Common Collector

The Common Collector or Emitter Follower circuit.

This time we have tied our collector directly to the supply voltage.

Our input can then be considered as a voltage between the base and collector, and our output can be considered as a voltage between the emitter and collector. We will get the same absolute results as if we choose the negative supply side as the common point.

There is no voltage gain with this circuit. There is actually a small voltage loss. There is a nice current gain. The input impedance is medium to high, and can be made higher in complex emitter follower circuits involving more transistors. The output impedance is low enough to drive most signal paths. The emitter voltage will "follow" the base voltage minus the diode drop of the base-emitter junction. This is typically 600mV for silicon and 100mV-300mV for germanium.

The output is in phase with the input.

This circuit is considered a "buffer" in that the input is mostly isolated from the output.

If the device is a power transistor, the load resistor could be an actual loudspeaker. A practical solid state power amp will typically use a complementary pair of transistors (one NPN and one PNP) as emitter followers to provide low impedance drive to loudspeakers.

Look for the emitter follower at the inputs and outputs of practical units where they ensure that the initial input is high impedance and the final output is low impedance.

Look for the emitter follower in power supplies where the transistor may be regulating the main supply voltage. Any kind of filtering or regulating in the base circuit will be multiplied into the collector-emitter circuit.