Reverse Bias:

Assume that another battery is associated to the base of the npn transistor at the point marked "control" therefore the base is negative with respect to the emitter. The adding up of this new battery will cause the E-B junction to be in a situation of reverse bias. Let's suppose that this new battery is not of such a high voltage which avalanche breakdown occurs at the E-B junction.

A signal may be injected to overcome the reverse-bias battery and the forward-breakover voltage of the E-B junction, though such a signal should have positive voltage peaks high sufficient to cause conduction of the E-B junction for part of the cycle. Or else, the transistor will stay cut off for the whole cycle.