JPS62173811A - Gain control circuit - Google Patents

Abstract

PURPOSE:To improve the linearity of gain characteristic by fixing the sum of currents of differential amplifying circuits in the input side and the output side and controlling the current of the differential amplifying circuit in the input side in a Gilbert amplifying circuit. CONSTITUTION:Currents I7 and I8 of transistors TRs Q7 and Q8 are changed in accordance with an output voltage V8 of a variable resistance 26, and the sum of them is always a current I0. When the current I7 of the TR Q7 is denoted as K.I0 (0<=K<=1), a current I1 of the first differential amplifying circuit 6 is K.I0 and a current I2 of the second differential amplifying circuit 12 is (1-K).I0. A current I9 of a current source 30 is controlled to K.I0/2. As the result, proportional constant of an input voltage Vi to an AC component V02 of an output voltage V2 is -RL(1-K)/2KRE (RL and RE are resistance values of resistances 15 and 16 and resistances 7 and 8 respectively), and it is changed approximately linearly for the change of K.

Description

DETAILED DESCRIPTION OF THE INVENTION A. Field of Industrial Application The present invention relates to a gain control circuit, and is suitable for use, for example, in a gain control circuit of an automatic color saturation adjustment (ACC) circuit of a color television receiver.

B. Summary of the Invention The present invention provides a gain control circuit having a so-called Gilbert type amplifier circuit configuration.
By keeping the sum of the currents of the differential amplifier circuit constant and controlling the current of the input side differential amplifier circuit, the change in the loop gain of the automatic gain adjustment circuit is made much smaller than before. This is what I did.

C. Conventional Technology Conventionally, for example, in a color television receiver, in order to keep the color saturation constant on the screen, an automatic color saturation adjustment circuit having an automatic gain adjustment circuit configuration as shown in Fig. 2 is used to adjust the color saturation. Subcarrier signal s1 given to a signal demodulation circuit (not shown)
I try to keep the level constant.

That is, the manual subcarrier signal S2 is amplified to a predetermined signal level in the automatic color saturation adjustment amplifier circuit 1, and is sent out as a subcarrier output signal S. The peak level of the color burst signal of this subcarrier signal S1 is detected by a peak detection circuit 2, and a DC output signal S corresponding to the detected peak level is outputted to an arithmetic circuit 3.

The DC output signal S3 is applied to the reference voltage ■,
. .

In this way, even if the peak level of the color burst signal fluctuates, the level of the subcarrier S1 is maintained at a constant value corresponding to the reference voltage Vref.

In order to configure the conventional automatic color saturation adjustment amplifier circuit 1 using RC, a so-called Gilbert type amplifier circuit (
For example, US Patent 3. f376.789) was used.

In FIG. 5, transistors Q1 and Q2 constitute a differential amplifier circuit 6, the emitters of which are connected to a current source 9 with a current value of 1 through resistors 7 and 8 with a resistance value RE, respectively. The collectors of transistors Ql and Q2 have a voltage tA
1o is connected to the base and the collector is connected to the emitters of so-called diode-connected transistors Q3 and Q4, which are connected to the power supply line L1, and thus the collectors of the transistors Q1 and Q2 are lowered by the base-emitter voltage from the output voltage of the power supply 10. It is maintained at a certain value.

The collectors of these transistors Ql and Q2 are connected to the emitters of a differential amplifier circuit 1 having a current source 11 with a current value of 12.
load resistors 15 and 16 with resistance value RL (voltage VCC
Output terminals TI and I2 are led out from the collector connected to the power supply line L1 (connected to the power supply line L1).

In the above configuration, when the input voltage V8 is supplied to the bases of the transistors Ql and Q2 from the signal #14 which is DC biased by the power supply 13, the base voltage of the transistor Q1 becomes the base-emitter voltage V Il
l! I and the current ■3 of the transistor Q1, the following formula % Formula % (1) Similarly, the base voltage VII2 of the transistor Q2 is determined by the base-emitter voltage V H4 and the transistor Q4.
According to the current I4, the voltage V of the signal source 14 is expressed by the following formula V r −V
B1 V B□ = (+3-I4) RE (VIIEI #VB!□) ・・・・・・(3)
It is expressed as

The sum of the currents l and I4 of transistors Q1 and Q2 is expressed by the following equation (4). From equation (3), the current I3 flowing through the transistor Ql is as follows. Similarly, the current ■4 flowing through the transistor Q2 is: The collector voltages of the transistors Ql and Q2 are held by the transistors Q3 and Q4 at the power supply IO and the base-emitter voltage of the transistors Q3 and Q4. Therefore, the current ■5 flowing through the transistor Q5 is calculated by the following formula lz +5= Is · 1+ 2 2RE r+ (7) Similarly, the current I6 flowing through the transistor Q6 is expressed as follows. Therefore, the voltage V at the output terminal T1 is obtained by subtracting the voltage drop across the resistor 15 from the voltage at the power supply line L1 (direct V+ = Vcc I5 ・ Rt...
(9) Similarly, the voltage V2 at the output terminal T2 is expressed by the following formula. Therefore, the voltage v0 of the differential output of the output terminals TI and T2 is expressed by the following formula (%), and the gain GI can be expressed as follows.

As a result, the gain of the gain control circuit 4 is expressed by the straight line LAI in FIG.
As shown, it changes in proportion to the change in the value of the current I2,
As shown by curve LA2, it changes in inverse proportion to the change in the value of current 11.

Therefore, by controlling the value of the current It or I2 using the control voltage S4 described above in FIG. 2 as necessary, it is possible to obtain an output voltage that maintains a predetermined level.

D Problems to be Solved by the Invention However, when using the automatic color saturation adjustment amplifier circuit configured as shown in FIG. (dB) ~ 20(d
B) Used in degrees. At this time, the voltage of the control signal Sa of the output signal S is set to ■. , when the gain of the operational amplifier circuit is β, it is desirable that the loop gain GL of the circuit system expressed by the following equation is constant.
This means that it is desirable to have control characteristics based on the current 2 as shown by the straight line LAI in the figure.

However, when the current I2 changes, as shown in the second term on the right side of equations (9) and (10), the voltages at the output terminals F1 and T2 change by the voltage 2 ・RL/2, which causes the output terminal The DC levels of 'r1 and T2 change, and as a result, the variable range of the current I2 cannot be set very wide, making it difficult to secure a practically sufficient gain control range.

Also, when a small signal is input, the current 2 must be increased to increase the gain, whereas when a large signal is input, the current I2 must be decreased to decrease the gain, so the dynamic range of the input signal must be sufficiently large. It was also difficult.

For this reason, in practice, in the past, the gain had to be controlled by controlling the current (1).In the end, when trying to construct a gain control circuit using a so-called Gilbert type amplifier circuit,
As shown by curve LA2 in FIG. 6, there is a problem in that it is impossible to avoid control characteristics with poor linearity. Incidentally, the current 11 that makes the gain G, l with the resistance value RE=RL is I.
+5to(ItsA.=■2), the gain G changes as expressed by the following formula % formula % (14).

Here, the current 1 is the control voltage V. Since it varies in proportion to 6 [dB]～
The value of X when changing in the range of 20 (dB) is (14
), x=2 (6(dB) ) −・”(
16) x = 0.1 (20 (dB))
-(17).

Therefore, the loop gain GL at this time is calculated as follows using equation (15): GL = 0.25β(-6(dB))
・・・・・・(18)GL = 100β(20(dB
) ) ...(19), the gain G is between -6 (dB) and 20 CdB),
This results in a change in the loop gain GL by a factor of 400.

The present invention has been made in consideration of the above points, and it is an object of the present invention to propose a gain control circuit that can effectively solve the problem of control characteristics in conventional gain control circuits.

E Means for Solving the Problem In order to solve this problem, in the present invention, a first transistor Q1 and a second transistor Q2 having a diode connected to the collector and inputting the input signal 14 to the base is provided. differential amplifier circuit 6, and first and second transistors Q.
1. A second differential amplifier circuit 1 consisting of third and fourth transistors Q5 and Q6, which connects the collector of Q2 to the base and connects the collector to the output terminals T1 and I2.
2, and the current 1 of the first and second differential amplifier circuits 6.12,
．． 1. A control circuit 21 that controls the sum of
The gain is controlled by controlling the current 11 of the first differential amplifier circuit 6.

F action current of first differential amplifier circuit 6 1. When , the sum of the currents of the first differential amplifier circuit 6 and the second differential amplifier circuit 12 is controlled to a constant value, so that the second differential amplifier circuit 12
The current I2 changes inversely to the change in the current r.

Therefore, by changing the current 11, it is possible to obtain a gain control circuit in which the gain changes uniformly and changes almost linearly when the gain is expressed logarithmically. An embodiment of the invention will be described in detail.

In FIG. 1, in which parts corresponding to those in FIG. 5 are denoted by the same reference numerals, a control circuit 21
By providing the current ■ of the differential amplifier circuits 6 and 12
, and I2.

In the control circuit 21, transistors Q7 and Q8 form a differential amplifier circuit whose emitters are connected to a current source 22 with a current value of 1° via resistors 23 and 24, and a voltage 7 is applied to the base of the transistor Q7. a25 is connected, and the power supply V of the power supply line LL is connected to the base of the transistor Q8.
A variable resistor 26 formed by dividing CC is connected. The collectors of transistors Q7 and Q8 are then connected to ground via diodes 27 and 29, respectively.

Thus, currents I7 and 1. flowing through transistors Q7 and Q8, respectively. The sum of is always the current ■ of the current source 22. and the current I? and the value of I6 is the voltage V of the power supply 25
, the output voltage of the variable resistor 26 is determined by dividing the voltage VCC of the power supply line LL with respect to .

In addition to this configuration, the emitters of the transistors Q1 and Q2 are connected via resistors 8 and 7, and the bases are connected to a diode 2.
A transistor Q9 is connected to the non-ground side of the transistor Q9, and forms a current mirror circuit together with the diode 27. In this way, the current 1 flowing through the differential amplifier circuit 6 is controlled so as to always be equal to the current I7 of the transistor Q7.

Similarly, a transistor QIO whose base is connected to the non-grounded end of a diode 29 is connected to the emitters of the transistors Q5 and Q6, forming a current mirror circuit together with the diode 29. In this way, the current I2 flowing through the differential amplifier circuit 12 is controlled so as to always be equal to the current (2) of the transistor Q8.

The control circuit 21 is provided with a current source 30, and its current ■
, is controlled to vary according to the current I of the transistor Q7. This current I is 19 = +7 /2
(=1./2). This current source 33 is connected to the midpoint between the transistor Q6 and the resistor 16, and is configured to shunt the current flowing into the collector of the transistor Q6.

In the above configuration, the currents I and I of the transistors Q7 and Q8 change according to the output voltage V8 of the variable resistor 26, and the sum thereof always becomes the current I0.

Therefore, the current I7 of transistor Q7 becomes 'l.

(O≦≦1), the current I of transistor Q8,
is (1-k) ・■. becomes.

Therefore, the value of the current I of the differential amplifier circuit 6 is k·■. Therefore, the value of the current ■2 of the differential amplifier circuit 12 is (t-k)
・To becomes the current [30(7) current■.

The value of is -to/z.

Therefore, the currents 13 and I4 of the transistors Q3 and Q4 of the differential amplifier circuit 6 are expressed by the following equations from equations (5) and (6), and the currents 5 and I6 flowing through the transistors Q5 and Q6 are calculated by the equations (7) and ( 8) According to the following formula, the voltage V2 at the output terminal T2 is the voltage obtained by subtracting the voltage drop across the resistor 16 due to the current 16l-I9 flowing through the resistor 16 from the voltage Vcc of the power supply line L1, and is calculated by the following formula: % Formula %) ...... (24) becomes. Here, the first and second terms on the right side of equation (24) are:
It is a DC component that is always constant regardless of the input voltage V and the constant k, and conversely, the third term represents the change in AC bone voltage that changes depending on the input voltage (2) and the constant k.

Therefore, the output signal of the gain control circuit 21 is ■. 2 is the following formula% formula%
(25) Similarly, the output signal VOI of the output terminal TI is expressed by the following formula...
・(26) Therefore, the differential output ■. and transfer function G.

is the following formula % formula % In other words, by varying the output voltage of the variable resistor 26, the current 1
By varying + (=-16), it is possible to obtain a control characteristic that changes in proportion to (1-k)/k, as shown in FIG. When the gain of this control characteristic is expressed in decibels, as shown by the curve LA3 in FIG. 4, the conventional current 1. It can be seen that it is possible to obtain control characteristics that change more linearly than the control characteristics obtained by .

The gain control circuit 20 having the above configuration is used in the automatic color saturation adjustment control circuit 1 in the automatic color saturation adjustment circuit shown in FIG. A control voltage S3 shown in FIG. 1 is applied to the base of the transistor Q8.

The gain G2 of the gain control circuit 20 is expressed by the following formula, and the loop gain GL is calculated from the formula (13) by the following formula GL = -□ β ...... (3
0).

Use equation (29) to find the value of k when the gain G2 changes in the practical range of -6 [dB] to 20 CdB]. I understand that.

Therefore, the loop gain GL at this time is calculated using the following equation (30): CL = -2,28β (-6 (dB))
...(33)GL=-123β (20(dB)
)......(34) An automatic color saturation adjustment circuit in which the loop gain GL changes by about 54 times when the gain G2 is between -6 (dB) and 20 [dB]. can be obtained.

According to the above configuration, when the gain control circuit is used under the same conditions as the conventional gain control circuit, that is, when the resistance value Rt=RL and the current 1=Iz, the gains G1 and Gt are 1, the loop gain GL is The width of change can be reduced to about 1/4 compared to the conventional method.

In addition, the variable range of the current It is also the conventional value [+ =X ・I+sto (x=2~
0.1.113. . -r2) to (31)2 and (3
2) Value T I = k-115TD (k = 0
．． 067 to 0.091, I, =r+5to). Therefore, it is possible to obtain an automatic gain adjustment circuit that has a greater degree of freedom in design and a wider range of application than the conventional one.

According to the above configuration, by changing the currents 1 and I2 simultaneously using a simple configuration, control that changes uniformly compared to a conventional gain control circuit in which the current 1 is changed alone. It can be made into a characteristic gain control circuit.

For this reason, variable resistor 2 is used in the circuit system shown in Figure 2.
By adding the control signal S4 in place of the output voltage 18 of 6, the loop gain of the circuit system can be kept almost constant.

In addition, the DC level of the output voltage (2) is always kept constant by the current source 30, so that when the circuit gain changes (when the current (2) changes), the DC level does not change. It can be prevented.

In the above-mentioned embodiments, the present invention was applied to an automatic color saturation control (ACC) circuit of a color television receiver, but the present invention is not limited to this. ) Can be widely applied to circuits, etc.

Effects of the Invention H As described above, according to the present invention, a gain control circuit in which the change in loop gain is much smaller than that of the prior art can be easily obtained.

[Brief explanation of drawings]

Fig. 1 is a connection diagram showing an embodiment of a gain control circuit according to the present invention, Fig. 2 is a block diagram showing an automatic color saturation adjustment circuit using the gain control circuit, and Figs. 3 and 4 are its control. FIG. 5 is a connection diagram showing a circuit of a conventional gain control circuit, and FIG. 6 is a characteristic curve diagram showing its control characteristics. 4.20...gain control circuit, 6.12...
・Differential amplifier circuit, 7.8.15.16.23.24・
...Resistance, 9.11.22.30...Current source, 10.13.25...Power supply, 14...
...Signal source, 27.29...Diode, Q
1~QIO...transistor.

Claims (1)

[Claims] A first differential amplifier circuit comprising first and second transistors having a diode connected to the collector and inputting an input signal to the base; and a base having the collectors of the first and second transistors. and the collector is connected to the output terminal.
and a second differential amplifier circuit including a fourth transistor, and a control circuit that controls the sum of currents of the first and second differential amplifier circuits to a constant value, A gain control circuit characterized in that the gain is controlled by controlling the current of a differential amplifier circuit.