JPS59126382A - Switching circuit - Google Patents

Abstract

PURPOSE:To block discharge of a capacitor for a prescribed time when a power supply is interrupted by providing an impedance means between a capacitor of a switching circuit and ground and blocking a forward bias from being applied to a transistor(TR) when the power supply is turned off to prevent a remarkable voltage drop of a connecting point of the switching circuit when the TR is turned on. CONSTITUTION:Since a capacitor C1 is charged via a resistor R5 and a diode D1 even at the receiving of a PAL signal where a TRQ1 is turned off, even if the circuit is switched to the receiving of an SECAM signal from this state, the value of the voltage drop at a connecting point (a) is decreased when the TRQ1 is turned on. When the power supply is turned on at the SECAM receiving state where a capacitor C5 is charged where the TRQ1 is turned on, a base potential of the TRQ1 becomes -E1 and a collector potential V2 becomes -E2 by the discharge of capacitors C5, C1, where E1 is the switching voltage at an input terminal (b) and E2 is the voltage at the connecting point (a), and the potential V1 after (t) seconds, becomes V1=-E1exp(-t/(R1C5). When the potential V1 is lower than the V2, the base and collector of the TRQ1 is biased reversely, allowing to block the discharge of the capacitor C1.

Description

Detailed Description of the Invention (a) Industrial Application Field The present invention relates to an automatic gain control circuit for a television receiver capable of receiving both positive modulation television signals (SECAM signals) and negative modulation television signals (PAL signals). (hereinafter abbreviated as AGC circuit), etc. (b) Prior art A television receiver capable of receiving both positive modulation SECAM signals and negative modulation PAL signals, which is equipped with a conventional switching circuit as shown in Figure 1. An example of an AGC circuit is shown below.

In FIG. 1, (1) is an AGC signal detection circuit that forms an AGC signal from a video signal amplification circuit (not shown);
) is a capacitor (
C1) a switching circuit that switches between an active state and a non-active state;
(3) is resistance (R2) (R3') % capacitor (C
2) Consisting of (C5) (C4) and coil (Ll) A
A smoothing circuit smoothes the output signal of the GC signal detection circuit (1), and (4) amplifies the C signal and sends it to a video intermediate frequency amplification circuit (not shown) and a high frequency amplification circuit (not shown).
The AGC signal amplification circuit that applies the signal, (5) is a voltage generation circuit consisting of the AGC signal detection circuit (1) and the smoothing circuit (31A G C signal amplification circuit (4); in this case, the voltage generated by the voltage generation circuit (5) The voltage applied is the AGC voltage. (
al indicates a connection point between the smoothing circuit (3) and the capacitor (C1).

(ICI) is an integrated circuit for video signal processing (for example, M51356P manufactured by Mitsubishi Electric Corporation), and ■■ is a terminal pin number of the integrated circuit (■C1). In FIG. 1, the value of the capacitor (C1) is 100 μF.

In the AGC circuit as shown in Fig. 1, negative modulation FAI-
When receiving a signal, a peak value type AGC operation is performed in which the transistor (Ql) is turned off, the peak value (synchronizing signal portion) of the video signal is detected, and the AGC signal is kept constant. However, since the positively modulated SECAM signal has a peak value or a luminance signal portion, when peak value type AGC operation is performed, luminance unevenness occurs depending on the content of the screen. Therefore, positive modulation SECAM
Signal reception Apply a switching voltage to the signal receiver terminal (bl) to turn on the transistor (Ql) and turn on the capacitor (C1
) is connected to the smoothing circuit (3) to increase the time constant of the smoothing circuit (3), perform average value type AGC operation, and perform positive modulation SEC.
This prevents AM signal reception signal failure from occurring.

However, in such a switching circuit (2), when the transistor (Ql) is turned on, the potential at the connection point (al) once drops to O■, rises as the capacitor (C1) is charged, and returns to its original state. During this time, the output voltage of the AGC circuit becomes low, so the amplification degree of the high frequency amplifier circuit (not shown) and the video intermediate frequency amplifier circuit (not shown) increases, and the video signal is amplified more than necessary. Therefore, the picture tube has the disadvantage that it becomes a blank state, which impairs the quality of the television receiver.Also, this symptom is caused by the transistor (Ql)
) is on, it also appears when the television receiver is turned off and then turned on again. This is due to the charge on the capacitor (C1) during the power off period.As shown in Figure 4, the AGC signal amplification circuit (4) and the switched voltage source impedance (R+), resistor (R1), transistor (Ql)
Because it is quickly discharged through the base-collector PN junction of the capacitor (C1
) is circulated again.

If the time between turning off and turning on the television receiver is long, the picture tube heater may have cooled down while the power was off, or the capacitor (C1) may have warmed up again before the image is displayed. Since the battery is being charged, the symptoms described above will not appear.

(c) Purpose of the Invention Book 1/y+ll was made in view of the above points, and it shows that when the transistor of the switching circuit is turned on from off, the potential at the connection point between the switching circuit and the voltage generating circuit is significantly increased. To provide a switching circuit that can prevent the voltage from dropping and prevent the capacitor from discharging for a certain period of time when the power is turned off to turn off the transistor and the voltage generating circuit.

2) Arrangement of the Invention The present invention provides a method for connecting a switching circuit capacitor in parallel with a switching circuit transistor between a switching circuit capacitor and ground to charge the capacitor when the transistor is off, or to charge the capacitor at a voltage connected to the switching circuit. impedance means that substantially do not affect the generating circuit;
A special feature characterized in that a blocking means is provided for blocking forward bias from being applied through a PN junction between the base, emitter, and ivy of the transistor when the power source is turned off to turn off the transistor and the voltage generating circuit. Switching circuit (Hata) Embodiment FIG. 2 shows an embodiment of an AGC circuit for a television receiver equipped with the switching circuit of the present invention. Note that parts corresponding to those in FIG. 1 are denoted by the same reference numerals, and redundant explanation will be omitted. In Figure 2, (R1) (Cs) is a transistor (Ql)
is turned off, the base of transistor 9 (Ql)
A capacitor (C
The resistor and capacitor (D2) that constitute the means for preventing the discharge of 1) are used to prevent the transistor (Ql) from being destroyed if a voltage higher than the reverse withstanding voltage is applied between the base, emitter, and ivy of the transistor (Ql). A diode to prevent (R
s) is a resistor to charge the capacitor (C1) even when the transistor (Ql) is off, and (Dl) is a diode to prevent discharging through the capacitor (C1) or resistor (R5). . This diode (Dl) and resistor (R5) constitute impedance means. Furthermore, the second
In the figure, the resistance (ILl) (5) is 39Ω, and the capacitor (C5) is 5.6Ω and 22μF.

%2 In the circuit shown in the figure, even when receiving a PAL signal with the transistor (Ql) off, the capacitor (C1) is connected to the resistor (R5).
) and the diode (Dl), so when switching from this state to SECAM signal reception and the transistor (Ql) turns on, the width of the voltage drop at the connection point (al) can be reduced. This prevents the screen from turning white temporarily.

Also, the switching voltage applied to the input terminal (b) is set to El(V
) If the voltage at the connection point (al is R2 (Vl), then in the SECAM receiving state where the transistor (Ql) is on and the capacitor (C5) is charged, when the power of the television receiver is turned off, the capacitor (Cs )(C1
), the base potential v1 of the transistor (Ql)
is -E1 (V), and the collector potential V2 is -E2 (V). The base potential v1 after [seconds is V1=-Eexp
(It, C4) ---- (Represented by equation 11. If the base potential v1 is lower than the collector potential ■2, the base and collector of the transistor (Ql) will be reverse biased, and the discharge of the capacitor (C1) will be inhibited. It can be prevented.In other words, V1=-E+exp (-E, c4) <-R2
−・−・−・−(While formula 21 is obtained, the capacitor (C1
) can be prevented from discharging.

However, in a circuit like the one shown in Figure 2, if the switching voltage applied to the input terminal (bl) is also used to switch other circuits of the television receiver, the time required to prevent the discharge of the capacitor (C1) is If you increase the value of the capacitor (C5) to increase the length of time, the time it takes for the capacitor (Cs) to be charged will become longer, the rise of the switching voltage applied to the input terminal (bl) will be delayed, and the switching of other circuits will be delayed. In addition, in order to prolong the discharge blocking time, the resistor (R1
) increases the voltage applied to the base of the transistor (Ql), which makes it difficult for the transistor (ql) to be completely turned on.

FIG. 6 shows another embodiment of the present invention made in view of the above. Note that parts corresponding to those in FIGS. 1 and 2 are designated by the same reference numerals, and redundant explanation will be omitted.

In Figure 3, (R1') (Cs') is a capacitor (
A resistor and a capacitor are used to prevent the discharge of C1), and (Q2) is a Darlington-connected transistor to the transistor (Ql). In addition, in Fig. 3, the capacitor (C5
) is 10 μF, and the resistors (R1) (It6) are each 2
20Ω and 4.7Ω.

As shown in FIG. 3, the transistor (Q1) can be completely turned on even if the value of the resistor (R1) is made very large by connecting the transistor (Q2) with the transistor (Q1) in Darlington connection. Furthermore, since the value of the resistor (R1) can be made large, the value of the capacitor (C5) can be kept small. Therefore, the switching voltage rises quickly (and the influence on other switching circuits can be reduced).Also, by appropriately selecting the switching voltage applied to the input terminal (b), the transistor shown in Fig. 2 The reverse protection diode (1) 2 ) at the base of (Ql) can be omitted.

(F) Effects of the Invention According to the present invention, the potential of the voltage supply terminal of the switching circuit is
When the transistor of the switching circuit is turned on, a large voltage drop can be prevented, and when the switching circuit and the voltage generating circuit are turned off, the discharge of the capacitor can be prevented for a certain period of time. Furthermore, if the present invention is implemented in the switching circuit of the AGC circuit of a television receiver as in the above embodiment (which can receive both PAL signals and SECAM signals), the picture tube will be in a bright white state when the transistor is turned on. It is also useful because it can prevent the picture tube from becoming blank even when the power of the television receiver is turned off and then turned on again.

[Brief explanation of the drawing]

FIG. 1 shows an embodiment of an AGC circuit including a conventional switching circuit, and FIGS. 2 and 3 show an embodiment of an AGC circuit including a switching circuit according to the present invention.
An example of a GC circuit is shown. FIG. 4 is a drawing for explaining FIG. 1. (2)...Switching circuit, (5)...Voltage generation circuit,
(C1) (Cs) (C5)... Capacitor, (Dl
)...Diode, (R1) (R1) (1'-5)
...Resistor, (Ql) (Q2)...Transistor.

Claims (1)

[Claims] (1) A voltage generating circuit that generates a voltage is connected to a reference potential point through a series connection between a capacitor and the second and third electrodes of the transistor, and a switching voltage is applied to the first electrode of the transistor. In a switching circuit configured to selectively set the transistor to one of two states, ie, on and off, the switching circuit is connected in parallel to the transistor between the capacitor and a reference potential point, and is connected between the capacitor and a reference potential point to indicate whether the transistor is in the off state or not. an impedance means for charging the capacitor or causing the capacitor to have no substantial effect on the voltage generation circuit; A switching circuit comprising a blocking means for blocking forward bias from being applied through the PN junction between the second electrodes. (2) The switching circuit according to claim 1, wherein the impedance means comprises a resistor and a diode. (31U) The switching circuit according to claim 1, characterized in that the blocking means comprises a parallel connection of a resistor and a capacitor inserted between the switching voltage supply point and the first electrode. (4) @ The switching circuit according to claim 3, wherein the blocking means includes a second transistor inserted between the parallel connection and the first electrode and Darlington connected to the transistor. 2. The switching circuit according to claim 1, which is an AGC signal detection circuit, a smoothing circuit, and an AGC signal amplification circuit.