Electronic – When are BJTs used in common base configuration

When we say that the base is "grounded" in a common-base amplifier, we're talking about the AC small-signal analysis of the circuit. The key concept is that there is no signal voltage on the base terminal.

This does not mean that there's no DC bias on that terminal, if it is required for the circuit to operate. But note that the base COULD be directly connected to ground if the emitter has a negative bias (assuming NPN transistor).

Kathi, it helps to realize that the signal voltage Vbc (between base and collector) is identical to the signal voltage Vbo (between base and common ground). This is because the internal ac resistance of the DC voltage source (connected to C) can be regarded as zero.

Secondly, in common-collector configuration there is a emitter resitor Re between the emitter node and ground. Hence, the input characteristics of this configuration contains the base-emitter path in series with the resistor Re.

In this context, it is important to realize that the current through Re is beta times larger than the current into the base node. Hence, the voltage drop across this resistor - resulting from the input signal at the base - is correspondingly larger.

This is the well known feedback effect which determines the whole input characteristics. As a result, the input resistance of the whole circuit is r(in)=rbe + beta*Re. (If the current through Re would be identical to the base current Ib the input resitance would be only rbe+Re)