Input/Output impedance

joshgreaves332

Jul 1, 2014
3
Could somebody please do me the favor of explaining the REASON that input impedance of say an amplifier would be high? What function does this serve?

Also, why would an amps output impedance preferably be low? Would this ever be reversed? What function would that serve?

Harald Kapp

Moderator
Moderator
Nov 17, 2011
13,722
This is not necessaryily so - also in many if not most cases you're right.

A high input impedance allows a source (e.g. MP3 player) to output the signal as a voltage while the current is very low (I=V/R). A low current serves to minimize the power that the source needs to drive the input.

A low output impedance allows the amplifier to drive a load (e.g. speaker) with high power, which typically goes hand in hand with high current (P=I²*Rload) while keeping the losses in the amplifier low (P=I²*Routput).

So far for your scenario. There are cases, however, where a source has a high impedance, acting more like a current source than a voltage source (you know you can replace any non.ideal voltage source by an equivalent non-ideal curretn source and vice versa, don't you?). If you were to drive a high-impedance input from such a source, the source would have to be able to source a high voltage (compliance voltage). This is often impractical (consider an MP3 player powered by a 1.5V battery). In this (admittedly rare) case you would want to have a low input impedance for the amplifier and you would control the amplifer by the current from the source, not by the voltage.

Similarly if the load impedance of the amplifier is high, you do not need a high current to drive the load, therefore you can afford a higher output impedance for the amplifier, possibly easing the design.

In a system design you will also have to look at mutual influences of the different impedances e.g. in connection with feedback mechanisms, stability considerations and noise - to name a few.

joshgreaves332

Jul 1, 2014
3
What is meant by the 'load' impedance of the amplifier? Is that the input, or output impedance seen by another load?

Harald Kapp

Moderator
Moderator
Nov 17, 2011
13,722
The load impedance is the impedance of the laod, e.g. a speaker or the input of another amplifier stage.
The source impedance is the impedance of the source, that is the output impedance of e.g. the preceding amplifier.

joshgreaves332

Jul 1, 2014
3
Much appreciated, you've cleared it up for me.

Supercap2F

Mar 22, 2014
550
Me to! Thanks Harald!

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