Transistor as amplifier and switch

Use of transistor as amplifier

To use a transistor as an amplifier it is used in common emitter configuration. The operating collector current is set nearly half the max collector current with the help of collector load resistance and base biasing resistance. transistor never go in saturation or cutoff value of collector current. Some short of negative feedback is also given by providing emitter resistance and emitter by pass capacitor. This negative feed back improve the quality of signal amplified.

Use of transistor as oscillator

A transistor connected in common emitter configuration just like an amplifier circuit can be converted to a oscillator just by providing a positive feedback from output circuit to input circuit. In common emitter circuit the output wave form is always 180° out of phase with input wave form. if some part of output is fed back to input in additive phase  then it becomes oscillator. below is diagram of resistance capacitance phase shift oscillator.

Use of transistor as switch 

A transistor can be used as a switch to control small current load. Relay can  be controlled by a transistor to control heavy loads. In switch mode transistor is biased to either cutoff current (for switch off) or to saturation current (for switch on).Set reset flip flop can be used with transistor to get latching property.

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