JPH04342305A - Gain control circuit - Google Patents

Abstract

PURPOSE:To realize a circuit suitable for gain control, for which it is necessary to load suppression such as aperture signal suppression in low luminance or chroma signal suppression in low luminance especially by rapidly changing the gain of a multiplier circuit with the less than + or -4VT change of a gain control signal, concerning the gain control circuit used for a video camera or the like. CONSTITUTION:For this circuit, a common constant voltage source 4 is connected through resistors 3 and 5 to the respective bases of first and second transistors 6 and 13 having emitter follower constitution, the gain control signal is inputted to the base of the first transistor 6, a differential voltage between the emitters of the first and second transistors 6 and 13 is inputted to a differential amplifier, and the gain of the main multiplier circuit is controlled. At such a circuit, the part of the gain control signal corresponding to an arbitrarily decided voltage range is extracted by a limiter circuit 33, converted to a current by a first voltage/current conversion circuit 34 and fed back to the emitter of the second transistor 13.

Description

[Detailed description of the invention]

0001

[Industrial Application Field] The present invention is applicable to a gain control signal of ±4V, especially for suppressing aperture signals at low brightness and suppressing chroma signals at low brightness, for video signal amplification circuits used in video cameras.
The present invention relates to a gain control circuit in which the gain of a multiplier circuit must be suppressed by rapidly changing the gain of a multiplier circuit when the change is less than or equal to T (T is absolute temperature).

0002

2. Description of the Related Art In recent years, with the rapid development of video cameras, there has been a growing demand for higher performance video signal amplification circuits.

A conventional gain control circuit used in a video camera will be explained below. As shown in Figure 5,
The transistor 6 and the constant current source 7 constitute a first emitter follower, and the base of the transistor 6 is connected to the constant voltage source 4 via the resistor 3 and to the input terminal 1 of the gain control signal via the capacitor 2. The collector is connected to the power supply 29. The transistor 13 and the constant current source 14 constitute a second emitter follower. The base of the transistor 13 is connected to the constant voltage source 4 via the resistor 5, and the collector is connected to the power source 29. transistor 9,1
1, diodes 8, 10, and constant current source 12 constitute a differential amplifier for gain control, the base of transistor 9 is connected to the emitter of transistor 6, and the base of transistor 11 is connected to the emitter of transistor 13. Connected. Transistors 15, 16, 18, 19
, 22, 23, constant current sources 17, 24, and resistors 20, 21 constitute a multiplication circuit. 31 and transistor 1
The base of 6 is connected to the constant voltage source 27 via a resistor 25. A signal inputted from an input terminal 31 is outputted from an output terminal 30 connected to a common connection point between the collector of the transistor 22 and the resistor 21.

The operation of the gain control circuit configured as described above will be explained below. First, the gain control signal Vgc input to input terminal 1 is applied to the base of transistor 6 and output from the emitter, and transistors 9 and 1
1, diodes 8 and 10, and a constant current source 12, which is input to a differential amplifier for gain control.
, 10 is changed. As a result, the gain G of the multiplication circuit consisting of transistors 15, 16, 18, 19, 22, 23, constant current sources 17, 24, and resistors 20, 21 is determined by the resistance value of resistor 20 being R20 and the resistance value of resistor 21 being R20. If R21, the current value flowing through diode 10 is I10, and the current value flowing through diode 8 is I8,

[0005]

[Math 1]

It is expressed as 0006. As can be seen from equation (1), the gain G of the multiplication circuit can be controlled by changing the ratio of currents flowing through the two diodes of the differential amplifier using a gain control signal.

[0007]

[Problems to be Solved by the Invention] However, in the above conventional configuration, it takes ±4VT (=± A gain control signal amplitude of 4 kT/q≈±100 mV was required. Here, k: Boltzmann's constant T: Absolute temperature q: Unit charge of electrons In FIG. 6, the horizontal axis shows the difference voltage Ve between the emitters of the transistors 6 and 13, and the vertical axis shows the gain.

Generally, in a video camera, the S/N deteriorates in dark scenes due to the aperture signal, so it is necessary to suppress the aperture signal when the brightness is low. In this aperture signal suppression at low brightness, the brightness signal is used as a gain control signal, and as shown in the characteristic curve (B) of FIG.
Above 0mV, the aperture signal is applied to the MAX of the multiplication circuit.
When the luminance signal is amplified and outputted with a gain, and the luminance signal becomes 100 mV or less, the gain of the multiplication circuit is lowered, and when the luminance signal becomes 0 mV, the gain is set to MIN, and the aperture signal is suppressed. In the above conventional example, as described above, due to the characteristic curve (A) in FIG. 6, the luminance signal which is the gain control signal is
At 0 mV or more, the gain of the multiplication circuit becomes the MAX gain, but when the luminance signal is 0 mV, it does not become the MIN gain, and the gain of the multiplier circuit becomes the MAX gain.
Since there is an X gain of -6 dB, the aperture signal cannot be suppressed completely, so the S/N deteriorates in dark scenes.

[0009] Similarly, in dark scenes, the S/N is degraded by the chroma signal, so it is necessary to suppress the chroma signal at low brightness times. However, if the chroma signal is suppressed as quickly as the aperture signal, the color will disappear and the image will look unnatural. Therefore, when suppressing the chroma signal at low luminance, the luminance signal is used as the gain control signal, and the characteristic curve shown in Fig. 6 ( As shown in C), when the luminance signal is 20mV or more, the chroma signal is amplified by the MAX gain of the multiplication circuit and output, and when the luminance signal becomes 20mV or less, the gain of the multiplication circuit is lowered and the luminance signal becomes 0mV. Then, the gain becomes MAX gain -6dB
to suppress the chroma signal by half and reduce the S/N of the video signal.
prevent deterioration. In the above conventional example, when the luminance signal, which is a gain control signal, is 0 mV, the gain of the multiplication circuit is MAX gain -6 dB, but in order for the gain of the multiplication circuit to reach the MAX gain, the luminance signal must be input at 100 mV or more.

The present invention solves the above conventional problems and provides a gain control circuit that can rapidly change the gain of a multiplier circuit with a change amount of ±4 VT or less in the gain control signal. The purpose is to

[0011]

[Means for Solving the Problems] In order to achieve this object, the gain control circuit of the present invention provides a common constant voltage source via a resistor to the bases of the first and second emitter follower transistors. A gain control signal is input to the base of the first emitter follower configuration transistor, a differential voltage between the emitters of the first and second emitter follower configuration transistors is input to the differential amplifier, and the main In the circuit for controlling the gain of the multiplication circuit, a portion of the gain control signal corresponding to an arbitrarily determined voltage range is taken out by a limiter circuit and converted into a current by a first voltage-current conversion circuit; It is arranged so as to feed back to the emitter of a transistor with an emitter follower configuration.

A gain control circuit according to a second embodiment of the present invention includes:
A common constant voltage source is connected to the bases of each of the first and second emitter follower configuration transistors via a resistor, a gain control signal is input to the base of the first emitter follower configuration transistor, and the gain control signal is input to the base of the first emitter follower configuration transistor. In a circuit that inputs the difference voltage between the emitters of transistors with a first and second emitter follower configuration to a differential amplifier to control the gain of the main multiplier circuit, it corresponds to an arbitrarily determined voltage range of the gain control signal. The portion that is
A circuit is provided for feeding back to the emitter of the transistor having the emitter follower configuration, and the output signal from the limiter circuit is fed to the emitter of the transistor having the first emitter follower configuration via an inverter circuit and a second voltage-current conversion circuit. They are arranged so that they can be connected.

[0013]

[Operation] With this configuration, it is possible to realize a gain control circuit that can rapidly change the gain of the multiplication circuit with a change of ±4 VT or less in the gain control signal.

[0014]

【Example】

(Embodiment 1) An embodiment of the present invention will be described below with reference to the drawings.

As shown in FIG. 1, a limiter circuit 33 and a voltage-current conversion circuit 34 for extracting an arbitrary voltage range of the gain control signal are provided between the base of the transistor 6 and the emitter of the transistor 13. is exactly the same as the circuit explained in the conventional example, and its explanation will be omitted.

The operation of the gain control circuit configured as described above will be explained below. First, the gain control signal Vgc input to the input terminal 1 is applied to the base of the transistor 6 and the limiter circuit 33. The gain control signal applied to the base of the transistor 6 is output from the emitter, inputted to a differential amplifier composed of transistors 9 and 11, diodes 8 and 10, and a constant current source 12, and the current flowing through the diodes 8 and 10 is Vary the ratio. As a result, the gain G of the multiplication circuit made up of transistors 15, 16, 18, 19, 22, 23, constant current sources 17, 24, and resistors 20, 21 is expressed by (Equation 1) as in the conventional example.

On the other hand, the gain control signal Vgc input to the limiter circuit 33 is taken out as a signal in an arbitrarily determined voltage range, converted to Ia by the first voltage-current conversion circuit 34, and outputted to the transistor 13. emitter current I
Change E(13). The emitter voltage Ve(13) of the transistor 13 is given by the voltage of the constant voltage source 4 as V(4).

[0018]

[Math 2]

It is expressed as: Here, IE(13): Emitter current of transistor 13 IS(13): Saturation current of transistor 13 Emitter voltage Ve(6) of transistor 6 is given by the voltage of constant voltage source 4 as V(4).

[0020]

[Math 3]

It is expressed as: Here, IE(6): Emitter current I of transistor 6
S(6): Saturation current of transistor 6 (Equation 2) and (Equation 3)
), the emitter of transistor 6 and transistor 13
The emitter difference voltage Ve is given by the assumption that the saturation current of transistor 6 and the saturation current of transistor 13 are equal.

[0022]

[Math 4]

It is expressed as: Since the input D range of the differential amplifier that controls the gain is ±4 VT, when the gain control signal input to the input terminal changes from 0 to 100 mV, the difference voltage between the emitter of transistor 6 and the emitter of transistor 13 increases. By changing from -4VT to 4VT, the gain of the multiplication circuit can be changed from MIN gain to MAX gain.

Since the base-emitter voltage VBE (13) of the transistor 13 changes depending on the emitter current IE (13) of the transistor 13, when the input signal Vgc increases, the increase ΔVgc causes the emitter of the transistor 6 to IE(13) may be changed such that the amount of change ΔVe in the voltage difference between the emitters of IE(13) becomes 8VT or more. That is,

[0025]

[Math 5]

The first voltage-current conversion circuit 34
By selecting the output current Ia of , it is possible to change the gain of the multiplier circuit from MIN gain to MAX gain within the voltage range of the gain control signal of 100 mV or less.

FIG. 2 shows a specific example of the gain control circuit of the present invention. The transistors 36 and 38, the resistors 35 and 37, and the constant current source 39 form a limiter circuit 33.
The transistors 41 and 43 and the constant current source 42 constitute a voltage-current conversion circuit 34. The circuit is exactly the same as the conventional example except that the constant voltage source 40 is connected to the base of the transistor 38.

The operation of the concrete example shown in FIG. 2 constructed as above will be explained. First, the gain control signal Vgc input to the input terminal 1 is applied to the base of the transistor 6 and the base of the transistor 36. A gain control signal applied to the base of transistor 6 is output from the emitter.

On the other hand, the gain control signal Vgc applied to the base of the transistor 36 is taken out as a signal in an arbitrarily determined voltage range by the limiter circuit 33, and converted into Ia by the first voltage-current conversion circuit 34. The emitter current IE(13) of the transistor 13 is changed.

In aperture signal suppression at low brightness, when the brightness signal, which is a gain control signal, changes from 100 mV to 0 mV, the gain of the multiplier circuit is changed from MAX gain to MIN.
It is necessary to change even the gain, but as an example, the current value I7 of the constant current source 7 and the current value I14 of the constant current source 14 are I7=I14, and the potential of the constant voltage source 40 is the constant voltage source. When the potential of the constant current source 42 is 100 mV higher than the potential of 10 and the gain control signal is 0 mV, all the current of the constant current source 42 flows into the emitter of the transistor 13, and the gain control signal becomes 100 mV.
Assume that the resistors 35 and 37 and the constant current source 39 are set so that all the current of the constant current source 42 flows to the ground when the voltage is 0 mV (for example, constant current source 39 = 100 μA, resistor 35 =
24kΩ, resistance 37 = 20kΩ).

When the gain control signal is 100 mV, all the current of the constant current source 42 flows to the ground through the transistor 43, so the gain of the multiplication circuit is M
This is the AX gain.

At this time, the difference voltage Ve between the emitter of transistor 6 and the emitter of transistor 13 (100 mV)
From (Math. 4),

[0033]

[Math 6]

It is expressed as: When the gain control signal is 0 mV, all the current I42 of the constant current source 42 flows into the emitter of the transistor 13 through the transistor 41, so the difference voltage Ve (0 mV) between the emitter of the transistor 6 and the emitter of the transistor 13 is as follows.

[0035]

[Math 7]

It is expressed as: Here, the current value I14 of the constant current source 14 and the current value I42 of the constant current source 42 are

003
7]

[Math. 8]

[0038] If the relationship is expressed as (Example: I42=
99 μA, I14 = 100 μA), from (Equation 7),

0
039]

[Math. 9]

When the gain control signal is 0 mV, the base potential of transistor 11 is higher than the base potential of transistor 9 by 4 VT (approximately 100 mV), so the gain of the multiplication circuit can be set to MIN gain.

As a result, the gain control signal of 0 to 100 mV
By changing , the gain of the multiplier circuit can be changed from MIN gain to MAX gain.

(Embodiment 2) A second embodiment of the present invention will be described below with reference to the drawings.

As shown in FIG. 3, the limiter circuit 3
Embodiment 3 is the same as described in Embodiment 1, except that an inverter circuit 44 for inverting the output of Embodiment 3 and a second voltage-current conversion circuit 45 are connected between the limiter circuit 33 and the emitter of transistor 6. be.

The operation of the gain control circuit configured as described above will be explained below. First, the gain control signal Vgc input to the input terminal 1 is applied to the base of the transistor 6 and the limiter circuit 33. The gain control signal applied to the base of the transistor 6 is output from the emitter, inputted to a differential amplifier composed of transistors 9 and 11, diodes 8 and 10, and a constant current source 12, and the current flowing through the diodes 8 and 10 is Vary the ratio. As a result, the gain G of the multiplication circuit made up of transistors 15, 16, 18, 19, 22, 23, constant current sources 17, 24, and resistors 20, 21 is expressed by (Equation 1) as in the conventional example.

On the other hand, the gain control signal Vgc inputted to the limiter circuit 33 is taken out as a signal in an arbitrarily determined voltage range, and converted into Ia by the first voltage-current conversion circuit 34 to obtain the emitter current of the transistor 13. IE(13)
is changed to Ib by the inverter circuit 44 and the second voltage-current conversion circuit 45, and the transistor 6
The emitter current IE(6) is changed. The emitter voltage Ve(13) of the transistor 13 is given by the voltage of the constant voltage source 4 as V(4).

[0046]

[Math. 10]

It is expressed as: Here, IE (13): Emitter current of transistor 13 IS (13): Saturation current of transistor 13 Also, emitter voltage Ve (13) of transistor 6 is
Assuming the voltage of V(4),

[0048]

[Math. 11]

It is expressed as: Here, IE(6): Emitter current I of transistor 6
S(6): When the gain control signal input to the saturation current input terminal of transistor 6 changes from 0 to 20 mV, the differential voltage between the emitter of transistor 6 and the emitter of transistor 13 changes from 0 mV to 4 VT. , the gain of the multiplication circuit can be changed from MAX gain -6dB to MAX
This means that even the gain can be changed.

From (Equation 10) and (Equation 11), the difference voltage Ve between the emitter of transistor 6 and the emitter of transistor 13 is determined by the saturation current of transistor 6 and the emitter of transistor 13.
Assuming that the saturation currents of are equal,

[0051]

[Math. 12]

It is expressed as: Base of transistor 6
The emitter voltage VBE(6) changes depending on the emitter current IE(6) of the transistor 6, and
Since the base-emitter voltage VBE (13) of the transistor 13 changes depending on the emitter current IE (13) of the transistor 13, when the input signal Vgc increases, the increase ΔV
IE(6) and IE(13) may be changed so that the amount of change ΔVe in the voltage difference between the emitter of transistor 6 and the emitter of transistor 13 at gc becomes 4VT or more. That is,

[0053]

[Math. 13]

The first voltage-current conversion circuit 34
output current Ia of the second voltage-current conversion circuit and output current Ia of the second voltage-current conversion circuit
By selecting b, the gain of the multiplication circuit can be changed from MAX gain -6 dB to MAX by changing the gain control signal from 0 to 20 mV.
It becomes possible to change even the gain.

Next, FIG. 4 shows a specific circuit example of the gain control circuit of the second embodiment. transistor 4
7, 49, resistors 46, 48, and constant current source 50 constitute a limiter circuit 33, and transistors 51, 53 and constant current source 52 constitute an inverter circuit 44 and first and second voltage-current conversion circuits 34, 35. The process is the same as that described in the conventional example except that.

Next, the operation of the above specific example will be explained. First, the gain signal Vgc input to the input terminal 1 is applied to the base of the transistor 6 and the base of the transistor 47,
The gain control signal applied to the base of transistor 6 is
Output from the emitter.

On the other hand, the gain control signal Vgc input to the base of the transistor 47 is taken out as a signal in an arbitrarily determined voltage range by the limiter circuit 33, and converted into Ia by the first voltage-current conversion circuit 34. It is output, converted into Ib by an inverter circuit 44 and a second voltage-current conversion circuit 45, and output.

In chroma signal suppression at low brightness, when the brightness signal, which is the gain control signal, changes from 0 mV to 20 mV, the gain of the multiplier circuit is changed from MAX gain -6 dB to MA
The current value I7 of the constant current source 7 and the current value I14 of the constant current source 14 are I7=I14, and the transistors 47 and 49 and the resistor 4
6, 48 and the constant current source 50, the gain of the limiter circuit 33 is 5 times, the gain control signal is 0 m
When V, half of the current from the constant current source 52 flows through the transistor 53 to the emitter of the transistor 6, and the other half flows through the transistor 51 to the transistor 1.
3, the difference voltage Ve(0) between the emitter of transistor 6 and the emitter of transistor 13
mV) is from (Equation 12),

[0059]

[Math. 14]

Therefore, the gain of the multiplication circuit is M
It becomes AX-6dB. When the gain control signal is 20 mV, the gain of the limiter circuit 33 made up of transistors 47 and 49, resistors 46 and 48, and constant current source 50 is five times, so transistors 51 and 53 and constant current source 52
first and second voltage-current conversion circuits 34,
The input to the transistor 35 is 100 mV, and all the current I52 of the constant current source 52 flows into the emitter of the transistor 6 through the transistor 53, so the difference voltage Ve(2) between the emitter of the transistor 6 and the emitter of the transistor 13 is
0mV) is

[0061]

[Math. 15]

It is expressed as: Here, the current value I7 of the constant current source 7 and the current value I52 of the constant current source 52 are

[0063]

[Math. 16]

[0064] If the relationship is expressed as (eg: I52=
96μA, I7=100μA), from (Equation 7),

00
65]

[Math. 17]

When the gain control signal is 20 mV, the base potential of transistor 9 is higher than the base potential of transistor 11 by 4 VT (approximately 100 mV), so the gain of the multiplication circuit can be set to the MAX gain.

As a result, by changing the gain control signal from 0 to 20 mV, the gain of the multiplication circuit can be changed from MAX gain -6 dB to M
Even the AX gain can be changed.

[0068]

As is clear from the above embodiments, the present invention can rapidly change the gain of the multiplication circuit with a change of ±4 VT or less in the gain control signal. This makes it possible to realize a gain control circuit suitable for suppressing chroma signals during brightness.

[Brief explanation of drawings]

FIG. 1 is a block diagram showing the basic configuration of a gain control circuit of the present invention.

[FIG. 2] Circuit diagram of a specific example of the gain control circuit of the present invention

[Figure 3
]Block diagram showing the basic configuration of a second embodiment of the present invention

FIG. 4: Specific circuit diagram of the second embodiment of the present invention

[Figure 5
] Conventional circuit diagram

[Figure 6] Gain control characteristics of multiplication circuit

[Explanation of symbols]

3, 5 Resistor 4 Constant voltage source 6 Transistor (first emitter follower) 13
Transistor (second emitter follower) 33 Limiter circuit 34 First voltage-current conversion circuit 44 Inverter circuit 45 Second voltage-current conversion circuit

Claims (2)

[Claims] 1. A common constant voltage source is connected to the bases of each of first and second transistors having an emitter follower configuration via a resistor, and a gain control signal is supplied to the base of the transistor having the first emitter follower configuration. and enter the first
This circuit controls the gain of the main multiplier circuit by inputting the difference voltage between the emitters of the transistor and the second emitter-follower configuration transistor to a differential amplifier, and corresponds to an arbitrarily determined voltage range of the gain control signal. A gain control circuit arranged so that a portion of the current is taken out by a limiter circuit, converted into a current by a first voltage-current conversion circuit, and fed back to the emitter of the transistor having the second emitter follower configuration. 2. A common constant voltage source is connected to the bases of each of the first and second transistors having the emitter follower configuration via a resistor, and a gain control signal is supplied to the base of the transistor having the first emitter follower configuration. and enter the first
This circuit controls the gain of the main multiplier circuit by inputting the difference voltage between the emitters of the transistor and the second emitter-follower configuration transistor to a differential amplifier, and corresponds to an arbitrarily determined voltage range of the gain control signal. A circuit is provided for extracting the output signal from the limiter circuit, converting it into a current using a first voltage-current conversion circuit, and feeding it back to the emitter of the transistor having the second emitter-follower configuration. a gain control circuit arranged to be connected to the emitter of the transistor having the first emitter follower configuration via the circuit and a second voltage-current conversion circuit;