Experiment 8 - The Emitter Follower Circuit

The common collector circuit is often referred to as an emitter follower circuit. Why this is, you learn when you look at the voltages in a common collector circuit.

Use the circuit shown.

Measuring voltage in a common collector circuit. (Enlarge)

Circuit diagram voltage measurement in a common collector circuit.

Turn the potentiometer P1, until a voltage of approx. V_in = 3.00 V applies to the base. At the same time you measure at the output a typical value of approx. V_out = 2.30 V.

Turn P1 a little further until a voltage of approx. V_in = 4.00 V applies to the base. Now you measure a typical value of approx. V_out = 3.30 V at the output.

A change of 0.01 V at the base results in an equal change of 1.00 V at the output. The output voltage follows the input voltage! Therefore, one speaks of an emitter follower circuit.

                           

The voltage gain of a common collector circuit is 1.

Since the current, but not the voltage is amplified, it can be stated...

                           

The power gain of a common collector circuit is equal to the current gain.

Now you compare these results to a common base circuit...?