JPH02135810A - gain control circuit - Google Patents

Abstract

(57) [Summary] This bulletin contains application data before electronic filing, so abstract data is not recorded.

Description

DETAILED DESCRIPTION OF THE INVENTION [Field of Industrial Application] The present invention relates to a gain control circuit, and particularly to a gain control circuit in a video signal amplification system for a color CRT monitor.

[Conventional technology]

Conventionally, a variable transconductance type multiplication circuit has been used for gain control (amplitude adjustment) in a video signal amplification system for a color CRT monitor.

In addition, as a conventional example related to the present invention, for example, Utsunomiya and Wakui (eds.): Image Electronic Circuits, edited by the Television Society,
Coronasha, July 1986, PP100-109.

[Problem to be solved by the invention]

In the variable transconductance type multiplication circuit having only the first quadrant of the prior art, the slope of the output signal with respect to the input signal obtained by the magnitude of the control signal, that is, the gain is the third
It will look like the figure.

On the other hand, the video signal of the video signal amplification system has a signal waveform peculiar to video in which the amplitude changes toward the white level with respect to the black level as shown in FIG.

Therefore, in FIG. 3, if the reference value (black level) of the video signal is temporarily set to VIB, there is a problem that an offset occurs in the output signal and the reference value of the video signal cannot be determined by this circuit alone.

An object of the present invention is to provide a gain control circuit in which the slope of an output signal with respect to an input signal, that is, the gain, which can be changed by the magnitude of a control signal, intersects at one point for all gains.

[Means to solve the problem]

The above purpose is to extract one output of a differential amplifier section in the form of a current in a variable transconductance type multiplier circuit, and operate a nonlinear correction section for correcting the nonlinear operation of the input transistor of the differential amplifier section to this output current. This is accomplished by adding a current linked to the current.

[Effect]

A current linked to the operating current of the nonlinear correction section is taken out and added to the output current of the differential amplifier section.

A bias amount is added to the slope of the output signal with respect to the input signal set by the control signal, that is, the gain. If this bias amount is obtained as an amount inversely proportional to the control signal, a large amount is applied when the gain is small, and a small amount is applied when the gain is large, so that all gains intersect at a constant value. This makes it possible to keep the normal value of the video signal constant, so that no offset occurs in the output signal.

〔Example〕

A specific embodiment to which the present invention is applied will be described below with reference to FIG.

In FIG. 1, 1 is an input terminal, 2 is a control terminal, 3 is an output terminal, 4 is a power supply voltage terminal, 100 is a differential amplifier section, 20
0 is a nonlinear correction section, and 300 is a current-voltage conversion section.

The differential amplifier section 100 is a variable transconductance type multiplication circuit, and includes transistors Q1. Q2 , Q3 , and Q
transistors Ql and Q2 form a differential pair, and the input terminal 1 is connected to the base of transistor QB.
is connected. The nonlinear correction section 200 is a transistor Q7.
~Qll, resistor R1, constant current sources CCI, CC2, control terminal 2 is connected to the base of transistor Q7, voltage source Vr is connected to the base of transistor Q6, and bias is applied to the bases of transistors Q9, T, and Qto. Power supply Vs is connected. The current-voltage converter 300 includes transistors Q4 and Qs and a resistor R2, and the emitters of the transistors Q4 and Qs are connected to the output terminal 3. Transistors Ql and Q of the differential amplifier section 100
The bases of z are connected to the emitters of transistors Q9 t QIO of the nonlinear correction section 200, respectively. Transistor Q3 of differential amplifier section 100 and current-voltage conversion section 300
It has a current mirror configuration with transistor Q4. In addition, the transistor Qll of the nonlinear correction section 200
The transistor Q5 of the current-voltage converter 300 also has a current mirror configuration, and this current mirror is for applying a bias current.

The operation of FIG. 1 in the above configuration will be explained.

First, the operation when no bias current is applied except for the current mirror configuration of transistors Q11 and Qs will be described.

In the differential amplifier section 100, let the collector currents of the transistors Ql and Q2 be 11 and Iz, respectively, the input signal applied to the input terminal 1 be vl, the base-emitter voltage of the transistor Qe be VBE, and the value of the resistor RE be RE.
Current 11+I2, which is the sum of collector currents flowing through transistors Q1 and 02 in response to input signal v1, is (Vt VBE
)/RE. If the differential voltage between the bases of the transistors Q and Qz is ΔV, the current ratio between the transistors Q1 and I2 is I, /I! = eAv/v”
- It can be obtained from the relationship (1). Here, vT indicates thermal voltage. As can be seen from equation (1), the linearity of 11/Iz is obtained only when ΔV is within the range of VT, and is approximately 26 mV.
And narrow.

Therefore, the nonlinear correction section 200 is used to obtain a wide linear operation range. Therefore, transistors Q1 and Qz
The differential voltage ΔV between the bases of transistors Qs and Ql
It is generated as a voltage obtained between the emitters of o. That is, ΔV is the collector current of transistors Q9 and 01o, respectively.

a Δv=vTQn-...(2) ■^ Obtained. Furthermore, the current ratio Ia/I^ in equation (2) is determined by assuming that the control signal applied to the control terminal 2 is vc, the reference signal of the reference voltage source ■ is vr, and the values of the constant current sources CCI and CG2 are Ice.
, and the value of the resistor R1 is R1, it is obtained as follows. Therefore, by substituting equation (2) into equation (1), the relationship Iz I^ is established, and the exponential current operation of the differential amplifier section 100 can be linearized by logarithmic transformation of the nonlinear correction section 200,
Furthermore, the current ratio Ix/It can be changed depending on the magnitude of the control signal Vc with respect to the reference signal V. That is, it shows that the gain is changed by the control signal.

The collector current It obtained in this way is obtained as an output signal at the output terminal 3 of the current-voltage converter 300 by the current mirror of the transistors Q8 and Q4. now,
The current ratio of the current mirror of transistors Q3 and Q4 is 1
:1 and the value of resistor Rz is R2, the output signal V
o becomes R2I2. At this time, the relationship between the input signal vI and the output signal Vo is expressed using the control signal Vc as a parameter as shown in FIG.

Next, a bias operation using a current mirror of transistors Qzt and Q5 will be described. This bias operation applies the collector current of transistor Q4 to transistor Qs.
collector current, that is, a bias current. Therefore, the output signal Vo now has a transistor Q
Considering the current ratio of current mirror of lt and Q5 to be 1:1, Vo=Rz(Iz+IF3)
...(5) is obtained, and when substituting this equation with reference to equation (3), Vo=Rz(Iz+Ice (Vc-Vr)/Rt)
...(6) is revised. When formula (6) is expressed as a graph, it becomes as shown in FIG. 2, and compared to FIG. 3, it shows that the output signal Vo intersects with Voa at one point for all control signals Vc.

In this operation, from equation (6), a bias amount of IOC is added to step 2 at V c '' V r, and Vc <
Since the gain Vo/Vi is small at V, a larger amount of bias is applied than Ice, and when V c > V r the gain is large, so a smaller amount of bias than ICC is applied. This bias amount is linked to In, which changes in inverse proportion to the control signal VC, so the output signal Vo
has an intersection point at a value greater than RzIz.

Although this intersection point has been described when the current ratio of the current mirrors of the transistors Qll and Q6 is 1:1, it can be realized with other current ratios, so that the intersection point can be obtained at any position.

Further, although this embodiment has been shown as an example using a bipolar transistor process having PNP and NPN, it can also be realized using an element process such as CMOS or BiCMO5.

According to this embodiment, by adding a current linked to an operating current that is inversely proportional to the control signal of the nonlinear correction section to an output current that is proportional to the control signal of the differential amplifier section, the input obtained by all the control signals is the slope of the output signal relative to the signal,
That is, the gains can intersect at one point.

An example of the configuration of a video signal amplification system for a color CRT monitor is shown in FIG. In FIG. 5, the applicable part of the present invention is 43
0 gain adjustment circuit. In Fig. 5, parts equivalent to those in Fig. 1 are given the same reference numerals. 1 is the input terminal, 3 is the output terminal, 2
is the control terminal.

In addition to applying the variable voltage source 431 shown in FIG. 5 to the control terminal 2, it is also possible to apply a divided voltage using a volume.

〔Effect of the invention〕

According to the present invention, the slope of the output signal with respect to the input signal obtained by the magnitude of the control signal, that is, the gain, can intersect at one point at all gains, so that the video signal for color CRT monitors and color television receivers can be amplified. The standard value of the system video signal, that is, the black level, can be stably obtained without offset.

Further, since the reference value can be set by the gain control circuit itself without offset, there is an advantage that it can be used as a directly connected gain control circuit.

[Brief explanation of the drawing]

Fig. 1 is a circuit diagram showing an embodiment of the present invention, Fig. 2 is an input/output characteristic diagram of Fig. 1, and Fig. 3 is a diagram showing the transistor Qz of Fig. 1.
FIG. 4 is a video signal waveform diagram, and FIG. 5 is a configuration diagram of a video signal amplification system to which the present invention is applied. DESCRIPTION OF SYMBOLS 1...Input terminal, 2...Control terminal, 3...Output terminal, 4...Power supply voltage terminal, 100...Differential amplifier section, 2
00... Nonlinear correction section, 300... Electricity-voltage conversion section, Qw~Qth... Transistor, R1, R2, R
E...Resistance, CCI, CC2... Constant current source, ■,...
...Reference voltage source, VB...bias power supply. Figure 2 Figure 3 Addition? l signal qi Figure 4

Claims (1)

[Claims] 1. In a variable transconductance type multiplication circuit,
The method is characterized in that one output of the differential amplifier section is extracted in the form of a current, and a current linked to the operating current of the nonlinear correction section for correcting the nonlinear operation of the input transistor of the differential amplifier section is added to this output current. Gain control circuit. 2. The gain control circuit according to claim 1, which is applied to a video signal amplification system for color CRT monitors and color television receivers.