JPH07326978A - Automatic gain control loop circuit - Google Patents

Abstract

PURPOSE:To stabilize loop gain and to prevent the oscillation of the loop circuit by controlling an RF amplifier circuit corresponding to an AGC voltage input by an amplification gain to a reception signal input so as to uniformize an AGC loop gain. CONSTITUTION:When a reception signal input level is increased, the output voltage level of a smoothing circuit 13 is also increased and the output voltage level of an AGC amplifier circuit 14 changes so as to start controlling the gain of the RF amplifier circuit 11. Then, when the output voltage of the circuit 13 becomes not less than a set value and the gain of the circuit 11 changes like an exponential function in a characteristic area, the gain control circuit 15 controls the gain of the circuit 14 so that the output voltage of the circuit 14 changes like a potential function such as the gain change of the circuit 11. Thereby, in the RF amplifier circuit, the amplification gain to the reception signal input is controlled corresponding to the AGC voltage input so as to control the level of the output signal level nearly constant. Consequently, the AGC loop gain is stabilized and there is no fear that the loop circuit is oscillated.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an automatic gain control loop circuit and is used for an automatic gain control loop of a high frequency amplification stage in a radio receiver such as a radio receiver mounted on an automobile.

[0002]

2. Description of the Related Art Conventionally, an automatic gain control (AG) of a high frequency (RF) amplification stage in an FM / AM radio receiver for a vehicle is used.
C) The loop is configured as shown in FIG. 3, for example. In FIG. 3, reference numeral 31 is an RF amplifier circuit for amplifying a received signal input, which has a gain control node. 32
Is a signal detection circuit for detecting the output signal level of the RF amplification circuit 31, 33 is a smoothing circuit for smoothing the signal output of the signal detection circuit 32 and converting it to a DC voltage, and 34 is the input of the output of the smoothing circuit 33. , An AGC amplifier circuit that amplifies this, outputs an AGC voltage, and supplies it to the gain control node of the RF amplifier 31.

In the AGC loop having the above structure, RF
In the amplifier circuit 31, the amplification gain for the received signal input is controlled according to the AGC voltage input of the gain control node, and the level of the output signal is controlled to be substantially constant.

As shown in FIG. 4, for example, the RF amplifier circuit 31 includes an N-channel field effect transistor 41 for RF amplification, in which a received signal is input to a gate and a source is connected to a ground node. Field effect transistor 4
1 and a NPN transistor 42 cascade-connected to the drain side of 1. This NPN transistor 42
Has an emitter connected to the drain of the field effect transistor 41, an AGC voltage input to the base, an RF amplified signal output from the collector, and the collector output is branched and input to the signal detection circuit 32.

RF in the open state of the AGC loop
As shown in FIG. 5, for example, the gain characteristic of the amplifier circuit 31 alone with respect to the input voltage of the gain control node is such that the gain changes exponentially with respect to the change of the input within a certain fixed range. The gain becomes saturated with respect to the change.

Further, the output voltage characteristic with respect to the input voltage of the AGC amplifier circuit 34 in the open state of the AGC loop has a linear output as shown in FIG. 6, for example, when the input changes within a certain range. The output voltage changes and the output voltage becomes constant with respect to the change of the input outside the fixed range.

However, as described above, if the RF amplification circuit 31 having a characteristic region in which the gain changes exponentially and the AGC amplification circuit 34 having a characteristic region in which the output changes linearly exist in the same AGC loop. During the AGC operation, the loop gain may become unstable under the condition of operating in the different characteristic regions, and the loop circuit may oscillate.

[0008]

As described above, in the conventional AGC loop circuit of the RF amplification stage, the characteristic region in which the gain of the RF amplification circuit changes exponentially and the output of the AGC amplification circuit change linearly. Since the characteristic region is mixed, the loop gain becomes unstable under a certain operating condition, and there is a problem that the loop circuit may oscillate.

The present invention has been made to solve the above problems, and an object of the present invention is to provide an automatic gain control loop circuit in which the loop gain does not become unstable and the loop circuit does not oscillate. To do.

[0010]

An automatic gain control loop circuit of the present invention has a gain control node to which an AGC voltage is input, and an amplification gain for a received signal input is controlled according to the AGC voltage input of this gain control node. An RF amplification circuit for amplifying a received signal input, a signal detection circuit for detecting an output signal level of the RF amplification circuit, a smoothing circuit for smoothing a signal output of the signal detection circuit and converting the signal output into a DC voltage, and a gain. An AGC amplifier circuit which has a control node, amplifies the input voltage from the smoothing circuit according to a gain control voltage input to the gain control node, outputs an AGC voltage, and supplies the AGC voltage to the gain control node of the RF amplifier circuit; The output voltage of the smoothing circuit is input, and a voltage obtained by converting the output voltage according to a predetermined input / output conversion characteristic is supplied to the gain control node of the AGC amplifier circuit to smooth the smoothed voltage. Becomes more than a certain set value the output voltage of the road, characterized in that the output voltage of the AGC amplifier circuit comprises a gain control circuit for controlling to change as well as the gain variation of the RF amplifier circuit.

[0011]

When the output voltage of the smoothing circuit exceeds a certain set value, the output voltage of the AGC amplifier circuit changes similarly to the gain change of the RF amplifier circuit, and the RF amplifier circuit has an amplification gain for the received signal input that is the AGC voltage input. The output signal level is controlled to be substantially constant.

During this AGC operation, the gain characteristic of the RF amplifier circuit, which changes exponentially, and the output change characteristic of the AGC amplifier circuit are matched, so that the AGC loop gain is made uniform. Therefore, the loop gain does not become unstable, and the loop circuit does not oscillate.

[0013]

Embodiments of the present invention will now be described in detail with reference to the drawings. FIG. 1 shows an AGC loop circuit used in an RF amplification stage in an on-vehicle FM AM radio receiver as an embodiment of the present invention.

This AGC loop circuit is a conventional AGC loop circuit.
Gain controllable AGC amplifier circuit 1 compared to loop circuit
4 is used, a gain control circuit 15 for controlling the gain of the AGC amplifier circuit 14 according to the output voltage of the smoothing circuit 13 is added, and when the output voltage of the smoothing circuit 13 exceeds a certain set value, the AGC is used. The difference is that the output voltage of the amplifier circuit 14 is controlled so as to change exponentially.

That is, in FIG. 1, reference numeral 11 denotes an RF amplifier circuit for amplifying a received signal input, which has a gain control node to which an AGC voltage is input, and an amplification gain for the received signal input is input to the AGC voltage input of the gain control node. Controlled accordingly.
Reference numeral 12 is a signal detection circuit for detecting the output signal level of the RF amplification circuit 11, which may directly detect the level of the RF signal, and may be an intermediate frequency (I
F) The signal level may be detected. A smoothing circuit 13 smoothes the signal output of the signal detection circuit 12 and converts it into a DC voltage.

Reference numeral 14 has a gain control node, and the smoothing circuit 13 is responsive to the gain control voltage input to the gain control node.
AGC by amplifying the input voltage (smoothing circuit output voltage) from
The AGC amplifier circuit outputs a voltage and supplies the voltage to the gain control node of the RF amplifier circuit 11.

The output voltage of the smoothing circuit 13 is input to 15 and the converted voltage according to a predetermined input / output conversion characteristic is supplied to the gain control node of the AGC amplifier circuit 14 to output the output voltage of the smoothing circuit 13. Is a gain control circuit that controls the output voltage of the AGC amplifier circuit 14 to change in the same manner as the gain change of the RF amplifier circuit 11 when the value exceeds a certain set value.

In the AGC loop having the above structure, RF
In the amplifier circuit 11, the amplification gain for the received signal input is controlled according to the AGC voltage input of the gain control node, and the level of the output signal is controlled to be substantially constant.

The RF amplifier circuit 11 is for RF amplification in which the received signal is input to the gate and the source is connected to the ground node, similarly to the RF amplifier circuit 31 of the conventional example described with reference to FIG. 4, for example. N-channel field effect transistor 41, and an NPN transistor 42 cascade-connected to the drain side of the field effect transistor 41.
Consists of. The NPN transistor 42 has an emitter connected to the drain of the field effect transistor 41, an AGC voltage input to the base, an RF amplified signal output from the collector, and the collector output is branched and input to the signal detection circuit 12. To do.

RF in the open state of the AGC loop
The gain characteristic of the amplifier circuit 11 alone with respect to the input voltage of the gain control node is, for example, the RF characteristic of the conventional example described above with reference to FIG.
Similar to the characteristic of the amplifier circuit 31, the gain changes exponentially with respect to the change of the input within a certain fixed range, and the gain becomes saturated with respect to the change of the input over the fixed range.

Further, the output voltage characteristic with respect to the input voltage of the AGC amplifier circuit 14 in the open state of the AGC loop has, for example, the conventional AGC described above with reference to FIG.
Similar to the characteristics of the amplifier circuit 34, the output changes linearly with respect to the change of the input within a certain fixed range, and the output voltage becomes constant with respect to the change of the input outside the fixed range.

FIG. 2 shows an example of the gain characteristic with respect to the gain control node input voltage (smoothing circuit output voltage) of the AGC amplifier circuit 14 in the state where the gain is controlled by the gain control circuit 15. In this gain characteristic, when the output voltage of the smoothing circuit 13 exceeds a certain set value, the output voltage changes exponentially.

In the AGC loop of this example, the RF
The exponentially changing direction of the amplifier circuit 11 is the rising direction,
The loop operation is designed to be stable in a direction (decreasing direction) in which the output voltage of the gain control circuit 14 changes exponentially.

According to the AGC loop circuit of the above embodiment,
When the received signal input level increases, the output voltage level of the smoothing circuit 13 also increases, the output voltage level of the AGC amplifier circuit 14 changes, and the gain of the RF amplifier circuit 11 starts to be controlled. Then, when the input level of the received signal further increases, the output voltage of the smoothing circuit 13 becomes a certain set value or more, and the gain of the RF amplifier circuit 11 enters a characteristic region in which it exponentially changes, the gain control circuit 15 causes the AGC The gain of the AGC amplifier circuit 14 is controlled so that the output voltage of the amplifier circuit 14 changes exponentially like the gain change of the RF amplifier circuit 11.
As a result, in the RF amplifier circuit 11, the amplification gain for the received signal input is controlled according to the AGC voltage input, and the level of the output signal is controlled to be substantially constant.

In this AGC operation, the RF amplifier circuit 1
Since the characteristic region in which the gain of 1 changes exponentially and the characteristic region in which the output of the AGC amplifier circuit 14 changes exponentially are matched, the AGC loop gain is made uniform. Therefore, the loop gain does not become unstable, and the loop circuit does not oscillate.

[0026]

As described above, according to the AGC loop circuit of the present invention, the loop gain does not become unstable, and the risk of the loop circuit oscillating can be eliminated.

[Brief description of drawings]

FIG. 1 is a vehicle-mounted FM / AM according to a first embodiment of the present invention.
The block diagram which shows the AGC loop circuit of the RF amplification stage in a radio receiver.

FIG. 2 is a characteristic diagram showing an example of a gain change with respect to a gain control node input voltage (smoothing circuit output voltage) of the AGC amplifier circuit in a state where the gain is controlled by the gain control circuit in FIG.

FIG. 3 is a block diagram showing an AGC loop of an RF amplification stage in a conventional vehicle-mounted FM / AM radio receiver.

FIG. 4 is a circuit diagram showing an example of an RF amplifier circuit shown in FIGS. 1 and 3.

5 is a characteristic diagram showing an example of a gain change with respect to a gain control node input voltage of the RF amplifier circuit alone of FIG. 4 in an open state of the AGC loop in FIG. 1 and FIG.

6 is an AG in an open state of the AGC loop in FIG.
The characteristic view which shows an example of the output voltage change with respect to the input voltage of a single C amplifier circuit.

[Explanation of symbols]

11 ... RF amplification circuit for amplifying received signal input, 12 ... Signal detection circuit, 13 ... Smoothing circuit, 14 ... AGC amplification circuit, 1
5 ... Gain control circuit.

Claims (2)

[Claims] 1. A high frequency amplifier for amplifying a received signal input, comprising: a gain control node to which an automatic gain control voltage is input, wherein an amplification gain for a received signal input is controlled according to the automatic gain control voltage input of the gain control node. A circuit, a signal detection circuit for detecting the output signal level of the high-frequency amplifier circuit, a smoothing circuit for smoothing the signal output of the signal detection circuit and converting it to a DC voltage, and a gain control node. An automatic gain control amplifier circuit that amplifies the input voltage from the smoothing circuit according to the gain control voltage input to output an automatic gain control voltage and supplies the automatic gain control voltage to a gain control node of the high frequency amplifier circuit; and an output of the smoothing circuit. A voltage is input, and a voltage obtained by converting it according to a predetermined input / output conversion characteristic is supplied to the gain control node of the automatic gain control amplifier circuit, and the output of the smoothing circuit A gain control circuit for controlling the output voltage of the automatic gain control amplifier circuit to change in the same manner as the gain change of the high frequency amplifier circuit when the pressure exceeds a certain set value. Loop circuit. 2. The automatic gain control loop circuit according to claim 1, wherein the gain characteristic of the high frequency amplifier circuit alone in the open state of the automatic gain control loop with respect to a gain control node input voltage is:
The gain changes exponentially with respect to a change in the input within a certain fixed range, and the gain control circuit outputs the output voltage of the automatic gain control amplifier circuit when the output voltage of the smoothing circuit becomes a certain set value or more. An automatic gain control loop circuit, which is controlled to change exponentially.