An emitter follower circuit shown in the figure is widely used in AC amplification circuits. The input and output of the emitter follower are the base and the emitter, respectively, while the collector is at AC zero, therefore this circuit is also called common-collector circuit.

**DC operating point**

Solving the second equation, we get :

and :

**Example**

Assume , , . Find so that the DC operating point is in the middle of the load line.

Solving the equation for , we get .

**AC small-signal equivalent circuit**

Based on this small signal model, we can find the three system parameters: voltage gain, input resistance, and output resistance:

**Voltage gain:**Typically and therefore can be ignored, and have

The voltage gain can be found to be:

As , the voltage gain is smaller than but approximately equal to 1. Note that , i.e., the output voltage is in phase with the input voltage.**Input resistance:**The input resistance is the parallel combination of and , the resistance of the circuit to its right including the load , which can be found as the ratio of the voltage and the current . We know

therefore we get

and

Comparing this with the input resistance of the common-emitter circuit , we see that the emitter follower has much higher input resistance.**Output resistance:**The output resistance is the parallel combination of and the resistance of the circuit to its left (including ), which can be found as the ratio of the open-circuit voltage (with ) and the short-circuit current (with ). As the voltage gain of the emitter follower is close to unity, the open-circuit output voltage is approximately the same as the source voltage . The short-circuit current is simply

Therefore we get

The overall output resistance can therefore be found to be

**Conclusion:**

The emitter follower is a circuit with deep negative feedback, i.e.,
100% of its output is fed back to become part of its input
. The fact that this is a negative feedback can be seen by:

Due to this deep negative feedback, it has the following properties:

- The voltage gain is smaller than but very close to unity.
- The input resistance is large .
- The output resistance is small .

Although the emitter follower circuit does not amplify the signal voltage, it drastically improves the input and output resistances, compared with the input resistance and output resistances of the common-emitter transistor circuit. In fact the emitter follower acts as an impedance transformer with a ratio of , i.e., the input resistance is times greater than and the output resistance is times smaller than .

Although the emitter follower does not amplify input voltage, due to its high input resistance and its low output resistance, it draws very little current from the source and can drive heavy load (low ), it is therefore widely used as both the input and output stages for a multi-stage voltage amplification circuit.