opamp

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.

 

I'll take homework questions for $200 please, Alex.

Dave

 

You might need a better understanding of voltage, current and
resistance before tackling op-amps. Try Googling for: "Ohm's Law" and
"potential dividers"

 

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

 

The most simple words that can clear ur doubts is as follows....


The input resistance must be very high and output resistance most be
low for any amplifiers and instrumentation and measurement circuits.

input resistance must be very high so as to absorb as much less power
as possible from the previous stage...

and output resistance must be very very low so as to deliver the
maximum possible power to the next stage...

high ip. res. high sensitivity.
low op. res. low power losses within the stage.

 

thanx a lot.u r as clear as mirror.