RADIO RECEIVER AND CONTROLLABLE AMPLIFIER CIRCUIT

Abstract: The radio receiver comprises a controllable amplifier circuit (7) for amplifying radio signals. This amplifier circuit (7) includes an amplifier element (59, 67) with an input electrode (57) and a voltage divider coupled thereto. This voltage divider comprises an impedance (3, 65) and a control element (49). The amplifier circuit (7) can suitably be used to amplify signals having frequencies of approximately 200 kHz. The control element (49) comprises a PIN diode (49). This has the advantage that relatively little power is necessary to operate the control element (49).

Description: Radio receiver and controllable amplifier circuit The invention relates to a radio receiver comprising a controllable amplifier circuit for amplifying radio signals, which amplifier circuit includes an amplifier element with an input electrode and, coupled thereto, a voltage divider with an impedance and a control element. The invention also relates to a controllable amplifier circuit for amplifying radio signals, and to the use of a three-layer semiconductor element as a control element in amplifier circuits. Such a radio receiver is known from US-A-2 417 844. Radio receivers often comprise an automatic gain control circuit (AGC-circuit) for amplifying radio signals. The object of such an AGC-circuit is to ensure that the amplitude of a signal fed to the output is more or less independent of the amplitude of the input signal supplied by an aerial, since the amplitude of the input signal is very variable and governed, inter alia, by the distance between the radio receiver and the radio transmitter. An AGC-circuit generally comprises an amplifier circuit including an amplifier element and a control element which is connected thereto. Dependent upon the amplitude of the input signal, the gain of the amplifier circuit is selected, via the control element, to be such that a signal of constant amplitude is present at the output of the amplifier circuit. In the radio receiver disclosed in the above-mentioned United States patent specification, the control element is an indirectly heated thermistor comprising a combination of a temperature-sensitive resistor and a heating element. The resistance value of the temperature-sensitive resistor can be controlled indirectly by varying the current through the heating element. The use of such a thermistor has the disadvantage that relatively much power is necessary to control this thermistor. As a result, the known radio receiver is less suitable for use in embodiments where the supply voltage originates from, for example, a battery or an accumulator. Another disadvantage of the known radio receiver is the slowness of the thermistor which, during controlling the resistance value, may cause a substantial delay. It is an object of the invention to provide a radio receiver of the type mentioned in the opening paragraph, in which relatively little power is required to operate the control element. To achieve this, the radio receiver in accordance with the invention is characterized in that the amplifier circuit can suitably be used to amplify signals having frequencies of approximately 200 kHz, and in that the control element includes a PIN diode. The use of a PIN diode as the control element has the advantage that relatively little power is necessary to control the resistance value. An additional advantage is that in such a configuration the resistance value of a PIN diode can be rapidly controlled. For all frequencies above a specific limiting frequency, PIN diodes behave as control elements with a substantially linear transfer characteristic. The resistance value is inversely proportional to the current flowing forward through the PIN diode. In hp associates application note 904 (15 Feb 66) a description is given of the use of a PIN diode as a control element. In said document, it is also noted that a PIN diode only behaves like a linear control element if the minority carrier lifetime of the PIN diode substantially exceeds the reciprocal value of the frequency of the applied signal. This application note further indicates that PIN diodes can only suitably be used as variable resistors in microwave applications. In this case, the frequency of the signals to be processed must be at least 10 MHz. In DE-B-1 135 968, a description is given of a controllable amplifier circuit in which also a diode is used as the control element. However, there are a number of differences between this amplifier circuit and the amplifier circuit in accordance with the invention: first, the diode used is not a PIN diode, and second, in this amplifier circuit the ordinary diode characteristic is used as the control characteristic. In addition, said patent specification describes the manner in which the non-linear character of said control characteristic can be compensated by also a non-linear transfer characteristic of a transistor. An embodiment of the radio receiver in accordance with the invention is characterized in that the PIN diode has a minority carrier lifetime of at least 20 microseconds. By virtue thereof, signals having frequencies of approximately 200 kHz can be transferred substantially without distortion. A further embodiment of the radio receiver in accordance with the invention is characterized in that the radio receiver is embodied so as to operate the PIN diode in the forward direction as well as in the reverse direction. This has the advantage that the impedance of the PIN diode can be controlled over a relatively wide range. By virtue thereof, this embodiment of the radio receiver can suitably be used with a large number of different types of aerials having substantially different output impedances. These and other aspects of the invention will be apparent from and elucidated with reference to the embodiments described hereinafter. In the drawings: Fig. 1 shows a block diagram of a radio receiver in accordance with the invention. Fig. 2 shows an electrical circuit diagram of an embodiment of an amplifier circuit in accordance with the invention. Fig. 3 shows an electrical circuit diagram of an alternative embodiment of an amplifier circuit in accordance with the invention. Fig. 4 shows the manufacture and construction of a three-layer semiconductor element for use in a radio receiver in accordance with the invention. A radio receiver as shown in Fig. 1 is specifically intended for receiving long- wave and medium-wave AM signals (frequency range from 150 kHz to 1.7 MHz). To this end, said radio receiver comprises an aerial 1 for receiving radio signals. The received radio signals are supplied, via a first band-pass filter 3, to an input 5 of a first amplifier circuit 7. The first band-pass filter 3 ensures that only signals within the designated frequency range are passed and that undesired frequencies are attenuated sufficiently. These undesired frequencies may originate, for example, from FM transmitters or mains interferences. The signals amplified by the first amplifier circuit 7 are supplied to a second band-pass filter 13 via an output 11 of this first amplifier circuit 7. This second bandpass filter 13 has a pass-range between 100 kHz and 1.7 MHz, and it ensures that the so- called image frequencies are suppressed. The signals filtered by the second band-pass filter 13 are mixed in a mixer 19 with a signal originating from a local oscillator 21. The resultant mixed signal has a frequency of 10.7 MHz. This mixed signal is supplied to a second amplifier circuit 25 via a third band-pass filter 23. Said third band-pass filter 23 has a pass-range between 10.55 and 10.85 MHz (10 dB). In a demodulator 27, the signals amplified by the second amplifier circuit 25 are first mixed-down to a frequency of 450 kHz, whereafter they are filtered (pass- range between 445 and 455 kHz, 10 dB) and subsequently demodulated. Next, the demodulated signals are amplified once more by a third amplifier circuit 29 and are finally supplied to a loudspeaker 31. The radio receiver shown in Fig. 1 comprises a mechanism for automatically controlling the gain of the radio signals received. For this purpose, an AGC- detector 15 is connected to an output 17 of the second band-pass filter 13. In this AGC detector 15, the amplitude of the output signal 17 is compared with a reference signal. Dependent upon the outcome of this comparison, the AGC detector 15 determines the value of a control signal 9 by means of which the gain of the amplifier circuit 7 is re-adjusted. The AGC detector 15 only becomes operative when the amplitude of the output signal 17 has reached a specific threshold value. If the amplitude of the output signal 17 is above this threshold value, said amplitude of the output signal 17 is maintained at a constant value by means of the AGC detector 15 and the amplifier circuit 7. If, however, the amplitude of the output signal 17 is below said threshold value, the amplifier circuit 7 is not influenced by the AGC detector 15. It is noted that the above-mentioned concept can also suitably be used to realize a radio receiver having a frequency range from 150 kHz to 30 MHz, without tunable input filters being required. In this case, it is required however that the first intermediate frequency is selected to be considerably higher than 10.7 MHz as used in the above- described radio receiver. A first intermediate frequency, for example, of 70 MHz could be used. Fig. 2 shows the electrical circuit diagram of an example of the amplifier circuit 7 shown in Fig. 1. Apart from this amplifier circuit 7, the aerial 1 and an electrical circuit diagram of the first band-pass filter 3 are also shown in this Figure. The band-pass filter 3 is formed by the combination of resistors 33 and 45, coils 35, 37, 41 and 43, and capacitors 39 and 47. As indicated in the description of Fig. 1 , the values of said components are selected to be such that only signals with frequencies in the frequency range from 150 kHz to 1.7 MHz are passed by this filter. The amplifier circuit 7 shown in Fig. 2 comprises an amplifier element 59 in the form of a field-effect transistor. Of course, the amplifier element may also be constructed differently, for example as a bipolar transistor. A PIN diode 49 and a variable current source 51 together form the control element. The control input 9 of the variable current source 51 is connected to the output of the AGC detector (see Fig. 1). Dependent upon the amplitude of the signal at the control input 9, a greater or smaller amount of direct current is supplied by the current source 51. As a result, also the direct current through the PIN diode 49 will vary. At the signal frequencies used (from 150 kHz) the PIN diode 49 behaves like a controllable resistor, the resistance value being inversely proportional to the current flowing forward through the PIN diode 49. The PIN diode 49 and the output impedance of the band-pass filter 3 together form the voltage divider. This voltage divider is connected, via a capacitor 53, to an input electrode 57 of the amplifier element 59. The capacitor 53 ensures that the input electrode 57 of the amplifier element 59 is decoupled from the direct voltage across said voltage divider. The bias point of the field-effect transistor 59 is determined by means of a resistor 55. A signal present on the input electrode 57 of the amplifier element 59 is converted by this amplifier element into an amplified signal which is present at the output 11 of the amplifier element. A coil 61 is incorporated in the amplifier circuit 7 to ensure that the complete supply voltage is present on the drain of the field effect transistor 59. A resistor 63 serves to determine the gain factor for signal frequencies of the amplifier circuit 7. In the amplifier circuit 7, the PIN diode 49 is operated in the forward direction as well as in the reverse direction. If the AGC detector 15 is passive, no current is supplied by the current source 51. As the positive supply voltage V2 is larger than the positive supply voltage VI , the PIN diode 49 is operated, in this case, in the reverse direction via a resistor 50. If the AGC detector 15 is active, then, dependent upon the value of the control signal 9, a current is supplied by the current source 51. If this current is large enough, the voltage drop across the resistor 50 becomes so large that the PIN diode 49 is operated in the forward direction. However, if this current is too small to operate the PIN diode 49 in the forward direction, then the PIN diode 49 is operated in the reverse direction. The coupling between the AGC detector 15 and the PIN diode 49 causes the PIN diode to be controlled in such a manner that the amplitude of the output signal 11 is more or less constant. Fig. 3 shows an electrical circuit diagram of an alternative example of the amplifier circuit 7 shown in Fig. 1. In Fig. 3, the amplifier element is formed by an inverting amplifier 67. An impedance 65 and a PIN diode 49 constitute the voltage divider. An input electrode 57 of the amplifier element 67 is connected to this voltage divider. Also in this case, the PIN diode 49 forms the control element, and the resistance value of the PIN diode can be controlled via a variable current source 51. Said current source 51 is controlled by the output 9 of the AGC detector. The gain factor of this amplifier circuit 7 is determined by the ratio between the impedance 65 and the resistance value of the PIN diode 49. Fig. 4 shows the manufacture and construction of a three-layer semiconductor element. For the starting material use is made of a 380 μm thick boron-doped silicon wafer having a resistivity of 1100 Ωcm (P " material). Boron is diffused onto one side of the wafer. This results in the formation of a P + layer 69 having a C s (= surface concentration) which exceeds 10 19 cm "3 . Phosphorus is diffused to a depth of 30 μm onto the other side of the wafer. This results in the formation of a N-layer 73, with a C s which exceeds 2.10 19 cm '3 . The P " layer 71 of the starting material remaining between the P + layer 69 and the N-layer 73 is also referred to as an I-layer. It is alternatively possible to use an N " layer instead of the P " layer 71. After said diffusion processes, the wafer is divided into crystals having a diameter of approximately 1 mm. The use of a suitable etching process causes the upper side to be smaller than the bottom side. The upper side and the lower side of the crystal are both metalized on account of electric connections. An advantage of this three-layer semiconductor element is the large control range of the resistance value.

Claims: Claims 1. A radio receiver comprising a controllable circuit (7) for amplifying radio signals, which amplifier circuit includes an amplifier element (59, 67) with an input electrode (57) and, coupled thereto, a voltage divider with an impedance (3, 65) and a control element (49), characterized in that the amplifier circuit (7) can suitably be used to amplify signals having frequencies of approximately 200 kHz, and in that the control element includes a PIN diode (49). 2. A radio receiver as claimed in claim 1, characterized in that the PIN diode (49) has a minority carrier lifetime of at least twenty microseconds. 3. A radio receiver as claimed in claim 1 or 2, characterized in that the radio receiver is embodied so as to operate the PIN diode (49) in the forward direction as well as in the reverse direction. 4. A controllable amplifier circuit (7) for amplifying radio signals, comprising an amplifier element (59, 67) with an input electrode (57) and, coupled thereto, a voltage divider with an impedance (3, 65) and a control element (49), characterized in that the amplifier circuit (7) can suitably be used to amplify signals having frequencies of approximately 200 kHz, and in that the control element includes a PIN diode (49). 5. A controllable amplifier circuit as claimed in claim 4, characterized in that the PIN diode (49) has a minority carrier lifetime of at least twenty microseconds. 6. A controllable amplifier circuit as claimed in claim 4 or 5, characterized in that the amplifier circuit (7) is embodied so as to operate the PIN diode (49) in the forward direction as well as in the reverse direction. 7. The use of a three-layer semiconductor element as a control element in amplifier circuits for amplifying signals having a frequency of approximately 200 kHz, said semiconductor element comprising a P layer (69) and an N layer (73), which are separated from each other by a layer (71) having a concentration of charge carriers which is substantially lower than the concentration of charge carriers in the P layer (69) and the N layer (73).

Inventors: MOONEN OSWALD JOSEF, MOONEN, OSWALD, JOSEF
Applicants: KONINKL PHILIPS ELECTRONICS NV, PHILIPS NORDEN AB, KONINKLIJKE PHILIPS ELECTRONICS N.V.
Application year: 1998
Main classification: H03G3
IPC codes: H03F3, H03G1