JPS6059879A - Video amplifier circuit - Google Patents

Abstract

PURPOSE:To prevent a screen from being made black by connecting a resistor between bases of both transistors (TRs) of a differential amplifier and setting the bases to almost the same potential in terms of DC in a video amplifier circuit of a color television receiver. CONSTITUTION:In setting a channel to an idle channel, no burst gate pulse appears at a terminal 13 and a base potential of a TR5 tends to fall down. Since a resistor 23, however, is connected between both bases of TRs 4, 5, the bases are brought into almost the same potential and the collector potential of the TR5 does not reach a voltage VCC applied to a DC power supply terminal 15. Thus, the differential amplifier 3 acts like differential operation and a potential appears at the base of the TR4 accordingly in response to a signal appearing via a contrast adjusting circuit 1.

Description

Detailed Description of the Invention (a) Industrial Application Field The present invention relates to a video amplification circuit for a television receiver, and in particular to a video amplification circuit with a novel configuration that eliminates the inconvenience of the television screen when it is set to an empty channel. Regarding.

(b) Prior Art Generally, as shown in Figure 1, the video amplification circuit of a color television receiver has an emitter follower transistor (2) at the output end of a contrast adjustment circuit (1), and a pair of differential amplifiers (y). A transistor (41 (5), a DC regeneration and brightness adjustment circuit (2) are connected in sequence, and a contrast adjustment circuit (Ll) is connected to a variable resistor (Ll) for contrast adjustment.
On the other hand, a DC regeneration and brightness adjustment circuit (a variable resistor (8) for brightness adjustment is connected to each side).

Terminal (9) is an input terminal to which a composite video signal is applied, 00
) is a coupling capacitor, 01) is a constant current circuit, 02 is a capacitor for pedestal level hold, (13) Hahurst gate pulse application terminal, (14) is a video output terminal, (19 is a DC power supply terminal, (16) is Earth terminal, 0η
(+81 indicates the emitter 7 oror transistor.

Next, to explain the operation shown in Figure 1, the above input terminal (
When a composite video signal is applied to 9), a burst gate pulse (hereinafter referred to as BGP) for extracting a color burst signal inserted in the back porch period of the video signal.
Gating the DC reproduction and brightness adjustment circuit (phrase), the vedestal level of the video signal is connected to the variable resistor (for brightness adjustment).
8) to match the predetermined voltage adjusted in step 8). During the scanning period, the same pedestal level as during the pulse period is maintained by the function of the capacitor a2). At this time, when the channel is empty, the BGP is not applied to the terminal α3), so the base potential of the transistor (5) drops and becomes close to the ground potential, and the collector potential of the transistor (5) reaches the power supply voltage vcc. As a result, the differential amplifier (y) does not operate as a differential amplifier, and the emitter current of the emitter follow-connected transistor (17) increases, and the base potential of the next stage transistor (18) also rises. Since the potential at terminal a4 increases, the pedestal level becomes high and the screen of the television receiver approaches pitch black.

Therefore, even if you change the setting position of the variable resistor (8) for brightness adjustment, the screen will not be bright (there is a drawback that the user may think that the TV receiver is malfunctioning). .

Therefore, in order to eliminate the above drawback, a horizontal blanking pulse (blanking pulse) is used outside the integrated circuit (IC) shown by the broken line in FIG. 1 to generate a pulse in place of the BGP, The DC regeneration and brightness adjustment circuit 16) was added to the DC regeneration and brightness adjustment circuit 16), but this method requires complicated circuit configuration and KI
There is a drawback that the advantages of C conversion cannot be fully utilized.

(c) Purpose of the Invention The present invention prevents the screen from becoming completely black due to the non-operation of the DC regeneration and brightness adjustment circuits due to the lack of burst gate pulses when the channel is empty, and maintains a predetermined brightness level. The purpose is to keep the screen as clear as possible.

(b) Structure of the Invention The present invention has a contrast adjustment circuit, a DC regeneration circuit, and a brightness adjustment circuit connected in sequence, and a differential amplifier connected to the output end of the contrast adjustment circuit and the input end of the DC regeneration circuit. , a pedestal level hold capacitor connected between the base of one of the transistors of the differential amplifier and ground, and a resistor connected between the bases of both transistors constituting the differential amplifier to connect the base to the DC current. They are set to approximately the same potential.

(E) Example The present invention will be explained according to the drawings. Fig. 2 shows a video amplification circuit of the invention, and the same elements as in Fig. 1 are given the same figure numbers. is the brightness adjustment circuit, c2H is the emitter resistor, and e is the resistor connected to both bases of each transistor (4) and (5) of the differential amplifier.
e (a) indicates a reference voltage application terminal.

Next, to explain the operation in Figure 2, the input terminal (9)
Adding a composite video signal to the contrast adjustment circuit (1)
The desired contrast is set using a variable resistor (7) connected to the . The output terminal of the contrast adjustment circuit (1) is connected to a differential amplifier ■ via an emitter follower transistor (2), and during normal reception, B is connected to terminal Q31.
Since GP is applied, in synchronization with this, the DC regeneration circuit U
is gated to match the pedestal level of the video signal to the reference voltage. This reference voltage is set so that the pedestal level becomes the blackest level when the brightness adjustment circuit (weight) is off.

Next, in the brightness adjustment circuit (Inc.), the pedestal level is made to match the voltage adjusted by the variable resistor (8) for brightness adjustment, so that a screen with optimum brightness is obtained.

A predetermined video signal (Yout) is obtained from the output terminal α4, and if a video output transistor (not shown) is connected to the terminal α4, the video output is taken out from the video output transistor.

If the channel is now set as an empty channel, BGP will not appear on the terminal, and the base potential of the two terminals connected to the capacitor (121, that is, the transistor (5)) will try to fall.

However, since the resistor (23) is connected between the bases of the transistors (4) and (5), the bases are at approximately the same potential, and the collector potential of the transistor (5) is equal to the DC power supply terminal as in the conventional case. (The voltage Vcc applied to 15+ is never reached, so the differential amplifier (3) performs a differential operation, and accordingly the transistor (4) responds to the signal appearing via the contrast adjustment circuit (1). A potential appears at that pace.

Here, when BGP appears on the terminal α group, the collector voltage of the transistor (5) when the pedestal level matches the reference voltage (Vref) is V. In this case, if the ratio of the values R1 and R2 of the resistors (21) (23) is selected to a predetermined value, the collector potential of the transistor (5) can be set to .

(F) Effects of the Invention According to the one-parameter amplifier circuit of the present invention, when an empty channel is set as in the conventional case, the base potential of one transistor of the differential amplifier decreases due to the fact that BGP does not appear. This invention can prevent the screen of a TV receiver from becoming completely black, and if the variable resistor and the capacitor for maintaining the pedestal level in the embodiment are implemented as an IC, there is no need to provide any external elements. It also greatly contributed to the use of ICs for video amplification circuits in color television receivers.

[Brief explanation of the drawing]

FIG. 1 shows a conventional video amplification circuit, and FIG. 2 shows the same circuit according to the present invention. Explanation of main drawing numbers

Claims (1)

[Claims] (1) A differential amplifier having a contrast adjustment circuit, a DC reproduction circuit, and a brightness adjustment circuit connected in sequence, and connected to the output terminal of the contrast adjustment circuit and the input terminal of the DC reproduction circuit, and one of the differential amplifiers. A pedestal level hold capacitor and a capacitor are connected between the transistor pace and the ground, and a resistor is connected between the transistor paces of both transistors constituting the differential amplifier, so that the paces are at approximately the same potential in terms of direct current. A video amplification circuit characterized by configurable settings.