JPH07118791B2 - ABL contrast circuit - Google Patents

Description

TECHNICAL FIELD The present invention relates to an ABL contrast circuit for a television receiver.

2. Description of the Related Art In recent years, in an automatic brightness limiting circuit of a television receiver, an automatic contrast limiting circuit (hereinafter, referred to as an ABL contrast circuit) which controls a contrast at the same time has been introduced in many receivers.

An example of the conventional automatic brightness limiting circuit as described above will be described below with reference to the drawings.

FIG. 2 shows a conventional automatic brightness limiting circuit.
In FIG. 2, 1 is a tuner / VLF / video processing circuit, 2 is a video control circuit, 3 is a video output amplifier circuit, 4 is a cathode ray tube, 5 is a high voltage generation circuit, 6 is a filter capacitor, and 7 is a current limiting resistor. , 8 is a bright control volume, 9 is a resistance, 10 is a contrast control volume, 11 is a resistance, 12 is a diode, 13
Are resistors, 14 and 15 are resistors, and 16 and 17 are power supply terminals. Again 1
8 and 19 are resistors and 20 is a diode.

The operation will be described below. When an electric current flows through the cathode ray tube 4,
An electric current is supplied through the power supply 17 and the resistor 7 through the high voltage generation circuit 5. Normally, the current flowing through the resistor 7 is larger than the current flowing through the cathode ray tube 4, so the remaining current is the resistance 1
Flow through diode 20 to 8,19. Therefore the diode
The 20 anodes are at a constant voltage. Therefore, the bright control voltage is constant between the volume 8 and the resistors 9, 14 and 15, and the contrast control voltage is also constant between the volume 10 and the resistors 11 and 13. Diode 12
Is normally biased off.

Now, when the beam current flowing through the cathode ray tube 4 increases, the current of the high voltage generating circuit 5 increases, the current flowing through the current limiting resistor 7 increases, and the anode potential of the diode 20 decreases.
The contrast control voltage is lowered, and the contrast is lowered first to reduce the current flowing through the cathode ray tube 4.
When the beam current further increases, the anode potential of the diode 20 further decreases, the diode 12 becomes conductive, the brightness control voltage also decreases, and the brightness control operates so that the brightness becomes constant.

Problems to be Solved by the Invention However, the above-described configuration has a problem that the effect of ABL contrast cannot be automatically switched.

In view of the above problems, the present invention provides a circuit in which a switch is provided in an IC and thereby the control level of ABL contrast can be automatically switched.

Means for Solving the Problems In order to achieve this object, the present invention connects the ABL terminal to a differential amplifier and extracts its output with a current mirror,
The collector is equipped with a switch, and is further grounded by a resistor.

Operation With such a configuration, the current of the current mirror is limited by turning the switch on and off, and the operation amount of the ABL contrast can be made variable.

Example An example of the present invention will be described below with reference to FIG.

In the figure, 21 is an ABL input terminal, 22 and 23 are differential amplification transistors, and the base of one transistor 22 is connected to the ABL input terminal 21. 24 and 25 are emitter resistors, 26 and 52 are constant current source transistors, 27 and 53 are emitter resistors, 28 and 29
Is a transistor for a current mirror, 30 is a load resistor, 31 is an emitter follower transistor, 32 is its emitter resistor, 33 is an emitter follower transistor, 34, 35, 36 are bias resistors, 37, 38 and 42, 43 are switching transistors, 39, 40, 44, 45, 41, 46 are resistors, 47 is a transistor 29
A contrast control circuit connected to the connection point between the collector of the load resistor 30 and the load resistor 30, 48 and 49 are transistors for differential amplification, and the base of one transistor 48 is connected to the ABL input terminal. 50 and 51 are its emitter resistors, 54 is a bias resistor, 55 and 58 are resistors, 56 and 57 are current mirror transistors, 59 and 60 are current mirror transistors, 70 and
Reference numeral 71 is its emitter resistance, and 61 and 64 are switch control transistors, the collectors of which are connected to the bases of the switching transistors 37, 38 and 42, 43, respectively. 62,6
3, 65 and 66 are resistors, and 69 is a bright control circuit, which is connected to the collector of the current mirror transistor 60. 67 and 68 are switching control terminals, which are connected to the bases of the switch control transistors 61 and 64, respectively.

Regarding the ABL contrast circuit configured as above,
The operation will be described.

Now, when the beam current increases and the voltage of the ABL input terminal 21 decreases, the differential amplifier composed of the transistors 22 and 23 operates first. The output is output to the resistor 30 by the current mirror circuit composed of the transistors 28 and 29, and the voltage thereof is supplied to the contrast control circuit 47 to reduce the contrast. Therefore, the beam current becomes stable. Here, the voltage output to the resistor 30 depends on the current. The maximum value of the current is the current of the constant current source composed of the transistor 26 and the resistor 27.

Now, for example, when the terminal 67 is set to low level, the transistor
61 is cut off, the switching transistors 37 and 38 are turned on, the current is shunted by the resistor 41, the voltage of the resistor 30 is reduced, and the amount of contrast reduction is reduced. Also, the terminal
When 68 is also set to low level, the transistor 64 is cut off,
Transistors 42 and 43 are turned on, current is shunted, and resistance 30
Voltage drops. For example, set both terminals 67 and 68 to low level so that the current flowing through the resistor 30 becomes zero.
If you set the value to 1,46, ABL contrast will not work. Therefore, the ABL contrast can be changed by a switch from 0 to 100%.

When the voltage of the ABL input terminal 21 further decreases, that is, when the beam current further flows, the differential amplifier composed of the transistors 48 and 49 operates and the bright control circuit 69 starts to operate.

As described above, according to this embodiment, the control amount of the ABL contrast can be changed by the switch from 0 to 100%.

As described above, according to the present invention, by switching the current of the current mirror so as to be divided by the switch, the ABC
It is possible to realize an ABL contrast circuit that can switch the amount of contrast extremely easily.

[Brief description of drawings]

FIG. 1 is a circuit diagram of an ABL contrast circuit according to an embodiment of the present invention, and FIG. 2 is a circuit diagram of a conventional ABL contrast circuit. 21 …… ABL input terminal, 22,23 …… Differential amplifier, 28,29 ……
Current mirror circuit, 37,38,42,43 …… Switch transistor, 47 …… Contrast control circuit, 61,64
...... Transistor for switch control.

Claims (1)

[Claims] 1. A differential amplifier in which a voltage according to a beam current is applied to one base and a reference potential for determining an operation start point is applied to the other base, and a current mirror circuit is provided at an output end of the differential amplifier. And connect a contrast control circuit to the output side of this current mirror circuit and connect a plurality of switch circuits in parallel with each other, and ground the other end of each of the switch circuits via a resistor, An ABL contrast circuit provided with means for controlling opening / closing of each of the above switch circuits.