reference to the attached drawings, in which: 
     FIG. 1 is a block diagram of a receiver section of a telephone according to the prior art; and 
     FIGS. 2-4 are block diagrams respectively illustrating an embodiment according to the present invention. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     In FIG. 2, elements identical or corresponding to those of FIG. 1 are indicated by the same reference numbers. A processing unit  11  with a balanced structure receives at its input a demodulated digital signal RX-IN, and supplies a balanced analog signal at two outputs. The outputs of the processing unit  11  are coupled to the inputs of a differential amplifier  12 . More particularly, each output is connected to one of the inputs of the amplifier through a respective resistor R 1 A, R 1 B in series with an n-channel MOS transistor M 1 A, M 1 B operating as an electrical switch. 
     Each of the differential outputs  15 ,  16  of the amplifier  12  is connected, respectively, to the inverting and non-inverting inputs of the amplifier through a resistor R 2 A, R 2 B in series with a MOS transistor M 2 A, M 2 B. Each MOS transistor M 2 A, M 2 B is biased for conduction. An electroacoustic transducer  13 , i.e., a speaker, is connected between the outputs  15 ,  16  of the differential amplifier  12 . 
     The processing unit  11  and the differential amplifier  12  are supplied power from a voltage source. The processing unit  11  and the differential amplifier  12  each have circuit means which enables or disables the power supply according to a signal applied to respective enabling terminals  17 ,  18 . 
     A control unit  20  includes a delay circuit  24  and a logic circuit  21 , and has an input terminal  19  to which is applied a binary power down PD signal generated by a central circuit control unit (not shown) of the telephone. The logic circuit  21  responds to the power down PD signal by causing the closing or opening of an electrical switch  22  and the closing or opening of an electrical changeover switch  23 . The switch  22  connects the terminal  19  to the enabling terminal  17  of the processing unit  11 . The changeover switch  23  selectively connects the terminal  19  to the enabling terminal  18  of the differential amplifier  12  directly or through the delay circuit  24 . 
     The changeover switch  23  is used to additionally apply the power down PD signal, directly or through the delay circuit  24 , to the gate electrodes of the n-channel MOS transistors M 1 A and M 1 B through an inverter  25 . The inverter is necessary because the n-channel transistors M 1 A and M 1 B must be closed when the amplifier  12  is activated. Since this happens when the binary power down PD signal is at a low level (PD=0), this signal must be inverted so that it has the necessary positive voltage to operate the n-channel transistors M 1 A and M 1 B. If p-channel transistors were used in place of the n-channel transistors, the inverter  25  would not be necessary. 
     The operation of the circuit will now be considered. The delay circuit  24 , the switch  22 , the changeover switch  23  and the logic circuit  21  form, in combination, a delay unit which produces control signals according to a predetermined timing program. In particular, the logic circuit  21  produces a signal Y=1 if the power down PD signal is changing from 1 to 0, i.e., from a power down state to a power up state. The logic circuit  21  produces a signal Y=0 if the power down PD signal is changing from the 0 to 1, i.e., from a power up state to a power down state. 
     When Y=1, the position of the switch  22  and that of the changeover switch  23  are as shown in the drawings. In other words, a 0 signal is applied to the terminal  17  at the instant of the closing of the switch  22 , and the 0 signal is applied to the terminal  18  after a period of delay fixed by the delay circuit  24 . The delay period is chosen to be sufficiently long to allow the processing unit  11  to settle. Thus the disturbances present at the output of the processing unit  11  have no effect on the amplifier  12  or on the speaker  13  because the amplifier remains off and the transistors M 1 A and M 1 B act as open circuits during the settling of the processing unit. 
     At the end of the delay period fixed by the delay circuit  24 , the amplifier  12  is also activated and the outputs of the processing unit  11  are connected to the inputs of the amplifier through the MOS transistors M 1 A and M 1 B when they are conducting. Since the input signal of the differential amplifier  12  is stable, any disturbances at the output of the amplifier is limited to the settling of the amplifier  12 . 
     However, since the structure of the amplifier  12  is balanced, the voltages present at the two outputs  15 ,  16  due to any settling disturbances will be in phase with each other, and therefore, will not have any effect on the differential output signal Vout. Consequently, there will not be any disturbances in the speaker  13 . 
     In the presence of a power down signal (PD from 0 to 1), the signal emitted by the logic circuit  21  is at low level (Y=0). This causes the opening of the switch  22  and the switching of the changeover switch  23 . Therefore, the amplifier  12  is disabled, the switches formed by the transistors M 1 A and M 1 B are opened and the processing unit  11  is disabled. 
     The settling of the amplifier  12  has no effect on the output since the structure is a differential amplifier. The processing unit  11  has no effect on the output of the receiver section because the connections between the processing unit  11  and the amplifier  12  are interrupted by the opening of the transistors M 1 A and M 1 B. The disabling of the amplifier  12  takes place simultaneously with the disabling of the processing unit  11  to avoid any disturbances in the speaker  13 . 
     The function of the transistors M 2 A and M 2 B biased for conduction is to introduce a supplementary resistance into the feedback circuit for the determination of the gain. The transistors M 2 A and M 2 B are biased for conduction. The supplementary resistance is equal to that of the transistors M 1 A and M 1 B when conducting. The gain of the amplifier  12  is determined from the ratio between the feedback resistance and the resistance in series with the input terminal. 
     In the design of an integrated circuit, it is not possible to precisely specify the resistance of a single resistor due to the wide variations in the manufacturing parameters, as readily understood by one skilled in the art. However, it is possible to produce two resistors having a precise ratio between their resistances. 
     The insertion of a respective transistor M 1 A, M 1 B in series with each of the inputs, and consequently with the respective resistors, modifies the ratio fixed at the design stage between the feedback resistance and the resistance in series with the inputs. The resistance of each of the conducting transistors M 2 A, M 2 B in series with the feedback resistors compensates for this modification of the ratio. 
     FIG. 3 shows a variation in which the controlled connection between the processing unit  11  and the amplifier  12  is by electrical switches M 1 A′, M 1 B′ connected before the resistors R 1 A and R 1 B instead of after them as shown in FIG.  2 . In this case, it is preferable for the switch to be formed by pairs of complementary transistors since the dynamic range of the signal is greater than that of the circuit shown in FIG.  2 . To compensate the supplementary resistances of the pairs of complementary transistors, pairs of identical transistors M 2 A′ and M 2 B′ are provided in series with the feedback resistors R 2 A, R 2 B, and are preferably positioned after them. 
     FIG. 4 shows another variation in which the processing unit  11  and the amplifier  12  have their corresponding enabling/disabling terminals  17 ,  18  both connected to the switch  22  so that they are enabled or disabled simultaneously. This may be done while the control terminals of the transistors M 1 A, M 1 B are connected through the inverter  25  to the changeover switch  23  as shown in FIG.  2 . 
     With this configuration, the power-up time of the receiver section is shorter than that of the embodiment shown in FIG.  2 . This is so since the power-up time is not the sum of the power-up time of the processing unit  11 , the delay period of the circuit  24 , and that of the amplifier  12 . Instead, the power-up time is determined by the delay period of the delay circuit  24 , i.e., by the settling time of the processing unit  11 . 
     The receiving section according to the present invention makes it possible to avoid disturbance in the speaker  13  since the propagation of the signal during the settling transients is substantially blocked. This is achieved without bulky external components, and is accomplished by using a few supplementary components which can easily be integrated.