JPH0251318B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION Field of Industrial Application The present invention relates to a white level adjustment circuit for video signals.

FIG. 1 shows a conventional white level adjustment circuit. This will be explained below with reference to the figures. The video signal detected by the detection circuit 2 after passing through the intermediate frequency amplification circuit 1 is applied to the video amplification circuit, but since there is always variation in the amplification factor of the detected video signal and the video amplification circuit, the white level will be different for each. Therefore, white level adjustment is required to adjust these. Conventionally, as shown in FIG. A system in which a voltage source 6 is added to adjust the DC level of the video amplification circuit 3 has been 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 and is also applied to the intermediate frequency amplification gain adjustment (IFAGC) circuit 8 at the same time. At this time, the operating function of the circuit is to compare the peak value of the video signal output and the predetermined voltage Vref of the reference voltage source 9 of the IFAGC circuit 8, and detect the difference, in the case of the peak AGC method. through the above intermediate frequency amplification circuit 1
Feedback control is applied to However, in this method, in order to prevent the reference voltage Vref from changing when adjusting the white level, the power supply 9 for the IFAGC amplifier circuit is normally provided separately from the fixed voltage power supply 6 of the voltage Vcc of the video amplifier circuit 3. Therefore, a fixed voltage source 11 is required. In particular, video amplification circuits and
When integrating the IFAGC circuit into an IC on the same chip, if separate fixed voltage sources are required, the power supply circuit will be complicated and separate terminals will be required for each, which is extremely inconvenient.

The present invention overcomes these difficulties and will be described in detail below with reference to an embodiment shown in FIG. In Fig. 2, the video intermediate frequency signal modulated and amplified at the intermediate frequency is transmitted through the transistor T ₁ and the resistor R ₁ .
Transistor through emitter follower
Added to the base of T ₂ and T ₃ .

At this time, the video intermediate frequency signal is usually several tens of MHz.
Therefore, the input signal 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 and demodulated by a detection circuit consisting of a transistor T ₂ , a constant current source ₁₂ , and a capacitor C ₃ , and is applied to the base of a transistor T ₄ .
Transistor through emitter follower
Added to T ₆ base. 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 an emitter follower consisting of a transistor _T5 and a resistor _R7 . At this time, when R ₈ = R ₉ = R and the current of the variable current source 14 is Io, when R>KT/qIo, the gain of the differential amplifier circuit using transistors T ₆ and T ₇ is R ₁₀ /2R. It is well known that the gain of this differential amplifier circuit is not affected much by Io. Further, the current Io of the variable current source 14 is Vcc-1/2・IoR ₁₀
gives the collector DC potential of transistor _T7 with the relationship, and this directly applies to the transistor
It is added to the base of _T8 to determine the white level of the video signal. The video signal is transmitted through the differential amplifier circuit described above, as well as transistor _T8 and resistor _11.
The signal is guided to the output terminal 7 through an output circuit consisting of the following.

Therefore, with this circuit configuration, the white level can be adjusted by changing the current Io of the variable current source 14 without affecting the gain of the video circuit.

On the other hand, the peak value of the video signal at output terminal 7 is
It is compared with the reference voltage Vref obtained by the reference voltage circuit 9 of the IFAGC circuit 8, but since this reference voltage Vref can be determined independently of the current Io of the variable current source 14 described above, even if Io is changed, Vref is It does not change. Therefore, in this example circuit, unlike the conventional circuit shown in FIG. 1, there is no need to provide two fixed voltage sources.
Only one power source 6 is sufficient.

[Brief explanation of the drawing]

FIG. 1 is a conventional white level adjustment circuit diagram, and FIG. 2 is a white level adjustment circuit according to an embodiment of the present invention. 1... Video signal intermediate frequency amplification circuit, 2... Detection circuit, 3... Video amplification circuit, 4, 7... External terminal, 5... Variable resistor, 6... Power supply, 8... Intermediate frequency amplification gain adjustment Circuit, 9...Reference power supply circuit, 10
...Filter, T ₁ to T ₈ ...Transistor, R ₁
~R ₁₁ ...Resistance.

Claims (1)

[Claims] 1 The emitters of a first transistor to which a video intermediate frequency detection signal is applied to the base, and a second transistor to which the average voltage of the video intermediate frequency output is applied to the base and whose collector is connected to the power supply via the first resistor. a differential amplifier configured by connecting a second and a third resistor in series through a series connection point of the resistors, and further connecting a variable current source to the series connection point of the resistors;
The base is connected to the collector of the transistor,
White level adjustment comprising an emitter follower transistor having an output terminal attached to its emitter, the emitter of the emitter follower transistor being coupled to a signal input terminal to be compared with a reference voltage of an intermediate frequency amplification gain adjustment circuit. circuit.