Improvements in voltage clamps


1,027,249. Transistor circuits; television. THORN ELECTRONICS Ltd. Oct. 8, 1963, No. 39684/63. Headings H3T and H4F. The output voltage of a D.C. amplifier having a series input capacitor is periodically clamped by rendering periodically active a negative feedback path which charges or discharges the capacitor through a constant current device at a rate dependent upon the difference between the amplifier output voltage and a reference voltage. Television signals are applied through input capacitor C1 to an amplifier comprising transistors VT6-VT9 and having negative feedback between the emitter of the output transistor VT9 and the base of VT8. During the back porch period an input pulse at the base of VT5 causes VT1, the common emitter load of comparator transistors VT2, VT3, to become conductive so that the output voltage of the amplifier is compared with the reference voltage applied to the base of VT2. The latter transistor is coupled to the base of a transistor VT4 forming with VT3 a constant current charge and discharge path for the capacitor which resets the D.C. input level and thus the output level. During the restoration period the amplifier is thus provided with a negative feedback path which effectively reduces the input impedance of the amplifier to a low value so that the capacitor voltage will be quickly reset. Diode MR prevents VT3 passing current when the restoration circuit is not in use and diode MR2 prevents forward conduction between collector and base of VT7 if the base is driven negative. Large positive input signals will cause VT6 to cut off and VT7 to bottom so that the signals will be amplitude limited.




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Cited By (10)

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    JP-S4852130-AJuly 21, 1973
    JP-S5136619-UMarch 18, 1976
    JP-S5152762-AMay 10, 1976Rca Corp
    JP-S5233932-B2August 31, 1977
    JP-S5515421-Y2April 09, 1980
    JP-S581869-B2January 13, 1983Rca Corp