Crash Course Electronics/Transistor

Transistors are often used to switch loads or to amplify signals. Other uses as variable resistor are also common (for example for level shifting on digital inputs).

Emitter Follower

The emitter follower useful whenever you have a voltage source which isn't capapble of delivering a desired current. For example a potentiometer wich should control the voltage of a motor is too weak to directly connect a motor. Amplifying a voltage coming out of the potentiometer (used as voltage divider) you can control a transistor which then controls the motor.

The output voltage is approximately equal to the input voltage of this circuit. (it is always slightly lower than the input). So this should not be used to switch a high voltage by a low voltage control signal. It can however be used to switch a load if the control circuit and the supply of the load are equal.

Emitter Circuit

Don't confuse it with the emitter follower. This circuit is useful to switch loads on and off by a small control voltage and a higher (or equal) supply voltage for the load. If a second resistor is added is also capable to amplify a voltage to a higher voltage like the emitter follower - but instead with a low voltage controlling a high voltage.

Links

Switching high current loads:
http://itp.nyu.edu/physcomp/Tutorials/HighCurrentLoads

Basic circuits with easy zo understand comments (german): http://www.rn-wissen.de/index.php/Transistor