GB1477568A - Video amplifier circuit for television receivers - Google Patents

Abstract

1477568 Transistor amplifiers INDESIT INDUSTRIA ELETTRODOMESTICI ITALIANA SpA 17 Get 1975 [22 Nov 1974] 42747/75 Heading H3T A video amplifier for a TV receiver comprises at least two amplifiers each containing at least one semi-conductor device 4, 5, 6 fed by unstabilized voltage V1, first means 82 which samples the bias voltage of each amplifier and provides one signal representative of the bias voltages, second means 79 receiving the representative signal and generating a control signal, and third means 70, 62, 60 transmitting the control signal to the semi-conductor devices for maintaining stable operation, e.g. against variations of supply voltage, transistor characteristics, tube current or temperature. In a receiver for colour TV a negative feedback loop (not illustrated) is provided as follows. The outputs of the amplifiers taken from the collectors of 4, 5, 6 corresponding to the red, green and blue signals respectively are joined through a divider to the base of transistor 82 via resistors 87, 88, 89 at terminals R, G, B respectively. Transistor 82 functions as a threshold device and is conductive only during the flyback periods, when the voltage at the common point of 87, 88, 89 is at its maximum value. Negative feedback provided by resistors 85, 86 ensures that the supply voltage V 3 to 82 need not be stabilized. The signal produced at the collector of 82, being representative of the bias voltages of the three colour amplifiers, is applied to the base of transistor 79 which acts as a comparator gate, being turned on only by the flyback pulses applied to the input terminal of a resistor 77. The power supply at V 2 is a stabilized reference voltage. A network including a capacitor 74 stores the output of 79 as a substantially D.C. control voltage for feeding back to the emitter circuits of transistors 4, 5, 6. An amplifier 62, 60 &c. for the luminance signal Y provides an output on the same paths to the emitters of 4, 5, 6, the function of 4, 5, 6 being to add the Y signal to the R-Y, G-Y and B-Y signals respectively.