Large-Signal Transient Response of Junction Transistors

Transient response in the active region for junction transistors can be calculated from the conventional small-signal equivalent circuit. This small-signal characterization is adequate to calculate turn-on time and decay time. Carrier storage time, or time for the transistor operating point to move from the current saturation region to the active region, is calculated from a small-signal characterization of the transistor in the current saturation region. Frequency cutoff of alpha, ωN (radians/second) is the most important single transistor parameter affecting switching speed. It is possible with moderate driving current to switch the operating point from collector current cutoff to collector current saturation in times of the order of 3/ωN. To switch from collector current saturation to collector current cutoff, in times of the order of 3/ωN, carrier storage effects must be avoided.