JPH0269010A - Time constant switching circuit - Google Patents

Abstract

PURPOSE:To reduce the influence of time constant switching to the picture and sound of a TV receiver by using the emitter follower circuits of an NPN and PNP transistors in the time constant switching circuit of the automatic gain control circuit of the TV receiver for receiving scrambled signals. CONSTITUTION:The emitter follower circuits of an NPN and PNP transistors 5 and 6 are provided between a terminal 4 for controlling the gain of an intermediate frequency amplifier circuit and the OFF terminal of a time constant changeover switch 3 so that the potential of capacitors for deciding time constant respectively provided at both terminals can be maintained at almost same levels. Even when a broadcasting mode is in a high-speed state and the switch 3 is in a turned-off state, the emitter potential of the PNP transistor 6 always becomes equivalent to the base potential of the NPN transistor 5 and an electrolytic capacitor 2 is maintained at the same potential as that of a ceramic capacitor 1 through the transistors 5 and 6. Even when the broadcasting mode is switched to a low-speed mode and the switch 3 is turned on, no transient disturbance takes place in the potential at the IF-AGC terminal, because the potential at the electrolytic capacitor 2 is maintained at almost the same level as that of the ceramic capacitor 1.

Description

DETAILED DESCRIPTION OF THE INVENTION (Industrial Application) The present invention relates to automatic gain control (AGC) of television (TV) receivers designed to receive scrambled signals.
) circuit's time constant switching circuit.

(Prior Art) The development of CATV, a community viewing television in North America, has been remarkable, and in order to reduce the deterioration of image quality, etc., the remote control channel selection function and other functions of TV receivers have been developed. In order to make full use of.

Many methods have been adopted in which scrambled signals are reproduced at baseband.

In most cases, this scramble signal has the synchronization signal part of the NTSC signal scrambled, so in TV receivers that operate AGC using the synchronization signal, the AGC signal is scrambled.
The GC time constant circuit will have great significance.

Hereinafter, a conventional time constant switching circuit will be explained with reference to FIG.

This is an intermediate frequency amplification circuit (hereinafter referred to as IF-AMP circuit) of the TV receiver due to the potential change of the IF-AGC terminal 4.
1 is a time constant determining ceramic capacitor that determines the follow-up speed of the IF-AMP circuit, and the time constant 1 is very small. 2 is a time constant determining electrolytic capacitor that determines the follow-up speed of the IF-AMP circuit like the ceramic capacitor 1, and the time constant is very large. 3 is a time constant changeover switch, When transmitting in (small) mode, a decoder (not shown) detects it and turns off the time constant changeover switch 3 by the time constant changeover signal S.
Increase the tracking speed of the C circuit.

Also, if the broadcasting station switches to low-speed (time constant = large) mode midway through, the decoder will similarly detect it, and the time constant selector switch 3
A switching signal S is issued to turn on (ON), and the tracking speed of the IF-AGC circuit of the TV receiver is delayed.

(Problem to be solved by the invention) However, in the time constant switching circuit as described above, when the time constant is switched from high speed to low speed, the ceramic capacitor 1
When the potential difference between the electrolytic capacitor 2 and
The potential of terminal 4 is disturbed.

The AGC circuit malfunctions, causing abnormalities in the video and audio of the TV receiver until the potential at the IF-AGC terminal 4 stabilizes.

SUMMARY OF THE INVENTION In view of the above problems, it is an object of the present invention to provide a time constant switching circuit that quickly stabilizes even when changing the time constant.

(Means for Solving the Problem) In order to solve this problem, the time constant switching circuit of the present invention combines emitter follower circuits of NPN and PNP transistors, and connects an electrolytic capacitor to a low impedance state without disturbing the DC potential. It is now possible to charge and discharge.

(Function) With this configuration, the time constant is fast (time constant changeover switch 3
Even when the electrolytic capacitor 2 is in an off state), the potential of the electrolytic capacitor 2 is always maintained at approximately the same potential as that of the ceramic capacitor 1 by the emic follower transistor circuit.

(Example) An example of the present invention will be described below with reference to the drawings.

FIG. 1 shows a time constant switching circuit in one embodiment of the present invention.

In Figure 1, 5 is an NPN transistor and 6 is a PN transistor.
P transistors 7 and 8 are emitter resistors, respectively, and constitute a two-stage emitter follower circuit.

Now, even if the broadcast mode is in the high speed state and the time constant changeover switch 3 is in the OFF state, the emitter potential of the PNP transistor 6 is always equal to the base potential of the NPN transistor 5, and the electrolytic capacitor 2 is in the NPN state. It is maintained at the same potential as the ceramic capacitor 1 via the transistor 5 and the PNP transistor 6.

Next, the broadcast mode switches to low speed, and the time constant changeover switch 3
Even if it is turned ON by the time constant switching signal S, the potential of the electrolytic capacitor 2 is maintained at almost the same potential as the ceramic capacitor 1, so the potential of the IF-AGC terminal 4 is not transiently disturbed. Abnormalities will no longer appear on the screen or audio of the TV receiver.

In the above circuit configuration, by selecting the constants of the emitter resistors 7 and 8 to appropriate values, it is possible to prevent the potential of the IF-AGC terminal 4 from being disturbed, thereby making it possible to eliminate the need for the ceramic capacitor 1. .

Although this embodiment has been described by taking the time constant switching circuit of a TV receiver as an example, it can be applied to any device that uses one time constant for switching.

(Effects of the Invention) As explained above, the present invention uses emitter follower circuits of NPN and PNP transistors to prevent transient abnormal phenomena during time constant switching. can significantly reduce the impact of

[Brief explanation of the drawing]

FIG. 1 is a circuit diagram of a time constant switching circuit according to an embodiment of the present invention, and FIG. 2 is a circuit diagram of a conventional time constant switching circuit. 1... Time constant determination ceramic capacitor (time constant =
small), 2... Time constant determining electrolytic capacitor (time constant =
small), 3... time constant changeover switch, 4...
IF-AGC terminal, NPN transistor, NP transistor, 7° emitter resistor.

Claims (1)

[Claims] In the time constant switching circuit in the automatic gain control circuit of a television receiver that is capable of receiving scrambled signals, an NPN terminal is connected between the terminal that controls the gain of the intermediate frequency amplifier circuit and the off terminal of the time constant switching switch. An emitter follower circuit of a PNP transistor is provided to maintain the potential of each capacitor for determining a time constant provided at both terminals at approximately the same potential, and to control the gain of the intermediate frequency amplifier circuit when the time constant selector switch is switched to the on terminal. A time constant switching circuit characterized in that it does not disturb the potential on the terminal side that controls the time constant.