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