Response at the Top of the Pulse

Once the pulse top is reached, E_a is dependent on the transformer OCL for its maintenance at this value. If the pulse stayed on indefinitely at the value E_a, an infinite inductance would be required to maintain it so, and of course this is not practical. There is always a droop at the top of such a pulse. The equivalent circuit during this time is shown in Fig. 233.

Fig. 233. Circuit for top of pulse.

Here the inductance L is the OCL of the transformer, and R₁ and R₂ remain the same as before. Since the rate of voltage change is relatively small during this period, capacitances C₁ and C₂ disappear. Also, since leakage inductance usually is small compared with the OCL, it is neglected. At the beginning of the pulse, the voltage e across R₂ is assumed to be at steady value E_a which is true if the voltage rise is rapid. Curves for the top of the wave are shown by Fig. 234.

Fig. 234. Droop at top of pulse transformer output voltage.

Several curves are given; they represent several types of pulse amplifiers ranging from a pentode in which R₂ is one-tenth of R₁ to an amplifier in which load resistance is infinite and output power is zero. In the latter curve, the voltage e has for its initial value the voltage of the source. All the curves are exponential, having a common point at 0, 1. Abscissas are the product of time τ and R₁/L_e, time τ being the duration of the pulse between points a and b in Fig. 226. The greater the inductance L_e the less the deviation from constant voltage during the pulse.