Class 12 Physics Semiconductor Electronics Transistor as a switch

Transistor as a switch

Circuit Diagram

  • Let us consider n-p-n transistor which is of common emitter configuration


Using Kirchhoff’s law for the above circuit,

For the input side,

-VBB +  IBRB + VBE  = 0

In other words, VBB  =   IBRB + VBE    ----------------------------------------------(1)

Similarly, in the case of output side,

-VCC + IcRc + VCE = 0

In other words, VCE=  VCC  -  IcRc ----------------------------------------------(2)

Assume VBB as the DC input voltage Vi and VCE as output voltage Vo

Hence (1) and (2) becomes,

V=   IBRB + VBE------------------------------------------------------------------------------------(3)

Vo=  VCC  -  IcRc------------------------------------------------------------------------------------(4)

Let us consider the change of Vo as Vi changes from 0 to higher value

Case 1 – When Vi < 0.6 V, there is no collector current. Hence, from (4), we get

Vo=  VCC 

The transistor is said to be in cut off state

Case 2 – When 0.6V <Vi <= 1.0 V, there will be some collector current.

 Hence, from (4), we get Ic increases. Hence, as Ic term is subtracted in (4) the value of Vo output voltage decreases.

In this range, as Ic increases almost linearly, so Vo decreases linearly. The transistor is said to be in active state at this stage

Case 3 –When Vi> 1.0 V, Vo is found to decrease towards zero but not zero. The collector current Ic becomes maximum and the transistor is in saturation state.


The relationship between Vi and Vo is called transfer characteristic curve of a base biased transistor in Common emitter configuration. The above mentioned regions – Cut off region, Active region and Saturation region are marked in the graph.

In cut-off state when Vi is low, the transistor is not working and hence said to be in switched off state.  In saturation state when Vi is high, the transistor is fully conducting as Ic is nearly equal to maximum or saturated. As the transistor is fully conducting, it is said to be in switched on state.

Thus the transistor acts as a switch.

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