JPH0589592A - Level control circuit - Google Patents

Abstract

(57) [Summary] [Object] To provide an adjustment circuit having a simple circuit configuration suitable for integration into a circuit and capable of excellent level adjustment. In a level control circuit for controlling a direct current level of an input signal, a limiter amplifier 11 for changing the input signal level according to a control signal, and the limiter amplifier 11
And a differential amplifier 12 to which the output of (1) is supplied are provided, and a level-controlled signal is obtained from the differential amplifier 12.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a level control circuit suitable for application to video signal level control.

[0002]

2. Description of the Related Art Conventionally, a video tape recorder (VTR)
In the above, in order to adjust the recording current of the recorded video signal, the circuit shown in FIG. 3 was configured. That is, terminal 1
The recorded video signal obtained in 1 is supplied to the frequency modulation circuit 2, frequency modulation (FM) processing is performed, and the processed video signal (frequency signal) is supplied to the volume 3. Then, the video signal is taken out from the movable terminal of the volume 3 and the taken-out video signal is supplied to the recording amplifier 4. Then, the video signal amplified by the recording amplifier 4 is supplied to the magnetic head 5 via a rotary transformer (not shown), and the video signal is recorded on the video tape T. in this case,
By adjusting the movable terminal of the volume 3, the magnetic head 5
The amount of current of the video signal supplied to the side is adjusted, and the recording current can be adjusted.

Since such an adjusting circuit requires a volume, there is a problem that it becomes an obstacle when it is integrated into a circuit. Therefore, the configuration shown in FIG. 5 has been proposed as an adjusting circuit that can be converted into an electronic volume.

This circuit supplies the video signal processed by the frequency modulation circuit 2 to an amplitude modulation circuit 6, and the amplitude modulation circuit 6 performs amplitude modulation corresponding to a predetermined modulated signal to obtain an amplitude-modulated video. The signal is supplied to the recording amplifier 4 side. In this case, the modulated signal supplied to the amplitude modulation circuit 6 is the level control signal obtained by adding the recording current adjustment voltage obtained at the terminal 7 and the shake compensation signal obtained at the terminal 8 by the adder 9. It The shake compensation signal is a signal for preventing so-called overmodulation in which the level drops in the high frequency part of the frequency-modulated video signal (that is, the high frequency part of the original video signal).
It is a signal that raises the level in the high frequency part.

[0005]

However, the level adjusting circuit using such an amplitude modulator has a very complicated circuit structure and cannot be easily integrated into an integrated circuit.

In view of the above points, the present invention has an object to provide a level adjusting circuit of this type which can perform good level adjustment with a simple circuit configuration suitable for integration into an integrated circuit.

[0007]

According to the present invention, for example, as shown in FIG. 1, in a level control circuit for controlling a direct current level of an input signal, a limiter amplifier 11 for changing the input signal level according to a control signal, This limiter amplifier 1
A differential amplifier 12 to which the output of 1 is supplied is provided, and a signal whose level is controlled is obtained from the differential amplifier 12.

[0008]

With this configuration, even if the voltage value of the control signal changes, the DC level of the output signal does not change.

[0009]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS An embodiment of the present invention will be described below with reference to FIGS. 1 to 3, parts corresponding to those in FIGS. 4 and 5 are designated by the same reference numerals, and detailed description thereof will be omitted.

This example is applied to the recording circuit of the video tape recorder as in the conventional example, and the output of the frequency modulation circuit 2 is supplied to the limiter amplifier 11, and this limiter amplifier 11 changes the current amount of the control signal. The output level is changed accordingly. Then, the output of the limiter amplifier 11 is supplied to the differential amplifier 12, and the output of the differential amplifier 12 is supplied to the magnetic head 5 side via the recording amplifier 4. In this case, the recording current adjustment voltage obtained at the terminal 7 and the shake compensation signal obtained at the terminal 8 are added by the adder 9, and the level control signal as the addition signal is supplied to the voltage / current conversion circuit 13. The conversion output (current signal) of the voltage / current conversion circuit 13 is supplied as a control signal for the limiter amplifier 11.

Here, the limiter amplifier 11 and the differential amplifier
12, a circuit configuration example of the voltage / current conversion circuit 13 is shown in FIG.
The recording signal is supplied from the frequency modulation circuit 2.
The input terminal 11a of the amplifier 11 is connected to an NPN type transformer.
Dista Q₁This Transis together with connecting to the base of
Q₁The emitter of NPN transistor Q₂Emi
Connect to the computer. In addition, the power source end where the power source voltage Vcc can be obtained
Child 11b, resistor R ₁And R₁′ Through Transis
Q₁And Q₂Connect to the collector. And both tigers
Register Q₁, Q₂The emitter of NPN type
Star Q₃Connect to the collector. This transistor Q₃
The emitter is grounded and the voltage-current conversion circuit 1 is connected to the base.
3 outputs are provided. This transistor Q₁, Q₂，
Q₃The limiter amplifier 11 is configured by a circuit that faces the above.

The limiter amplifier 11 is configured as follows.
Langista Q₁And Q₂The collectors of NPN
Type transistor Q_FourAnd Q_FiveConnect to the base and power
Terminal 11b is both transistor Q_Four, Q_FiveContact the collector of
To continue. In addition, the transistor Q_FourAnd Q_FiveThe emitter of
, And a predetermined constant current source is connected to each. And the tiger
Register Q_FourAnd Q_FiveNPN type emitters
Transistor Q₆And Q₇Connect to the base of both tigers
Register Q₆, Q₇The emitter of the resistor R₂Connect with
It And both transistors Q₆And Q₇The emitter of
And a separate constant current source I₁And I₁’Connect.
In addition, the power supply terminal 11b is connected to the transistor Q ₆Collector of
And the transistor Q₇To the collector of
Resistor R₃Connect through. This transistor Q₆，
Q₇The differential amplifier 12 is composed of a circuit facing the above.

Then, the transistor Q₇The collector of
NPN type transistor Q₈Connect to the base of. Well
The power supply terminal 11b is connected to the transistor Q₈Collect
Connected to the transistor Q₈Predetermined constant current for the emitter of
Connect the source, and use the transistor Q ₈Differential from emitter
Pull out the output terminal 12a of the pump 12.

To explain the structure of the voltage / current conversion circuit 13, the input terminal 13a to which the level control signal from the adder 9 is supplied is connected to the PNP transistor Q.
₉ is connected to the base, the collector of this transistor Q ₉ is grounded, and the emitter is connected to PN via resistor R _4.
It is connected to the emitter of a P-type transistor Q ₁₀ . Then, the emitters of both transistors Q ₉ and Q ₁₀ are connected to one ends of separate constant current sources I ₂ and I ₂ ′, respectively, and the base of the transistor Q ₁₀ is connected to the power source of potential V ₀ . Further, the collector of the transistor Q _{10 is connected to} the collector of the NPN type transistor Q _{11 and} the base of the NPN type transistor Q ₁₂ , and the other ends of the constant current sources I ₂ and I ₂ ′ are
Connect to the collector of transistor Q ₁₂ . In addition, the emitter of the transistor Q ₁₁ is grounded, the base of the transistor Q ₁₁ and the emitter of the transistor Q ₁₂ are grounded via a predetermined resistor, and the base of the transistor Q ₃ on the limiter amplifier 11 side is connected.

The operation of the circuit thus constructed will be described. In this example, the limiter amplifier 11 shapes the output of the frequency modulation circuit 2 and adjusts the output level. Then, the variation of the DC component of the output of the limiter amplifier 11 is canceled by the operation amplifier 12, and the secondary distortion component due to the vertical asymmetry is also canceled at the same time. Next, this will be described using an equation.

First, the input terminal 11 of the limiter amplifier 11
When the signal obtained in a is v _i , the gain G ₁ of this limiter amplifier 11 is expressed by the following equation.

[0017]

[Equation 1]

Where v₁Is the transistor Q₁Collect
Potential, r₁Is a resistor R₁, R₁′ Resistance, I_CIs electric
Output current value of the pressure / current conversion circuit 13 (that is, transistor
Q₃Current value). Based on this formula
Waveform shaping is performed. That is, for example, the input shown in FIG.
One output of the limiter amplifier 11 when there is a force signal
Is the transistor Q₁Collector potential v₁In Figure 3
It changes as shown in B, and the other output of the limiter amplifier 11 changes.
Power transistor Q₂Collector potential v₂Is shown in FIG.
Changes as shown in C. In this case, both output levels v
₁And v₂Is the power supply voltage Vcc to r₁・ I_CChanges with the amplitude of
It is moving. This output level v₁And v ₂Is the next [number
2] and [Equation 3].

[0019]

[Equation 2]

[0020]

[Equation 3]

The maximum value of the output amplitude of both output levels v ₁ and v ₂ (that is, the limiter level) is given by the following [Equation 4].
It is shown by the formula.

[0022]

[Equation 4]

Therefore, by changing the output current I _C of the voltage / current conversion circuit 13, it becomes possible to adjust the output level of the limiter amplifier 11. The outputs v ₁ and v ₂ of the limiter amplifier 11 are the transistors Q ₄ and Q.
It is supplied to the differential amplifier 12 composed of the transistors Q ₆ and Q ₇ via ₅ , and the gain G ₂ of the differential amplifier 12 is represented by the following [Equation 5].

[0024]

[Equation 5]

In the differential amplifier 12, the outputs v ₁ and v _{2 of} the limiter amplifier 11 are at the potential (r ₁
.I _C ), which varies with the amplitude of (I _c ), as shown in C of FIG. 3, with the potential (r ₃ · i ₁ ) as the center (v ₁ −
v ₂ ) It is converted into a signal v ₃ with a fixed DC component that swings with an amplitude of G ₂ . Here, r ₃ is the resistance value of the resistor R ₃ , and i ₁ is the current value of the constant current sources I ₁ and I ₁ ′. Also, a whisker component x ₁ generated at the outputs v ₁ and v ₂ of the limiter amplifier 11
And x ₂ become vertically symmetrical beard components y ₁ and y _{2 at} the output v ₃ of the differential amplifier 12, and the secondary distortion component is improved.

On the other hand, the output current I _C of the voltage / current conversion circuit 13 is represented by the following equation (6), where V _c is the potential of the level control signal supplied to the input terminal 13a.

[0027]

[Equation 6]

Where r ₄ is the resistance value of the resistor R ₄ , i ₂
Is the current value of the constant current sources I ₂ and I ₂ ′. This output current I _C is supplied to the limiter amplifier 11 as it is because the current mirror circuit is composed of the transistors Q ₃ , Q ₁₁ , and Q ₁₂ , and the current source of the current I _C is connected to the limiter amplifier 11. Becomes Therefore, the output amplitude of the differential amplifier 12 is expressed by the following expression (7) based on the expressions (4) and (6).

[0029]

[Equation 7]

From this equation (7), it is understood that the output level can be changed based on the level control signal V _c .

Since the output of the differential amplifier 12 can be controlled in this manner, it is possible to control the DC component not to change even when the output level is changed, and the video signal recorded from the magnetic head 5 to the video tape T is obtained. The recording current is set to a good value. In this case, the circuit of this example can be realized with a simple configuration including a limiter amplifier and a differential amplifier, and since it is a circuit that does not require an external capacitor, it can be easily integrated into an integrated circuit. Further, even if the limiter amplifier has some poor characteristics, the secondary distortion is canceled on the side of the differential amplifier, so that good characteristics are secured. This also has the advantage of reducing the current for improving distortion of the limiter amplifier.

In the above embodiment, the recording current adjusting circuit of the video signal recording circuit of the video tape recorder is used, but it goes without saying that the present invention can be applied to various level control circuits. Further, it can be applied to a circuit that handles a pulse signal such as a PCM signal, in addition to a circuit that handles an analog signal as in the above-described embodiment.

[0033]

According to the present invention, it is possible to realize a level control circuit having a simple structure suitable for integration into an integrated circuit, in which the DC level of the output signal does not change even if the voltage value of the control signal changes.

[Brief description of drawings]

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

FIG. 2 is a circuit configuration diagram showing a circuit example of an embodiment.

FIG. 3 is a timing diagram for explaining one embodiment.

FIG. 4 is a configuration diagram showing an example of a conventional level control circuit.

FIG. 5 is a configuration diagram showing an example of a conventional level control circuit.

[Explanation of symbols]

 11 Limiter amplifier 12 Differential amplifier 13 Voltage / current conversion circuit

Claims (1)

[Claims] 1. A level control circuit for controlling a direct current level of an input signal, comprising: a limiter amplifier for changing the input signal level according to a control signal; and a differential amplifier to which an output of the limiter amplifier is supplied. A level control circuit adapted to obtain a level-controlled signal from the differential amplifier.