I'm using this Ebay item : 253088080650 ($2.27).
It is not really a compressor but rather an automatic gain MIC amplifier. It is very sensitive and your sound quality remains clear. You don't have to shout in the microphone to generate some output ... :)
In my Bitx40 it replaces the original MIC amplifier. The AGC function prevents overloading of the TX chain, so your power remains clean and within the limits.
I'm curious wether a real compressor circuit would still be a better solution for this kind of radio.
Herman - ON7HC