Electronic – Cascaded common emitter amplifier

amplifierbjtRF

Im trying to design a cascaded amplifier with 3 common emitter stages.
As long as the input AC voltage is below about 20mV the amplification is fine, as you can see on AMP1.
But the output of AMP2 is also maximum at 20mV and and goes much below zero. AMP3 is also maximum 20mV.
The bias of each stage is at 773mV.
How can I modify the bias of the second and third stage for higher input /output amplitude than 20mV?

Many thanks![CascadedAmplifier]1

Best Answer

You can't.

A swing of 20mV seems like a pretty small signal -- but it's not small enough to have a reasonably linear current through a typical silicon diode. So the emitter current of one of those transistor stages will be severely distorted with that large of a signal.

You need feedback to linearize the circuit. Emitter degeneration would be easiest, but you may be able to do it using your existing shunt feedback and putting a resistor in series with each DC coupling capacitor.