Transistor operating modes and How to tell if a transistor is in Active, Saturation or Cut off mode?


A transistor, be it BJT or FET operates in either of the mode namely Active mode, Saturation mode, or Cut-off mode. In this post, we revise some basics of the operating modes of a transistor and then we go into details of how practically we can check if the transistor in your design is working in the desired mode.

Consider a transistor as a variable resistor. The value of this resistor changes depending on the bias (the base current) you provide it. Depending on your bias the resistor operates or takes 3 types of different values and hence 3 different regions of operation.

Transistor as a variable resistor

The value of resistor R1 in the above figure varies depending on the base current Ib. This value can either be Zero (close of Zero in the practical scenario), Infinite (as high as possible in practical case), in-between zero and infinity. These 3 values represent 3 working regions of the transistor. Thus we can summarize the 3 regions as:

1. Cut off region: In this region, the resistance is infinite in the ideal case and as large as possible in the practical case. Since the resistance is very high the transistor acts as an open switch and hence Ic=0 (Ideal case).

2. Saturation region: In this region, the resistance is zero in the ideal case and as small as possible in the practical case. Since resistance is very low the transistor acts as a closed switch and Ic= Vcc/Rc (Ideal case), where Rc is the collector Resistor.

3. Active region: In this region, the transistor has a finite value of resistance and it depends on base current. The amount of base current decides the resistance and hence the collector current. Thus the collector current is indirectly controlled by the base current. Ic = Vcc/(Rc + R1).

How to know if the transistor is in Active, Saturation or Cut off ?

It is simple, Just measure the Vce by a using a multimeter. By measuring Vce you are indirectly measuring the R1 resistor as state above as Vce = Ic x R1.

If the Vce value is close by the Vce(sat) value given in the datasheet, you can say the transistor is in the saturation region. Vce(sat) value in the datasheet is very low. Since R1 in saturation is very low, the product of Ic x R1 is very low which is Vce(sat).

If Vce = Vcc of your circuit, then the transistor is in cut off region.Since there is no Ic. Although Vce won't be exactly equal to Vcc but very close to Vcc since there will be always some leakage Ic in the practical case.

If the Vce > Vce(sat) and Vce < Vcc, if both these conditions are satisfied, then the transistor is in Active region.