Why You NEED An Emitter Follower

In this video I explain what an emitter follower circuit is and why you need one in your circuits.

These emitter follower amplifiers can be used in many circuits I've used them a huge number of times.

In essence, an emitter follower is a common-collector configuration of a bipolar junction transistor or BJT. It's called an "emitter follower" because the emitter voltage closely follows the base voltage, even though there's a small voltage drop across the base-emitter junction.

In its very simplest form, the circuit consists of a transistor and an emitter resistor. When a signal is applied to the base, it causes a corresponding current to flow through the collector and emitter.

The emitter voltage closely tracks or follows the base voltage, there's just the base emitter junction difference which is about 0.6V for a silicon transistor.

I look at the key characteristics of the emitter follower:

1. It has a high input resistance: This means the circuit draws very little current from the input signal source. It reduces the loading on the previous stage. This is typically beta, the transistor current gain times the overall emitter resistance including the load.

2. It has a low output resistance and this makes it suitable for driving lowish-impedance loads.

3. The voltage gain of an emitter follower is unity or 1 and

4. Finally the current gain - this is greater than 1, providing current amplification.

I look at where emitter followers are used:

a. Their chief use is as a buffer to reduce the load on a source. It could be an oscillator or anything where the loading may be an issue.

b. In a similar vein they are used in impedance matching where it's necessary to match the impedance of a high-impedance source to a low-impedance load.

The circuits can be directly coupled. Here the bias voltage for the device is taken from the previous stage. Alternatively they can be AC coupled, but here the bias network needs to be set up and the coupling capacitors selected to give the required frequency response.

More information about the transistor emitter follower:
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