Question

The emitter of a swamped amplifier __________

(a) is grounded

(b) has no dc voltage

(c) has an ac voltage

(d) has no ac voltage

I got this question in an interview.

This is a very interesting question from Voltage Amplifiers in portion Differential and Multistage Amplifiers of Electronic Devices & Circuits

Answer 1

The correct choice is (c) has an ac voltage

Best explanation: The resistance of the emitter diode r’e equals 25mV / IE and depends on the temperature. Any change in r’e will change the voltage gain in CE amplifier. In some applications, a change in voltage is acceptable. But in many applications a stable voltage gain is required.

To make it stable, a resistance rE is inserted in series with the emitter and therefore emitter is no longer ac grounded.

Because of this the ac emitter current flows through rE and produces an ac voltage at the emitter. If rE is much greater than r’e almost the entire ac input signal appears at the emitter, and the emitter is bootstrapped to the base for ac as well as for dc.