JPH0738550B2 - Gain control circuit - Google Patents

Description

The present invention relates to a gain control circuit, which is suitable for use in a gain control circuit of an automatic color saturation adjustment (ACC) circuit of a color television receiver, for example.

B Outline of the Invention The present invention is a gain control circuit having a so-called Gilbert-type amplifier circuit configuration.
By making the sum of the currents of the differential amplifier circuit of 1 constant and controlling the current of the differential amplifier circuit on the input side, the change of the loop gain of the automatic gain adjustment circuit can be made much smaller than the conventional one. It was done.

C. Conventional Technology In a conventional color television receiver, for example, in order to keep the color saturation on the screen constant, a color signal is obtained by using an automatic saturation adjusting circuit having an automatic gain adjusting circuit configuration as shown in FIG. The level of the subcarrier signal S ₁ given to the demodulation circuit (not shown) is kept constant.

That is, the input subcarrier signal S ₂ is amplified to a predetermined signal level in the automatic color saturation adjustment amplifier circuit 1 and transmitted as the subcarrier signal S ₁ . The peak level of the color burst signal of the subcarrier signal S ₁ is detected by the peak detection circuit 2, and the DC output signal S ₃ corresponding to the detected peak level is output to the arithmetic circuit 3.

The DC output signal S ₃ is compared with the reference voltage Vref in the arithmetic circuit 3, and the control signal S ₄ according to the difference voltage between the reference voltage Vref and the DC output signal S ₃ is sent to the automatic color saturation adjustment amplifier circuit 1 by the difference voltage. Is output as a level adjustment signal.

Thus, even if the peak level of the color burst signal fluctuates, the level of the subcarrier signal S ₁ is maintained at a constant value corresponding to the reference voltage Vref.

Conventionally, a so-called Gilbert-type amplifier circuit (for example, disclosed in US Pat. No. 3,676,789) shown in FIG. 5 has been used to configure the automatic color saturation adjustment amplifier circuit 1 by an IC.

In FIG. 5, transistors Q1 and Q2 form a differential amplifier circuit 6, and the emitters thereof are connected to a current source 9 having a current value I ₁ via resistors 7 and 8 having a resistance value R _E , respectively. The collectors of the transistors Q1 and Q2 are connected to the emitters of so-called diode-connected transistors Q3 and Q4, which connect the power supply 10 to the base and the collector to the power supply line L1, and thus the collectors of the transistors Q1 and Q2 are connected to the power supply 10. The output voltage is maintained at a value that is reduced by the base-emitter voltage.

The collectors of the transistors Q1 and Q2 are connected to the bases of the transistors Q5 and Q6 of the differential amplifier circuit 12 in which the current source 11 having the current value I ₂ is connected to the emitter, respectively, and the resistance value is
From the collector connected to the load resistors 15 and 16 of R _L (connected to the power supply line L1 of voltage V _CC ) to the output terminal T1
And T2 have been derived.

In the above configuration, when the input voltage Vi is supplied to the bases of the transistors Q1 and Q2 from the signal source 14 that is DC biased by the power supply 13, the base voltage V _{B1 of the} transistor Q1 is supplied.
Is its base-emitter voltage V _BE1 and transistor
The current I _{3 of} Q1 is expressed by the following formula V _B1 = I ₃ · R _E ＋ V _BE1 ＋ V _S …… (1).

Similarly, the base voltage V _B2 of the transistor Q2 is expressed by the following formula V _B2 = I ₄ · R _E + V _BE2 + V _S ... (2) by the base-emitter voltage V _BE2 and the current I _{4 of the} transistor Q4. It

Accordance connexion, the voltage vi of the signal source 14 is represented by the following formula _{Vi = V B2 -V B1 ≒ (} I 4 -I 3) R E (V BE1 ≒ V BE2) ...... (3).

Since the sum of the currents I ₃ and I ₄ of the transistors Q1 and Q2 is the following formula I ₃ + I ₄ = I ₁ (4), the current I ₃ flowing in the transistor Q1 can be calculated from the formula (3) as follows. Becomes

Similarly, the current I ₄ flowing in the transistor Q 2 is Becomes

Here, the collector voltage of the transistors Q1 and Q2 is
The current I _{5 that} is held in the base-emitter voltage of Q4 and therefore flows in transistor Q5 is Similarly, the current I ₆ flowing in the transistor Q6 is Becomes Therefore, the voltage V _{1 at the} output terminal T1 is the value obtained by subtracting the voltage drop across the resistor 15 from the voltage V _{CC at the} power line L1. Similarly, the voltage V ₂ at the output terminal T2 is Becomes Therefore, the voltage v ₀ of the differential output of the output terminals T1 and T2 is And the gain G ₁ is Can be expressed as

As a result, the gain of the gain control circuit 4 changes in proportion to the change in the value of the current I ₂ as shown by the straight line LA1 in FIG.
As indicated by LA2, the current I ₁ changes in inverse proportion to the change in the value.

Therefore, by controlling the value of the current I ₁ or I ₂ by the control voltage S ₄ described above in FIG. ₂ as necessary,
It is possible to obtain an output voltage that maintains a predetermined level.

D Problem to be solved by the invention However, when the automatic color saturation adjusting / amplifying circuit having the structure shown in FIG. 5 is used, the gain with respect to the change of the control voltage S ₄ is
The change range of G ₁ is, for example, about −6 [dB] to 20 [dB]. At this time, the voltage of the control signal S ₄ of the output signal S ₁
When V _C and the gain of the operational amplifier circuit are β, It is desirable that the loop gain G _{L of the} circuit system represented by is constant, which means that the straight line LA of FIG.
It means that it is desirable to have the control characteristic by the current I ₂ as shown by 1.

However, when the current I ₂ changes, the voltage at the output terminals T1 and T2 becomes the voltage as shown in the second term on the right side of the expressions (9) and (10).
Only I _{2 RL} / 2 changes, which changes the DC level of output terminals T1 and T2.As a result, the variable range of current I ₂ cannot be set too wide. Was difficult to secure.

Also, when a small signal is input, the current I ₂ is increased to increase the gain, whereas when a large signal is input, the current I ₂ must be decreased to reduce the gain.Therefore, the dynamic range of the input signal is made sufficiently large. It was also difficult to take.

For this reason, in reality, conventionally, the gain must be controlled by controlling the current I _1, and when a gain control circuit is eventually configured using a so-called Gilbert-type amplifier circuit, the curve LA2 in FIG. As shown in, there is a problem that the control characteristic with poor linearity cannot be avoided. Because of the resistance value R _E
＝ _RL and the current I ₁ that makes the gain G _{1 equal to 1} is I _1STD (I _1STD = I ₂ ), the gain G ₁ becomes Changes as represented by.

Since the current I ₁ changes in proportion to the control voltage V _C ,
When I ₁ = V _C , the loop gain G _L shown in equation (13) is given by the following equation when the gain of the operational amplifier circuit 3 is β. It is represented by.

Here, the gain G ₁ is in a practical range, that is, -6 [dB] to 20
When the value of x when changing in the range of [dB] is calculated from the equation (14), x = 2 (-6 [dB]) …… (16) x = 0.1 (20 [dB]) …… (17) It will change in the range of.

Therefore, the loop gain G _{L at} this time is calculated by using Eq. (15) G _L = −0.25β (−6 [dB]) …… (18) G _L = −100β (20 [dB]) …… (19 ), The loop gain G _L changes 400 times when the gain G ₁ is between −6 [dB] and 20 [dB].

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

E Means for Solving the Problems In order to solve the problems, in the present invention, the first and second transistors Q1 and Q2 are connected to the collector and the input signal 14 is input to the base. Differential amplifier circuit 6 and the collectors of the first and second transistors Q1 and Q2 are connected to the bases, and the collectors are connected to the output terminals T1 and T2.
The second differential amplifier circuit 12 consisting of Q6 and one of the output terminals T2
A current source 30 is connected, and the current value I ₉ is selected to be one-half of the current value I ₁ of the first differential amplifier circuit 6,
And a control circuit 21 for controlling the sum of the currents I ₁ and I ₂ of the first and second differential amplifier circuits 6 and 12 to be a constant value.
The gain is controlled by controlling the current I ₁ of the differential amplifier circuit 6.

F action When the current I _{1 of} the first differential amplifier circuit 6 changes, 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. 2 differential amplifier circuit 12 current
I ₂ becomes opposite to the change of the current I ₁ . At this time, the DC level at the output terminal T2 is kept constant by the current source 30.

Therefore, by changing the current I ₁ , it is possible to obtain the gain control circuit 20 in which the gain changes uniformly, and when the gain is expressed in logarithm, it changes substantially linearly.

G Embodiment One embodiment of the present invention will be described in detail below with reference to the drawings.

In FIG. 1 in which parts corresponding to those in FIG. 5 are designated by the same reference numerals, the current I of the differential amplifier circuits 6 and 12 can be obtained by providing the control circuit 21 in place of the current sources 9 and 11 of FIG. ₁ and I ₂
To control.

In the control circuit 21, the transistors Q7 and Q8 form a differential amplifier circuit in which the emitter is connected to the current source 22 having the current value I ₀ via the resistors 23 and 24, and the base of the transistor Q7 has the power supply 25 of the voltage V ₇ . Is connected, and the variable resistance 26 is formed by dividing the power supply V _CC of the power supply line L1 into the base of the transistor Q8.
Are connected. The collectors of transistors Q7 and Q8 are then connected to ground via diodes 27 and 29, respectively.

Thus, the current I ₇ flowing through the transistors Q7 and Q8, respectively
The sum of and I ₈ is always equal to the current I ₀ of the current source 22, the value of the current I ₇ and I _8, the power supply line for the voltage V ₇ of the power supply 25
It changes according to the output voltage V ₈ of the variable resistor 26 obtained by dividing the voltage V _CC of L1.

In addition to such a configuration, the emitters of the transistors Q1 and Q2 are connected through resistors 8 and 7 to a transistor Q9 in which the non-ground side of the diode 27 is connected to the base,
A current mirror circuit is configured with the diode 27. Thus, the current I ₁ flowing through the differential amplifier circuit 6 is controlled so as to be always equal to the current I _{7 of the} transistor Q ₇ .

Similarly, the emitters of the transistors Q5 and Q6 are connected to the transistor Q10 whose base is connected to the non-earth side end of the diode 29, and together with the diode 29 form a current mirror circuit. Thus, the current I ₂ flowing through the differential amplifier circuit 12 is controlled so that it is always equal to the current I _{8 of the} transistor Q ₈ .

A current source 30 is provided in the control circuit 21, and its current I ₉ is controlled so as to change according to the current I _{7 of the} transistor Q ₇ . The current I ₉ is selected _{I 9 = I 7/2 (} = I 1/2).
The current source 30 is connected to the midpoint of the connection between the transistor Q6 and the resistor 16 so that the current flowing into the collector of the transistor Q6 is shunted.

In the above configuration, the currents I ₇ and I ₈ of the transistors Q7 and Q8 change according to the output voltage V ₈ of the variable resistor 26, and the sum thereof is always the current I ₀ .

Therefore, the current I _{7 of the} transistor Q7 is k · I ₀ (0 ≦ k ≦ 1)
Then, the current I ₈ (1-k) · I _{0 of} the transistor Q8 is obtained.

Therefore, the value of the current I ₁ of the differential amplifier circuit 6 becomes k · I ₀ ,
The value of the current I ₂ of the differential amplifier circuit 12 is (1-k) · I ₀ ,
The value of the current I ₉ of the current source 30 is k · I _0/2 .

Therefore, the current I ₃ of the transistors Q3 and Q4 of the differential amplifier circuit 6
And I _{4 From} equations (5) and (6), And the currents I ₅ and I ₆ that flow through the transistors Q5 and Q6 are given by the following equations from the equations (7) and (8). Becomes

Therefore, the voltage V ₂ of the output terminal T2 is the voltage V _{CC of the} power line L1.
Is the voltage obtained by subtracting the voltage drop across the resistor 16 due to the current I ₆ + I ₉ flowing through the resistor 16 from the following formula Becomes Here, the first term and the second term on the right side of the equation (24) are constant DC components regardless of the input voltage vi and the constant k, and conversely, the third term changes according to the input voltage vi and the constant k. Represents the voltage change of the alternating current component. Therefore, the output signal v ₀₂ of the output terminal T2 is It is represented by.

Similarly, the output signal v ₀₁ of the output terminal T1 is Therefore, the differential output v ₀ and the transfer function G ₂ are Will be represented by.

That is, by changing the output voltage V ₈ of the variable resistor 26, the current I ₁ (=
By varying k · I ₀ ), it is possible to obtain a control characteristic that changes in proportion to (1-k) / k as shown in FIG. When the gain is expressed in decibels in this control characteristic, the curve LA4 is obtained as shown by the curve LA3 in FIG.
It can be seen that a control characteristic that changes more linearly can be obtained as compared with the conventional control characteristic by the current I ₁ shown by.

The gain control circuit 20 having the above configuration is used for the automatic color saturation adjustment control circuit 1 in the automatic color saturation adjustment circuit of FIG.
The resistance value is R _E = R _L , and the control voltage S4 shown in FIG. 2 is applied to the base of the transistor Q8 instead of the resistor 26.

The gain G ₂ of the gain control circuit 20 is The loop gain G _L is given by the following equation from equation (13). It is represented by.

When the value of k when the gain G ₂ changes within the practical range of -6 [dB] to 20 [dB] is obtained by using the equation (29), the following equation k = 0.067 (-6 [dB] ) (31) k = 0.091 (20 [dB]) ... (32) It turns out that it changes in the range.

Therefore, the loop gain G _{L at} this time is expressed by the following formula G _L = −2.28β (−6 [dB]) using the equation (30). (33) G _L = −123β (20 [dB]) It is possible to obtain an automatic color saturation adjustment circuit in which the loop gain G _L changes about 54 times when the gain G ₂ changes from -6 [dB] to 20 [dB] by changing in the range of (34). it can.

According to the above configuration, the gain G _{1 under the} same condition as the conventional gain control circuit, that is, when the resistance value R _E = R _L and the current I ₁ = I ₂
When G and G ₂ are used under the condition of 1, the change width of the loop gain G _L can be reduced to about 1/4 as compared with the conventional case.

Further, the variable range of the current I ₁ is also changed from the conventional value I ₁ = x · I _1STD (x = 2 to 0.1, I _1STD = I ₂ ) shown in the equations (16) and (17) to the equation (31) and The value I ₁ = k · I _1STD (k
= 0.067 to 0.091, I ₁ = I _1STD ). Therefore, it is possible to obtain an automatic gain adjustment circuit having a wider design freedom and a wider range of application than the conventional one.

According to the above configuration, the currents I ₁ and I _{2 can be}
The gain control circuit having a control characteristic that changes uniformly compared to the conventional gain control circuit in which the current I ₁ is changed independently can be obtained by simultaneously changing the gain control circuit.

Therefore, the variable resistor 26 is used in the circuit system as shown in FIG.
The loop gain of the circuit system can be kept substantially constant by adding the control signal S ₄ instead of the output voltage V 8 of.

Further, the DC level of the output voltage V ₂ is always kept constant by the current source 30, and when the gain of the circuit changes (when the current I ₂ changes) as in the conventional case, the DC level changes. Can be prevented.

In the above embodiment, the case where the present invention is applied to the automatic color saturation adjustment (ACC) circuit of the color television receiver is described, but the present invention is not limited to this.
For example, it can be widely applied to an automatic gain adjustment (AGC) circuit and the like.

H Effect of the Invention As described above, according to the present invention, it is possible to easily obtain a gain control circuit in which the change in loop gain is much smaller than in the conventional case.

[Brief description 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 adjusting circuit using the gain control circuit, and FIGS. 3 and 4 show the control thereof. FIG. 5 is a characteristic curve diagram showing characteristics, 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 amplification circuit,
7,8,15,16,23,24 …… Resistance, 9,11,22,30 ……
Current source, 10, 13, 25 ... Power source, 14 ... Signal source, 27, 29 ...
… Diodes, Q1 to Q10… Transistors.

Claims (1)

[Claims] 1. A first differential amplifier circuit comprising first and second transistors having a collector connected to a diode and inputting an input signal to the base, and the collectors of the first and second transistors being the base. Third, connecting and connecting the collector to the output terminal
And a second differential amplifier circuit including a fourth transistor and one of the output terminals and having a current value of the first differential amplifier circuit.
A current source selected to be a half of the current value of the differential amplifier circuit, and a control circuit for controlling the sum of the currents of the first and second differential amplifier circuits to be a constant value. A gain control circuit comprising: a gain control circuit for controlling a gain by controlling a current of the first differential amplifier circuit.