Impedance matching

[autir]

I am trying to design a class-A audio preamplifier. The very classic design we all know, with 1 transistor, 4 resistors and 3 capacitors.
Should the ideal input impedance be of equal value to the output impedance of the previous stage (e.g. electret microphone)?
Should the ideal output impedance be of equal value to the input impedance of the next stage (e.g. power amplifier)?
Or should the ideal Zin=infinite and Zout=0?

 

[walid1]

Hi autir
I think you cannot achieve these extrem limits in practicle
but i think to have a maximum power transfer the impedances between the stages must be equal

 

[audioguru2]

Good audio circuits never match impedances from one stage to the next. A 10k load on a 10k source reduces the signal voltage to half. If you have more than a few stages then the signal is reduced nearly to zero.

A transistor's collector and its load resistor are a medium impedance. It usually connects to another transistor that has negative feedbck from having an emitter resistor, so its input impedance is very high.

Opamps are frequently used in audio amps. They have an extremely low output impedance so you would never match a load to them.

 

[autir]

Dear audioguru
The fact that English is not my native language gives me the right to ask for clarifications regarding your previous answer  ;D
What I understand is that I should go for as much high input impedance and as much low output impedance as I can? Like in the ideal operational/differential amplifier model, where Zin=infinite, Zout=0?

 

[audioguru2]

Yes, any load on a transistor stage reduces its output level and increases its distortion. I showed this transistor discussion in your other post but here it is again:
http://www.dself.dsl.pipex.com/ampins/discrete/discrete.htm