Transistor modes of operation

In this lecture we will discuss three major transistor modes of operation. Before you go directly on transistor applications, I recommend you to understand modes of operation of transistor.

There are three transistor modes

1. Cutoff mode
2. Active mode
3. Saturation mode

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BJT Cutoff Mode

In cutoff mode there is no current flow in transistor. In other words the base-emitter as well as the base-collector junctions are reversed biased.The transistor works just like an open circuit.

The following water tap example will explain you the exact working of cutoff mode.

Assume the water tap is a transistor. One end is emitter (E) and the other end is collector (C). The valve is the base (B). You can observe when valve is closed there would be no water flows from emitter (E) to collector (C).

Similarly if the base current (Ib) is zero, the current in the transistor would be zero. However a small collector leakage current flows. But this is negligible.

Fig-1: Cutoff mode

Cutoff Mode Conditions for NPN Transistor

1. As emitter is n-type and base is p-type. So to reverse bias the E-B junction the emitter voltage must be greater than base voltage
2. As collector is again n-type so to achieve reverse bias at B-C junction the collector voltage must also be greater than base voltage.

Cutoff Mode Conditions for PNP Transistor

1. As emitter is p-type so to reverse bias B-E junction, the base voltage must be greater than emitter voltage
2. The collector is p-type so again base voltage must be greater than collector voltage to reverse bias B-C junction

BJT Active Mode

In active mode B-E junction is forward biased and B-C junction is reversed biased. In this mode transistor behaves like a closed switch.

Fig-2: Active Mode

Active Mode Conditions for NPN Transistor

1. To forward bias the B-E junction, the base voltage must be greater than emitter voltage.
2. Similarly to reverse bias B-C junction the collector voltage must be greater than base voltage  

Active Mode Conditions for PNP Transistor

1. To forward bias the B-E junction, the base voltage must be lesser than emitter voltage.
2. Now collector is p-type so to reverse bias B-C junction the collector voltage must be lesser than base voltage.

BJT Saturation Mode

In saturation mode the maximum amount of current passes through the transistor. For this both B-E junction and B-C junction should be forward biased.

In short the transistor behaves like a short circuit. So we can’t control the current using base anymore.

Fig-3: Saturation Mode

Saturation Mode Conditions for NPN Transistor

1. Emitter is n-type and base is p-type so base voltage must be greater than emitter voltage to forward bias the B-E junction.
2. Like emitter collector is also n-type so again base voltage must be greater than collector voltage to forward bias the B-C junction.

Saturation Mode Conditions for PNP Transistor

1. Emitter is p-type and base is n-type so base voltage must be lesser than emitter voltage to forward bias the B-E junction.
2. Collector is also p-type so again base voltage must be lesser than collector voltage to forward bias the B-C junction.

 

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