Emitter Follower Current Gain


I have two electronics textbooks and for some reason, both of them just gloss over configurations that are not "Common Emitter".
Regarding the emitter follower, I understand that there's no voltage gain. It was very easy to see why. But then the author just says, it has a very high current gain. Even though he didn't bother to show why, I went on to calculate the ratio and indeed there was a large current gain. The problem is my mind is tuned to "Common Emitter" circuits where the base current is multiplied by beta. This current is large and so the output voltage is large. Hence the voltage gain. That is quite understood.
But when it comes to the emitter follower, where is the current "hiding" because I read that it can supply loads that require large current. I know I'm confusing things. That's why I am asking for an explanation.

Best Answer

Let me try a short - and more descriptively - explanation without formulas (which you already know): The input signal causes a signal output current Ic. Now it is important how this current is translated to voltage:

1.) Common emitter: There is a collector resistor which produces a corresponding output voltage Vc, which does NOT react back to the input. As a consequence, we can have a rather good voltage gain.

2.) Common Collector: Now, there is a emitter resistor which also "translates" the output current (forget that Ie is little larger than Ic) into a voltage Ve. However, this voltage strongly reacts back to the input because it is a part of the current-controlling quantity Vbe=Vb-Ve. More exact: It follows the input voltage at the base Vb - and, thus, does not allow any voltage amplification. This is the result of negative feedback.

(May I add the following - although I am aware that not all forum members are happy? The explanation under 2.) clearly shows that the BJT is a voltage-controlled device and that the base current Ib is not the controlling quantity. The working principle of an emitter follower cannot be explained using the current-control model.)