JPH0623320U - AGC circuit of RF amplifier in AM receiver - Google Patents

Abstract

(57) [Abstract] [Objective] Regarding an AGC circuit of an RF amplifier in an AM receiver, in order to prevent cross-modulation by applying sufficient AGC during normal reception, increase sensitivity at small signals to obtain sufficient gain. It is an object of the present invention to provide an AGC circuit of an RF amplifier according to the above. [Configuration] Source-grounded FET at the input stage of the RF amplifier 3
A source resistance 33 for gain adjustment is connected in series to the source of the FET, and a variable impedance element 32 is connected in series to the drain. The impedance of the variable impedance element is variably controlled by an AGC signal. By changing the drain current of the FET by applying the high frequency AGC RF in the AM receiver
In the AGC circuit of the amplifier, a source resistance switching circuit 7 which is controlled to be opened / closed by an AGC signal is connected in parallel with the source resistance 33, and the source resistance switching circuit 7 causes the source resistance switching circuit 7 to operate when the AGC signal exceeds a predetermined level. 33 is configured to be short-circuited.

Description

[Detailed description of the device]

[0001]

[Industrial applications]

 The present invention relates to an AGC circuit of an RF amplifier in an AM receiver.

[0002] In AM broadcasting, since the amplitude of a carrier wave contains an audio component, it is not possible to apply an amplitude limiter as in an FM receiver. Therefore, it is essential to apply AGC (automatic gain control) to the RF (high frequency) amplifier and IF (intermediate frequency) amplifier according to the reception level. The present invention relates to the AGC circuit of the RF amplifier for high frequency amplification, of these two AGCs.

[0003]

[Prior art]

 FIG. 2 shows an AGC circuit of a conventional RF amplifier. In the figure, 1 is an antenna, 2 is an antenna sensitivity switching circuit, 3 is an RF amplifier, 4 is an RF tuning circuit, 5 is an RF AGC circuit, and 6 antenna sensitivity control circuit. The antenna sensitivity switching circuit 1 lowers the antenna sensitivity when the received electric field strength is too high, and is composed of a capacitor 21 and diodes 22 and 23. At the time of sensitivity switching, the antenna sensitivity control circuit 6 gives an antenna sensitivity switching signal via the diode 23 to make the diode 22 conductive, and a part of the high frequency signal received by the antenna 1 is grounded via the capacitor 21 and the diode 22. It is shunted to and the antenna sensitivity is switched.

The RF amplifier 3 is composed of a FET 31 forming a source-grounded input amplifier, a transistor 32 forming a variable impedance element for AGC, and a source resistor 33 for gain adjustment. The transistor 32 has its emitter-collector impedance variably controlled by the AGC signal supplied to its base, thereby controlling the gain by changing the drain current of the FET 31, and the high frequency signal output from the RF amplifier 3 is controlled. AGC is applied so that the level is always constant.

The RF tuning circuit 4 takes out the amplified high frequency signal output from the RF amplifier 3 by using the transformer 41 and the capacitor 42 and sends it to a mixer (not shown). The mixer mixes the local oscillation output and converts the high frequency signal into an IF signal.

The RF AGC circuit 5 detects a high frequency signal output from the RF amplifier 3, supplies an AGC signal that changes in inverse proportion to the signal level to the base of the transistor 32, and is proportional to the signal level. The antenna sensitivity control signal, which changes accordingly, is supplied to the antenna sensitivity control circuit 6.

The antenna sensitivity control circuit 6 monitors the level of the antenna sensitivity control signal sent from the AGC circuit 5 for RF, determines that the input is too large for AGC when the set level is exceeded, and switches the antenna sensitivity. The antenna sensitivity switching signal is sent to the circuit 2.

In the above structure, when the level of the high frequency signal output from the RF amplifier 3 is high, the value of the AGC signal output from the RF AGC circuit 5 decreases in inverse proportion to the level of the high frequency signal. Therefore, the base bias voltage of the transistor 32 is reduced, and the emitter-collector impedance of the transistor 32 is increased. As a result, the drain current of the FET 31 decreases and its amplification degree decreases, so that AGC is applied so that the output signal decreases.

On the other hand, when the level of the high frequency signal output from the RF amplifier 3 decreases, the value of the AGC signal output from the RF AGC circuit 5 increases in inverse proportion to the level of the high frequency signal. Therefore, the base bias voltage of the transistor 32 increases and the emitter-collector impedance of the transistor 32 decreases. As a result, the drain current of the FET 31 increases and its amplification degree increases, so that AGC is applied to increase the output signal. As a result of the operation as described above, the level of the high frequency signal output from the RF amplifier 3 is always kept constant.

When the received radio wave is too strong, the antenna switching control circuit 6 outputs an antenna switching signal, and the diode 22 in the antenna sensitivity switching circuit 2 is turned on to make the high-frequency signal received by the antenna 1. Part of this is shunted to the ground side to prevent excessive input.

[0011]

[Problems to be solved by the device]

 A conventional RF amplifier realizes AGC for a high frequency signal as described above, but if the gain of the RF amplifier is too large, cross modulation may occur. Therefore, conventionally, as shown in FIG. 2, a source resistor 33 for gain adjustment is inserted in series with the source of the FET 31, and a negative feedback is applied by the source resistor 33 to obtain the gain of the RF amplifier. Was lowered to prevent the occurrence of cross modulation.

However, if the gain of the RF amplifier is lowered by the source resistor 33 as described above, a new problem arises that the sensitivity is lowered by the amount of the lowered gain.

The present invention has been made in view of the above circumstances, and an object thereof is to increase the sensitivity by receiving a small signal while sufficiently suppressing the cross modulation by applying sufficient AGC during normal reception. It is to provide an AGC circuit of an RF amplifier adapted to obtain

[0014]

[Means for Solving the Problems]

 In order to achieve the above-mentioned object, the present invention is provided with a source-grounded FET at an input stage of an RF amplifier for high frequency amplification, a source resistance for gain adjustment is connected in series to a source of the FET, and A variable impedance element is connected in series to the drain, and the drain current of the FET is changed by variably controlling the impedance of the variable impedance element by the AGC signal, and the high frequency AGC is applied to the AGC of the RF amplifier in the AM receiver. In the circuit, a source resistance switching circuit that is controlled to be opened or closed by an AGC signal is connected in parallel with the source resistance, and the source resistance switching circuit short-circuits the source resistance when the AGC signal exceeds a predetermined level. It is the one.

[0015]

[Work]

 When the level of the received high frequency signal is sufficient, the source resistance switching circuit does not operate. Therefore, a source resistor for gain adjustment is inserted in series to the source of the RF amplifier as in the conventional case, and the same AGC operation as that of the conventional RF amplifier is performed. Therefore, sufficient AGC is applied in the normal reception state, and cross modulation is prevented.

On the other hand, when the level of the received high frequency signal decreases, the level of the AGC signal increases in inverse proportion to this. Then, when the AGC signal becomes larger than a predetermined set level, the source resistance switching circuit operates to short-circuit both ends of the source resistor for gain adjustment. As a result, the negative feedback due to the source resistance is not applied to the EFT, so that the gain of the RF amplifier is increased and the sensitivity is improved.

[0017]

【Example】

 Hereinafter, embodiments of the present invention will be described with reference to the drawings. FIG. 1 shows an embodiment of the AGC circuit of the RF amplifier according to the present invention. In the figure, 1 is an antenna, 2 is an antenna sensitivity switching circuit, 3 is an RF amplifier, 4 is an RF tuning circuit, 5 is an RF AGC circuit, and 6 is an antenna switching circuit. The configuration of the portions indicated by reference numerals 1 to 6 is exactly the same as that of the conventional example (FIG. 2) described above. Therefore, the same parts as those of the conventional example are designated by the same reference numerals, and detailed description thereof will be omitted.

According to the present invention, in the same circuit configuration as the conventional RF amplifier described above, a source resistance switching circuit 7 is additionally provided in parallel with a source resistor 33 for gain adjustment connected to the source of the FET 31, and an AGC circuit for RF is provided. When the AGC signal output from 5 exceeds a predetermined value, that is, when the received high frequency signal becomes smaller than a preset value, the source resistance switching circuit 7 is activated to short-circuit the source resistance 33. It was done.

In the illustrated example, the source resistance switching circuit 7 is composed of a transistor 71 that constitutes a switching element, a Zener diode 72, a base resistor 73, and a level shift resistor 74. The emitter and collector of the transistor 71 are connected to each other. The source resistor 33 is connected to both ends of the source resistor 33, and the base of the transistor 71 is supplied with an AGC signal via a zener diode 72 and a base resistor 73. The Zener diode 72 is an element for judging the level of the AGC signal, and its Zener voltage is selected so as to conduct when the level of the AGC signal exceeds a predetermined value.

Next, the operation of the above embodiment will be described. When the level of the received high frequency signal is equal to or higher than a predetermined level, the value of the AGC signal output from the RF AGC circuit 5 does not exceed the Zener voltage of the Zener diode 72 in the source resistance switching circuit 7. . Therefore, in the normal reception state, the transistor 71 in the source resistance switching circuit 7 is always off, and the RF amplifier 3 operates exactly as in the case of the conventional example described above.

On the other hand, when the received radio wave becomes weak and the level of the received high frequency signal becomes small, the AGC signal output from the RF AGC circuit 5 becomes inversely proportional to this. Then, when the AGC signal exceeds a preset level, the zener diode 72 becomes conductive, current is supplied to the base of the transistor 71, the transistor 71 is turned on, and the emitter-collector becomes conductive.

As a result, the source resistance 33 is short-circuited by the transistor 71, and the source S of the FET 31 bypasses the collector-emitter of the transistor 71 and is directly connected to the ground. Therefore, the FET 31 does not receive the negative feedback due to the source resistance 33, so that the gain of the FET 31 is increased and the sensitivity of the RF amplifier 3 is improved accordingly.

[0023]

[Effect of device]

 As is clear from the above description, in the case of the AGC circuit of the RF amplifier according to the present invention, the source resistance switching circuit is connected in parallel with the gain adjusting source resistance inserted in the FET of the input stage, Since this source resistance switching circuit is controlled to open and close by the AGC signal, sufficient AGC is applied during normal reception to prevent intermodulation, while sensitivity is automatically increased during small signal reception, and even weak radio waves are received. It can be possible.

[Brief description of drawings]

FIG. 1 is a circuit diagram showing a configuration of an embodiment of an AGC circuit of an RF amplifier according to the present invention.

FIG. 2 is a circuit diagram showing a configuration of an AGC circuit of a conventional RF amplifier.

[Explanation of symbols]

 1 Antenna 2 Antenna Sensitivity Switching Circuit 3 RF Amplifier 4 RF Tuning Circuit 5 RF AGC Circuit 6 Antenna Sensitivity Control Circuit 7 Source Resistance Switching Circuit 31 FET 32 Transistor 33 Source Resistance 71 Transistor 72 Zener Diode 73 Base Resistance 74 Level Shift Resistor

Claims (1)

[Scope of utility model registration request] 1. A grounded source FET is provided in the input stage of an RF amplifier for high frequency amplification, a source resistor for gain adjustment is connected in series to the source of the FET, and a variable impedance element is connected in series to the drain. In the AGC circuit of the RF amplifier in the AM receiver, in which the drain current of the FET is changed by variably controlling the impedance of the variable impedance element by the AGC signal to apply high frequency AGC, the source impedance is connected in parallel with the source resistance. In the AM receiver, a source resistance switching circuit that is controlled to be opened and closed by the AGC signal is connected, and the source resistance is short-circuited by the source resistance switching circuit when the AGC signal becomes a predetermined level or higher. RF amplifier AG
C circuit.