Explain the action of transistor as a switch.
To answer this question (with proper understanding) you should have an idea of the three regions in the VI characteristics of a transistor. viz; Cutoff region, Active region and the saturation region.
![transistor_switch](http://plustwophysics.com/wp-content/uploads/2011/02/transistor_switch-300x224.gif)
When the input bias of a CE transistor (Vbe) is very low, the transistor does not conduct. So the output voltage (Vce) will be almost equal to Vcc, the output bias voltage.
![tran46](http://plustwophysics.com/wp-content/uploads/2011/02/tran46.gif)
By increasing the input voltage, the transistor can be brought to saturation region when the transistor will be conducting well. Now the pd across the transistor (Vce) will be very low.
![tran47](http://plustwophysics.com/wp-content/uploads/2011/02/tran47.gif)
This is the working principle of transistor as a switch.
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