Input and output impedance

[giri_lp]

Hi,
How should I understand input and output impedance in general? For example, the input impedance of an amplifier is this much and output impedance is this much. And what does that imply?

Say, input impedance of hte amp is very low/very high. And how does that matter when I design my basic block?

and similarly for output impedance

 

[Kral]

giri_lp,
The input impedance of an amplifier forms a voltage divider with the output (source) impedance of the source that is providing the input voltage to the amplifier. For simplicity, assume that the impedances are purely resistive. Suppose the amplifier has an input resistance of 10K Ohms, and the source has an output resistance of 20K Ohms. This means that the voltage appearing on the input of the amplifier will be 1/3 the open circuit output voltage of the source. For low frequency, e.g audio applications it is desirable to have a low output impedance and a high input impedance. For RF applications, it is desirable that the output impedance of the source matches the input impedance of the load. This is necessary to prevent ringing due to reflections.
Regards,
Kral