EXPERIMENT No.7

EXPERIMENT No. 7

Transistor as a switch

Components: 1 x Small Signal NPN transistor, 2 resistors.

Exercise:  Connect the circuit as shown in Fig 12 and switch on the power supply.

Fig 12

Connect the multimeter between base and emitter.

Note the voltage reading and explain what this reading is indicating.

The voltage between base and emitter is 0.8v which is the voltage required to push electric charge through the insulating layer from the base to emitter. A small current appears.

Connect the multimeter between collector and emitter.

Note the voltage reading and explain what this reading is indicating.

The voltage between collector and emitter is 0.6v which means the base has free holes for the electrons to move along and are closer so you don’t need a higher voltage like .8v to move electrons. A higher current appears (20mA) because electrons can move/flow easier.

In the plot given below what are the regions indicated by the arrows A & B?

Fig 13

How does a transistor work in these regions? Explain in detail:

A – Cutoff = Transistors that are fully off

B - Saturated = Transistors that are fully on

What is the power dissipated by the transistor at Vce of 3 volts?

P= I x V

   =15 x 2.7

   =15mA x 2.7(Vce)

   =0.4micro W / 400 mW

   =0.0405 Watts

What is the Beta of this transistor at Vce 2,3 & 4 volts?

2Vce - B=Ic/Ib = 20mA/0.8mA = 25

3Vce - B=Ic/Ib = 15mA/0.6mA = 25

4Vce - B=Ic/Ib = 5mA/0.2mA = 25