Mumbai University > Electronics and telecommunication engineering > Sem 3 > Analog electronics 1

Marks: 5M ,4M

Years: Dec 15, May 16

Both the npn&pnp type bipolar transistors can be made to operate as “ON/OFF” type solid state switch.

The areas of operation for a transistor switch are known as the Saturation Region and the Cut-off Region.

The shaded area at the bottom of the curves represents the “Cut-off” region while the area to the left represents the “Saturation” region of the transistor. Both these transistor regions are defined as:

Cutoff Mode:

In this mode, both collector base junction and emitter base junction are reverse biased. This in turn not allows the current to flow from collector to emitter when the base-emitter voltage is low.

In this mode device is completely switched off as the result the current flowing through the device is zero.

Here the operating conditions of the transistor are zero input base current

( $I_B$), zero output collector current ($I_C$ ) and maximum collector voltage ($V_CE$ ) which results in a large depletion layer and no current flowing through the device. Therefore the transistor is switched “Fully-OFF”.

Saturation Region:

Here the transistor will be biased so that the maximum amount of base current is applied, resulting in maximum collector current resulting in the minimum collector emitter voltage drop which results in the depletion layer being as small as possible and maximum current flowing through the transistor.

Therefore the transistor is switched “Fully-ON”.