A Pulse Amplifier

Author: J.B. Hoag

The time constant of the grid circuit becomes of considerable interest when the input signal consists of a succession of pulses. In particular, the question arises as to the rapidity with which the pulses may follow each other and still be recognized as separate pulses. The limit, in seconds, is called the resolving time of the amplifier. Suppose, at the start, that each pulse were of very short duration. Then the grid condenser would be charged up quickly when a pulse was applied, and would discharge between impulses. In RC seconds it would be nearly discharged. The second pulse would then repeat the process and be recognized as a separate pulse. But, if the discharge time (which is arbitrarily taken as RC seconds) were too long, the grid condenser would still retain an appreciable percentage of the charge of the first pulse at the time the second pulse arrived. After a few pulses, the continued charging of the condenser (with too little time for discharge) would prevent the pulses from being distinguished one from the other. Hence in pulse counters, it is customary to use small grid condensers. For example, at the input of the first stage C = 10 µµf.; at the second stage C = 100 µµf.