Input and output resistance

[ssayan79]

Hi
I'm a studenti in elctronic engineer and ihave some questions for my next exams. I need to know why in some cases is better to have a high input resistance of amplifier in general, why is good to have a low output resistance and when is good to have a low imput resistance.
Thanks a lot for your replies!!

 

[kennyg]

It depend on what kind of load your amp drives, and what kind of source you apply to the amp.

Please check some classic microelectronic books.

 

[ptmsl]

Output impedance should be as low as possible to make it independent from load impedance except of course if the output is a current source in which case you need a high output impedance.
If your input is voltage driven it is good to have as high input impedance as possible so that it will not load the source driving it. Only exception is for very low noise circuits when high imput impedance is usually achieved at the cost of noise performance. If the input is current driven you might consider low input resistance to avoid demanding high voltage from your source.

 

[peterwang]

this is a nice book for your to study, your question is in Ch11:

Op Amps for Everyone

Author: Ron Manchini
Publication: Texas Instruments
Volume: 463 pages

**broken link removed**

 

[coolguy_ar]

A variation of the question is this:
What is Impedance Matching? When is it required?

I badly need to understand this. When does "loading" happen in a circuit?

 

[ptmsl]

Impedance matching has different meanings depending on the application. In High Frequency circuits (digital or analog) and long lines ("long" always depends on the frequency) impedance matching is used to reduce reflections on the transmission line by matching the transmitter and receiver impedances to the line impedance. In power applications matching the generator (or amplifier) output impedance to the load impedance maximizes power transfer.
Other than that if you have a source with a high, complex (and sometimes non linear) output impedance the input impedance of the next stage determines the frequency response and harmonic distortion performance.
Also the output impedance of the amplifier when driving a complex load determines the frequency response and the power dissipation of the amplifier. In voltage driven circuits the output impedance of the first stage and the input impedance of the next form a voltage divider so the lower the input impedance of the first stage and the higher output impedance of the next stage the less frequency (and sometimes harmonic) distortion and power loss.
The only case when this is reversed is when circuits are current driven.

 

[chacha]

depends on the out put of the opamp.
if it is current output then output resistance should be high.

refer Microelectronic circuits, by Jacob Millman.