REVERSE BIAS

Suppose that another battery is connected to the base of the npn transistor at the point marked “control” so that the base is negative with respect to the emitter. The addition of this new battery will cause the E-B junction to be in a condition of reverse bias. Let’s assume that this new battery is not of such a high voltage that avalanche breakdown takes place at the E-B junction.
A signal might be injected to overcome the reverse-bias battery and the forward-breakover voltage of the E-B junction, but such a signal must have positive voltage peaks high enough to cause conduction of the E-B junction for part of the cycle. Otherwise, the transistor will remain cut off for the entire cycle.