Apparatus for reducing and maintaining constant overshoot in a high

Abstract

Disclosed is a method and circuit for reducing and maintaining constant overshoot in a high speed driver. The circuit includes a predriver circuit which is driven single endedly and a driver circuit which is differentially driven by the predriver outputs. The predriver and the driver are differential pairs, with commonly controlled individual transistor current sources. A diode has been added in series with each emitter of the differential pairs. Schottky diodes are preferable because of their low capacitance. The diodes increase the input switching voltage (the smallest input voltage swing that will cause the outputs to fully switch) of the differential pair because they must also be switched on and off. The increase results in an increase in effective transition time, which results in smaller overshoots because the circuit is being switched slower. The output amplitude of the driver is set by a voltage which controls the current source currents of the commonly controlled current sources. As the amplitude is decreased the input switching voltage decreases because the current through the devices decreases which results in smaller base-emitter and diode voltages. Due to the commonly controlled current sources, the predriver amplitude decreases as the driver amplitude decreases. The predriver is designed such that its variable output supplies the driver with the proper input switching voltage at any driver amplitude. This keeps the effective input transition time constant which results in constant output overshoot.