Variable-Width Pulse Generation Using Avalanche Transistors

Abstract

The extremely fast ionization process in semiconductors offers a means of generating pulses that have subnanosecond rise times and high peak powers. There are several important applications of these pulses which require the duration of the generated pulse to be variable. This report investigates three methods of producing variable-width pulses using transistors operating in the avalanche mode. The first circuit studied is used with a capacitor as the collector load. It produces pulse rise times of less than one nanosecond but has the disadvantage of a relatively slow RC discharge fall time. Distributed and lumped parameter delay lines were used in the second form of circuits studied. With the artificial delay line the pulse width can be controlled by opening the line at different sections whereas, with the cable, the length must be altered to change the pulse width. These circuits produced fast rise, flat top, and fast fall pulses. In the third method studied, a composite circuit is used in which the fast rise of an avalanche pulse is added to a slower rise, but variable duration pulse, generated by a saturating transistor. This method is shown to produce fast rise, variable-width pulses with overshoot and ringing easily held to less than five per cent.