Temperature-compensated voltage clamp with forced pass transistor

Abstract

A voltage clamp for protecting a load from transients in a supply line comprising pass transistors coupled to a bias circuit path. The bias circuit path determines the clamp turn-on voltage of the voltage clamp and comprises transistors configured as zener diodes and transistors configured as forward biased pn junctions. The pass transistors are coupled to the bias path so as to conduct current from the supply line to ground when the voltage drop across the bias circuit path reaches the clamp turn-on voltage. The collector-to-emitter voltage drops of the pass transistors during current conduction are equal to one another and sum up to the clamp turn-on voltage, and therefore, the pass transistors advantageously share equally the clamp turn-on voltage drop across their collector-to-emitter junctions. The bias circuit path is temperature compensated so that the clamp turn-on voltage is substantially independent of temperature. Additional bipolar transistors are coupled between the pass transistors and bias circuit path to supply bias current to the pass transistors so that negligible current is drawn from the bias path when the pass transistors are conducting current.