Emitter Follower2

7/28/2019 Emitter Follower2

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Emitter Follower

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

:


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Example

Assume , , find so that the DC operating point is in the middle of

the load line.

Solving for , we get .

AC small-signal equivalent circuit

Voltage gain:

We assume and therefore can be ignored, and have

Now the voltage gain can be found to be:


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As , 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 in parallel with the resistance of the circuit to its right including

the load , which can be found as the ratio of the voltage and the current . But as

we have

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 in parallel with the resistance of the circuit to its left including

the source but excluding , which can be found as the ratio of the open-circuit voltage

( ) and the short-circuit current ( ). 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 can be found as


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Therefore the output resistance is

The overall output resistance can therefore be found to be

Conclusion:

The emitter follower is a circuit with deep negative feedback, i.e., all 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, the voltage gain of the emitter follower is smaller than unity.

However, the circuit is drastically improved in terms of its input and output resistances. 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 .

Comparing this with the input resistance and output resistances of the

common-emitter transistor circuit, we see that the emitter follower circuit has very favorable

input/output resistances.

Although the emitter follower does not amplify voltage, due to its high input resistance drawing

little current from the source, and its low output resistance capable of driving heavy load, it is

widely used as both the input and output stages for a multi-stage voltage amplification circuit.

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Emitter Follower2