op-amp

[denian]

i know that output impedance of an op-amp is very small compared to its input impedance. is there any purpose of having a high input impedance and small output impedance?

[chroot]

Yes, there's a purpose. You want the op-amp to avoid pulling much current from whatever is supplying its input. This is called loading the source and is usually a bad thing. If you were trying to amplify the signal from a delicate sensor, for example, the sensor might not be able to supply much current at all without disturbing its accuracy. You therefore don't want the op-amp to load the sensor much at all.

On the other hand, whatever is receiving the sensor signal after the op-amp might be a very poorly-behaved device. Maybe it's an analog-to-digital converter or a meter that pulls tons of current. You wouldn't want to connect this current-hungry device directly to the sensor, but you could connect it to the op-amp. The op-amp has a very low output impedance and doesn't mind sourcing a lot of current.

This is why op-amps wired up in the voltage-follower configuration are sometimes called "buffers." They essentially buffer or protect their input from whatever's on their output.

- Warren

[chroot]

Moved to EE..

- Warren

[turin]

To elaborate on the utility of high input impedence, consider the theory behind the ideal opamp in a feedback configuration. It is essential to the idealization that negligible current flows into the input, thereby allowing you to approximate the feedback mesh and develop those convenient equations for gain.

Low output impedance is desirable for basically the same reason that low internal resistance in a battery is desirable (and what chroot said).

[denian]

thank you for all the explainations. it helps.