Emitter Follower

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Emitter Follower

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

emitterfollower.gif

DC operating point

img335.png

Solving these equations, we can get img85.png, img77.pngand img87.png

img336.png

img337.png

img338.png

AC small-signal equivalent circuit

emitterfollower1.gif

We assume img339.pngand therefore can be ignored, and have

img340.png

Voltage gain:

img341.png

As img342.png, img343.pngis smaller than but approximately equal to 1.

The input resistance:

The input resistance is img194.pngin parallel with the resistance of the circuit to its right including the load img344.png, which can be found by img345.png. But as

img346.png

we have

img347.png

Comparing this with the input resistance of the common-emitter circuit img348.png, we see that the emitter follower has a very large input resistance.

The output resistance:

The output resistance is img349.pngin parallel with the resistances of the circuit to its left including the source, which can be found by img350.png, where

img351.png

and we have

img352.png

Alternatively, the output resistance of the emitter follower (treated as either a voltage or current source) can also be found as the ratio img353.pngof the open-circuit voltage img354.pngand the short-circuit current img355.png, where img354.pngis the output voltage when the load is open-circuit img356.png, and img355.pngis the output current when the load is short-circuit img357.png.

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 img358.png. The short-circuit current img355.pngcan be found as

img359.png

Now the output resistance can be found as

img360.png

The overall output resistance is

img361.png

Conclusion:

The emitter follower is a circuit with deep negative feedback, i.e., all of its output img362.pngis fed back to become part of its input img268.png. The fact that this is a negative feedback can be seen by:

img363.png

Due to this deep negative feedback, the voltage gain of the emitter follower is smaller than unity. However, many other important properties of the circuit, such as the input and output resistances, are drastically improved. In fact the emitter follower acts as an impedance transformer with a ratio of img364.png, i.e., the input resistance is img364.pngtimes greater than img365.pngand the output resistance is img364.pngtimes smaller than img366.png.

Comparing this with the input resistance img367.pngand output resistances img368.pngof 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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