The Transistor as a Switch

In the circuit below we use a rheostat as a variable potential divider to apply a variable voltage across the base and emitter of the transistor to see how this affects the voltage across the collector and emitter.

Adjust the variable potential divider so that V_be = zero.

Slowly increase V_be. Notice that the LED lights when V_be = (about) 0·6v and that V_be does NOT increase much above this figure no matter what we do with the rheostat.

Conclusion

V_be < 0·6v, transistor is OFF and V_ce = the voltage of the battery

V_be > 0·6v, transistor is ON and V_ce = about 0·2v

When we say that the transistor is ON, we mean that it allows current to flow easily into its collector and out of its emitter.

Transistors used as switches are found in nearly all modern electronic equipment, e.g. computers, calculators, T.V.’s.

• The Transistor
• Three important transistor switching circuits
• The Transistor as an AMPLIFIER

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