Mode of Operation

BJT can operate in three different modes according to the external voltage applied at each junction. The three modes of operations are

• Active region,
• Saturation
• Cutoff

To determine which mode a transistor is working in, we will check to look at the voltages on each of the three pins, and their relation to each other as shown in the diagram

This quadrant graph above shows how positive and negative voltages at those terminals affect the mode.

1.      Saturation Mode

Saturation mode is also called on mode or conducting mode of a transistor. In saturation mode acts it act like a short circuit between collector and emitter.

In saturation mode both of the “diodes” in the transistor are working in forward biased. We can say that V_B must be higher than both V_E and V_C.

Because the junction from base to emitter looks just like a diode, in reality, V_BE must be greater than a threshold voltage to conduct in saturation mode.

2.      Cutoff Mode

A cutoff mode is the reverse of saturation. A transistor in cutoff mode is also called off mode of a transistor in which transistor is not conducting,   there is no collector current, and therefore no emitter current so it works as an open circuit.

To make transistor into a cutoff mode, the base voltage must be less than both the emitter and collector voltages that is V_BC and V_BE must both be negative.

Active Mode

To operate in active mode, a transistor’s parameter V_BE must be greater than zero and V_BC must be negative. Therefore the base voltage must be less than the collector voltage, but greater than the emitter voltage. That also means the collector must be greater than the emitter.