Overvoltage protection circuit for general purpose amplifier

Abstract

An output stage and feedback circuit for a buffer amplifier are disclosed. The output stage protects an operational-amplifier circuit preceding it from overvoltages appearing at the load. The circuit includes a pair of complementary transistors in a push-pull arrangement that sources and sinks current to and from the load. Both the transistors are in common-emitter configurations, and each one has a corresponding diode connected between its collector and the load in an orientation opposite that of the collector-base junction. The diodes prevent the reverse current that would otherwise flow in response to an output overvoltage of the polarity that forward biases the collector-base junction of the transistor, and the over-voltage is thereby prevented from being seen at the operational-amplifier output terminal. Overvoltages of the other polarity are not seen because they reverse bias the base-collector junction. Each amplifier transistor also has associated with it an auxiliary transistor whose base is connected through a zener diode to the output terminal of the circuit. The auxiliary transistor is connected between the output-stage amplifier and a source voltage. An output overvoltage of the right polarity and sufficiently large to turn on the zener diode therefore turns on the auxiliary transistor. This turns off the amplifier transistor that would otherwise be on, thereby preventing thermal destruction of the amplifier transistor.

Dependencies are inferred from the claim text.