JPS5897979A - White level adjustment circuit - Google Patents

Abstract

PURPOSE:To control the output level of a differential amplifier and to prevent an adjustment of a white level from exerting an influence upon the gain of a video circuit by composing a detecting and amplifying circuit connecting to a video signal intermediate frequency circuit and a circuit for amplifying a DC signal component and a detection signal and a constant current source. CONSTITUTION:A video intermediate frequency signal from a video signal intermediate frequency amplifying circuit 1 is applied to bases of transistors (TRs) T2 and T3 through the emitter follower composed of a TR T1 and a resistance R1. At this time, the signal applied to the TR T3 is smoothed by the filter of a resistance R2 and a capacitance C2 into a DC signal, and the signal applied to the TR T2 is made into an intermediate frequency signal. The TR T2, a constant current source 12, and a capacitor C3 constitute a detecting circuit, whose detection signal is applied to a TR T4. The DC signal applied to the TR T3 is applied to the base of a TR T7 through an emitter follower TR T5. Then, DC levels of outputs of the TRs T6 and T7 constituting a differential amplifier are controlled to prevent an adjustment of a white level from exerting an influence upon the gain of an intermediate frequency amplification gain adjusting circuit 8.

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a white level adjustment circuit for a video signal.

FIG. 1 shows a conventional white level adjustment circuit. The explanation will be given below according to the figures. The video signal detected by the detection circuit 2 passes through the intermediate frequency amplification circuit 1 and is applied to the video amplification circuit, but since there are always variations in the detected video signal and the amplification factor of the video amplification circuit, Each level is different. Therefore, white level adjustment is required to adjust these. Conventionally, as a method for such white level adjustment, as shown in FIG. 6 to adjust the DC level of the video amplification circuit 3 was widely used. This is because when the AM modulated intermediate frequency signal has negative polarity, the white level is almost the same as the DC level of the video amplification circuit. However, on the other hand, the signal output of the video amplification circuit 3 is sent to the drive circuit via the video output terminal 7 at the same time as the intermediate frequency amplification gain adjustment (IFAGC) circuit 8.
It can also be added to At this time, the operating function of the circuit is the peak value of the video signal output and the above I in the case of the peak AGC method.
Predetermined voltage Vr e f of the reference voltage source 9 of the FAGC circuit 8
The difference is detected and feedback control is applied to the intermediate frequency amplification circuit 1 through the filter circuit 10.

However, with this method, when adjusting the white level, the reference voltage V
In order to prevent the ref from changing, the above IFA is usually
The power supply 9 for the GC amplifier circuit needs to be a separate power supply from the power supply 6 of the video amplifier circuit 3, and a fixed voltage source 11 is required. In particular, when integrating a video amplification circuit and an IFAGC circuit into the same chip, separate fixed voltage sources are required, which adds to the complexity of the power supply circuit and requires separate terminals for each, which is extremely time consuming. It's inconvenient.

The present invention overcomes such difficulties.
The invention will be explained in detail by means of embodiments shown in the figures. In FIG. 2, an image intermediate frequency signal modulated at an intermediate frequency and amplified is applied to the base of a transistor T2T3 through an emitter 7 lower comprising a transistor T1 and a resistor R1.

At this time, the signal sent to the base of the transistor T3 becomes a DC signal smoothed by a filter composed of a resistor R2 and a parasitic capacitor C2. On the other hand, the video intermediate frequency signal is directly applied to the base of the transistor T2 by a high-pass filter consisting of a resistor R3 and a capacitor C1. The signal is detected by a detection circuit consisting of a transistor T2, a constant current source 12, and a capacitor C3 (applied to the base of a transistor T4, and the detected signal is applied to the base of a transistor T6 through an emitter follower consisting of a transistor T4 and a resistor R6. On the other hand, the smoothed DC signal on the side of the transistor T3 is applied to the base of the transistor T7 through the emitter and the lower of the transistor T6 and the resistor R7. The gain of the amplifier circuit is not affected much.
gives the collector DC potential of the transistor T7, which determines the white level of the video signal. Then, the video signal is transmitted to the differential amplifier circuit and the transistor T.
8 and a resistor R11, the output terminal 7
guided by.

Therefore, according to this circuit configuration, adjustment of the white level does not affect the gain of the video circuit.

-The peak value of the PIJJ signal at power output terminal 7 is IFAGC.
It is compared with the reference voltage Vref obtained by the reference voltage circuit 9 of the circuit 8, but since this reference voltage Vref can be determined independently of the variable current source 0 described above, the
Even if Vref is changed, Vref does not change. Therefore, in the circuit of this embodiment, only one power supply 6 is sufficient without providing two fixed voltage sources as in the conventional circuit shown in FIG.

[Brief explanation of the drawing]

FIG. 1 is a conventional white level adjustment circuit diagram, and FIG. 2 is a white level adjustment circuit diagram according to an embodiment of the present invention. 1...Video signal intermediate frequency amplification circuit, 2...
1 detection circuit, 3... video amplification circuit, 4, 7...
...External terminal, 6...Mountain variable resistor, 6...
・Power supply, 8...Intermediate frequency amplification gain adjustment circuit, 9
...Reference power supply circuit, 10...Filter, T1-T8...Transistor, R1-R1
11,””Resistance.

Claims (1)

[Claims] A video detection signal is input to the first transistor of the differential amplifier circuit consisting of the first and second transistors, and a DC level signal obtained by smoothing the video intermediate frequency modulation signal is used to control the current of the constant current circuit. 2. A white level adjustment circuit, characterized in that the white level adjustment circuit controls the DC level of the output from the differential amplifier circuit by inputting the signal level of the identification current circuit to the second transistor.