JPH07273574A - Agc amplifier - Google Patents

Abstract

PURPOSE:To keep the response speed to be a proper speed over a wide range of an input level in the AGC amplifier. CONSTITUTION:A 2nd response speed adjustment circuit 10 generates a prescribed reference voltage Vref by using resistors R1, R2, the reference voltage is compared with a tuner AGC voltage Vc by using a TR Q1 and when the tuner AGC voltage Vc is less than a prescribed voltage, a TR Q2 is turned on to add a capacitor C1 in parallel with a capacitor Ca. When the capacitor C1 is added in parallel with the capacitor Ca, since the resulting time constant is increased more than that of a 1st response speed adjustment circuit 9 using only a resistor Ra and the capacitor Ca, the response speed of the tuner AGC voltage is much delayed thereby preventing the response speed of the entire tuner AGC loop from being increased.

Description

Detailed Description of the Invention

[0001]

The present invention relates to AGC (Automat)
The present invention relates to a circuit that adjusts the response speed of an ic gain control (Amp).

[0002]

2. Description of the Related Art FIG. 3 shows an example of a general AGC circuit.
In this figure, the input signal is amplified by the variable gain amplifier 21 and output. Further, the output of the variable gain amplifier 21 is compared with the reference voltage (Vref) in the comparison circuit 22, and when the output level is small, the gain control voltage of the variable gain amplifier 21 (hereinafter abbreviated as “control voltage”) is raised. Conversely, when the output level is high, a control voltage that lowers the control voltage is generated. Then, this control voltage is fed back to the variable gain amplifier 21 through the response speed adjusting circuit 23 composed of a CR time constant circuit, and is controlled so that the output level of the variable gain amplifier 21 always becomes a predetermined target value.

[0003]

In such an AGC amplifier, the response speed of the output signal level with respect to the change of the input signal level becomes a problem. When the input signal level changes as shown in FIG. 4A, Δt as shown in FIG.
It is ideal that the output level stabilizes at a predetermined output level after (= t1−t0), but if this response speed is too fast, FIG.
As shown in (c), oscillation occurs, and the output level response becomes ringing, resulting in poor convergence. On the contrary, if the response speed is too slow, the time (t2-t0) until the output level stabilizes with respect to the change of the input level becomes long as shown in FIG. Therefore, in order to obtain a quick and stable response, it is necessary to adjust the response speed of the control voltage to an appropriate value.

However, in the variable gain amplifier, the relationship between the control voltage and the magnitude of the gain is not generally linear as shown in FIG. That is, the change width of the gain with respect to the constant change width of the control voltage is different between when the gain is large and when the gain is small. In the conventional AGC amplifier, since the characteristic of the response speed adjusting circuit for the control voltage is fixed, the changing speed of the control voltage becomes constant, and thus the changing speed of the gain is not constant. For this reason, the rate of change of the gain changes depending on the relationship between the control voltage of the variable gain amplifier and the gain.
The response speed of the GC amplifier could not be maintained at an appropriate value.

The present invention has been made in view of such a problem, and makes the response speed of the AGC amplifier appropriate over a wide range of input level by making the characteristic of the response speed adjusting circuit of the control voltage variable. The purpose is to keep the value.

[0006]

In order to solve the above problems, the present invention compares an output voltage of a variable gain amplifier with a predetermined reference voltage to generate a control voltage, and the control voltage is used to generate the control voltage of the variable gain amplifier. The AGC amplifier configured to control the gain is provided with a response speed adjusting circuit that changes the response speed according to the level of the control voltage.

Here, the response speed adjusting circuit is, for example, a first response speed adjusting circuit composed of a resistor and a first capacitor, and a first response speed adjusting circuit when the control voltage exceeds a predetermined level.
And a second response speed adjusting circuit including a switching circuit in which the second capacitor is added in parallel to the second capacitor. Alternatively, a second response speed adjustment circuit including a resistor and a first capacitor, and a second capacitance element that is connected in parallel to the first capacitor and has a capacitance that changes according to the level of the control voltage.
Response speed adjusting circuit.

The object of gain control may be the RF variable gain amplifier of the television receiver, and the control voltage may be generated by comparing the video detection output voltage with a predetermined reference voltage.

[0009]

According to the AGC amplifier of the present invention, the AGC operation is stabilized by changing the response speed of the control voltage according to the level of the control voltage, and the response speed of the AGC amplifier is adjusted to an appropriate level over a wide input level range. Can be held at a value.

[0010]

Embodiments of the present invention will be described in detail below with reference to the drawings. (First Embodiment) FIG. 1 shows an embodiment in which the present invention is applied to gain control of an RF amplifier of a television receiver.

In the figure, the signal input from the RF input terminal T1 of the tuner is passed through the bandpass filter 1 to extract the desired channel signal, amplified by the RF amplifier 2 and input to the frequency converter 3, where the intermediate signal is output. It is converted into a frequency (hereinafter referred to as “IF”) signal.

This IF signal is the IF output terminal T of the tuner.
2 from the video IF IC (hereinafter referred to as “VIF I”) from the input terminal T4 through the bandpass filter 4.
"C"). The IF signal input to the VIF IC is amplified by the IF amplifier 5 and detected by the detection circuit 6 to become a baseband video signal, which is output from the detection output terminal T5.

In the receiving circuit of this embodiment, AGC is applied by a double AGC loop. The first AGC loop is I
F-Amp 5-Detection circuit 6-Comparison circuit 7 is a loop that is completed only inside VIF IF and is called IF AGC.
The AGC loop is a large loop that returns from the comparison circuit 7 to the RF amplifier 2 inside the tuner through the comparison circuit 8 and is called a tuner AGC.

In the IF AGC, the comparison circuit 7 compares the output of the detection circuit 6 with the reference voltage Va generated inside the VIF IC to generate the control voltage of the IF amplifier 5 so that the detection output becomes 1 V _PP , for example. In the tuner AGC, the comparison circuit 8 compares the output of the comparison circuit 7 with the reference voltage Vb supplied from the outside to generate a control voltage. Then, for example, by applying AGC to the IF amplifier 5 during a weak / medium electric field and applying AGC to the RF amplifier 2 during a strong electric field,
The S / N ratio of the receiver as a whole is kept good.

In order to stabilize this tuner AGC and obtain good response characteristics, normally, a resistor Ra and a capacitor C are used.
The response speed of the tuner AGC voltage (control voltage of the RF amplifier 2) is adjusted only by the first response speed adjusting circuit 9 composed of a.

However, the tuner RF gain amplifier 2
The relationship between the tuner AGC voltage input to the gain and the gain is not linear. Especially when the RF input level is large, that is, when the gain is small, the change width of the gain with respect to the change width of the tuner AGC voltage becomes very large. Response speed will be faster. Therefore, the response speed of the tuner AGC loop cannot be maintained at an appropriate value only with the fixed time constant of the resistor Ra and the capacitor Ca.

Therefore, in this embodiment, in order to always maintain the response speed of the tuner AGC at an appropriate value and improve the response speed, the transistors Q1 and Q2 and the resistors R1, R2 and R are used.
A second response speed adjusting circuit 10 including R3, R4, and capacitor C1 is added.

The second response speed adjusting circuit 10 has a resistor R
1 and a resistor R2 form a predetermined reference voltage Vref, and this is compared with the tuner AGC voltage Vc by a transistor Q1. When the tuner AGC voltage Vc is lower than a predetermined value, the transistor Q2 is turned on to turn on the capacitor C1.
Is added in parallel with the capacitor Ca. When the capacitor C1 is added in parallel with the capacitor Ca, the time constant becomes larger than that of the first response speed adjusting circuit including only the resistor Ra and the capacitor Ca.
The response speed of the C voltage will be greatly delayed,
It is possible to prevent the response speed of the entire tuner AGC loop from increasing. As a result, an appropriate response speed can be obtained over a wide input level range.

(Second Embodiment) FIG. 2 shows the structure of a response speed adjusting circuit according to a second embodiment of the present invention. Here, the parts from the antenna to the VIF IC are the same as in FIG.

In this embodiment, the second response speed adjusting circuit 10 ', which is added in parallel to the response speed adjusting circuit 9 composed of the resistor Ra and the capacitor Ca, is composed of the varicap diode Cv. In this way, tuner A
It is also possible to continuously change the characteristics of the response speed adjustment circuit with respect to changes in the GC voltage. This allows finer adjustment and eliminates the need for a switching circuit. Here, the buffer 12 is for performing impedance matching.

The first response speed adjusting circuit 9 in the first and second embodiments may be composed of only the capacitor Ca.

[0022]

As described in detail above, according to the present invention, even if a variable gain amplifier in which the relationship between the control voltage and the gain is largely deviated from the straight line is used, the response speed of the AGC amplifier is wide in the input level. It can be maintained at an appropriate value over the range, and the operation of the AGC amplifier can be made quick and stable.

Further, since the control voltage itself is detected and the response speed of the control voltage is changed, even if the variable gain amplifier or the comparison circuit is a black box or is already fixed and cannot be changed, the circuit can be easily changed. It can be added externally and the operation of the AGC amplifier can be improved.

[Brief description of drawings]

FIG. 1 is a diagram showing a first embodiment of the present invention.

FIG. 2 is a diagram showing a second embodiment of the present invention.

FIG. 3 is a block diagram showing a configuration of a conventional general AGC amplifier.

FIG. 4 is a diagram showing a response characteristic of a conventional general AGC amplifier.

FIG. 5 is a diagram showing a relationship between a control voltage and a gain of a conventional general AGC amplifier.

[Explanation of symbols]

2 ... RF amplifier, 8 ... Comparison circuit, 9 ... First response speed adjusting circuit, 10, 10 '... Second response speed adjusting circuit

Claims (4)

[Claims] 1. An AGC amplifier configured to compare an output voltage of a variable gain amplifier with a predetermined reference voltage to generate a control voltage and control the gain of the variable gain amplifier by the control voltage. AGC characterized by having a response speed adjusting circuit whose response speed changes according to the level
Amplifier. 2. The response speed adjusting circuit includes a first response speed adjusting circuit including a resistor and a first capacitor, and a second capacitor connected in parallel to the first capacitor when the control voltage exceeds a predetermined level. The AGC amplifier according to claim 1, further comprising a second response speed adjusting circuit including a switching circuit added to the. 3. The response speed adjusting circuit comprises a first response speed adjusting circuit composed of a resistor and a first capacitor, and a capacitance which is connected in parallel to the first capacitor and which has a capacitance according to the level of the control voltage. The AGC amplifier according to claim 1, wherein the AGC amplifier comprises a second response speed adjusting circuit that is composed of a variable capacitance element that changes. 4. The variable gain amplifier is an RF of a television receiver.
An amplifier, and the control voltage is generated by comparing a video detection output voltage with a predetermined reference voltage.
The described AGC amplifier.