Transistor as a switch

Thread Starter

alexmath

Joined May 2, 2014
17
When setting up a transistor as a switch is there any difference between putting the load on the collector or emitter?
 

MrChips

Joined Oct 2, 2009
30,706
Yes.

With the load on the collector, the configuration is called "common emitter amplifier".
This circuit provides both voltage gain and current gain.
A base-emitter voltage (Vbe) greater than 0.7V will turn on the transistor as a switch.
The standard practice is to use a current gain (beta) = 10. Hence if you want to switch 100mA through the load, make sure the source driving the base current can provide 10mA, i.e. Ib = Ic/beta.

A "common collector" or "voltage follower" configuration is used when you need to match to a low impedance load.
There is no voltage gain and Vb (voltage at the base) must exceed the voltage out at the load.
 
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