Hello,

I am doing a assesment in which i need to design common emitter amplifier and i am stuck with one question which i can not answer.

In the assesment is asked to explain why common-emitter amplifiers gain reduces at regions below the low cut-off frequency.

If anyone could explain it to me I would be very thankful.

 

Quite often the common emitter design includes a resistance in the emitter leg added to improve DC bias stability. This resistor is normally completely or partially bypassed by a capacitor.

In this situation the common emitter gain depends upon the total AC impedance in the emitter leg, having regard also to the total collector impedance.

Does this suggest any ideas, given the emitter bypass capacitance will have an impedance related to frequency?

 

If you meant frequencies above the cutoff for the divice this is the explanation: The cut off frequency is due to internal parameters of the device. The output at the collector has a resitance of about 100kohms when operating at one milliamp.

But the capacitance at the collector is the real culprit. The common emitter amplifier suffers from an effect called Miller capacitance. This is the input capacitance amplified by the gain of the circuit. More if you want.

If this was not what you were looking for, I'm sorry.