why the input resistance and output resistance of opamp is high(or
infinite) and low(or zero) respectively? wat is the advantage if
input(and o/p) resis. is high(is low for o/p resis)? Wat is loading
effect? thank a lot in advance.
Think of the following situation:
input signal -------.
|
[R1]
|
o--- output
|
[R2]
|
GND
Your input signal has some resistance of its own, which is modelled by
R1. The input of the opamp measures voltage across some resistance,
which is modelled by R2.
Now, what is the relationship between the input signal and the output
signal? The output will be 'near' the input only if R1 is much less
than R2. So, opamp designers ensure that this is true by making inputs
have a VERY high impedance.
As to the output resistance, the situation is now reversed. The opamp
wants to have the SMALLEST output resistance, because it doesn't know
how bit the resistance the next element in the signal path will have,
and so wants to ensure that the signal the next guy sees is as big as
possible.
--
Regards,
Bob Monsen
Kiss girls all you want to - it beats the hell out of card games."
~ Jubal Harshaw