...........................
Now my next question is, and something else that has always looked like a Dark Art to me, I know there are several types of feedback (current, voltage, series, parallel), but supposing I have for a very simple example an amp with 3 bipolar transistors (I'll stick with those because I understand them better) common emitter, biased very boringly with a p.d. to the base, and say I want it to have a closed loop gain of 100, how do I work out the value of a resistor taking negative feedback from the collector of the third tranny to the base of the first? Or supposing I have only one transistor, and I want a gain of 10, I can bias it with a resistor from collector to base, but how do I work out the value to get the required gain?
I'm assuming from the principle you explained above it will decrease the input impedance in this arrangement.
It's just one of those things I never understood, mainly because it gets very mathematical very quickly (and I only do slightly mathematical, sorry!)