OP Amp Input Impedance Explained

[hisham.i]

Hello

Am reading about amplifiers, and the articles i read suggests that for a successfull amplification the op amp should have an input impedance and zero output impedance.

I want some explanation about this information, why the input and output impedance are important factors?

 

[skeptic2]

Do you mean an INFINITE input impedance and zero output impedance?

An infinite input impedance would allow the opamp to sense the input voltage without loading its source, that is without drawing any current from the the source that would cause a voltage drop across the internal resistance of the source.

Having zero output resistance means that the opamp can act as a voltage source without dropping any voltage across its internal resistance.

 

[gnurf]

The Thevenin equivalent series resistance (or output impedance, Z_out) of your amplifier forms a voltage divider with the load it is driving. If Z_out is large relative to the input impedance of the load (or next stage---which might, for instance, be another amplifier), the loading effect causes a reduction of signal, like skeptic2 explained.

A low output impedance can also be thought of as the realization of a "stiff" voltage source in the sense that a varying input impedance of your load will not affect the output voltage of the source.