Why the input resistance of an ideal OPAMP is infinite and output resistance is zero? secondly, how can we measure these resistances(input and output) in case of an ideal OPAMPand Real OP-AMP in the following conditions when 1- load is not connected. 2- load is connected.

Why the input resistance of an ideal OPAMP is infinite and output resistance is zero? secondly, how can we measure these resistances(input and output) in case of an ideal OPAMPand Real OP-AMP in the following conditions when 1- load is not connected. 2- load is connected.
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1 Answer

Answer :

Ans- Ideal OP-AMP is a power ful concept. If one goes through the history of amplifying devices it becomes clear that less it loads the previous stage better the output signal. ie. it is a trend toward higher and higher input impedances. Also it is better to maintain the signal level irrespective of the LOAD connected at the ouput. Lesser the output impedance less the signal amplitude reduction when load is connected. ie. it is a trend toward lower and lower output impedances. As ideal opamp is a concept by virtue it has zero output impedance and infinite input impedance and they are not physically measurable but has to be taken for granted for the use in theoritical analysis. To measure input resistance of non ideal opamp connect a known source at the input and give a signal (within Specifications) through a current meter of sufficient precision. Input voltage by current gives the input impedance. To measure input resistance of non ideal opamp connect a known source at the input and give a signal (within Specifications) through a current meter of sufficient precision. Input voltage by current gives the input impedance. To measure output resistance of non ideal opamp connect a known source at the input such that the output does not saturate and connect a load (within Specifications ) through a current meter of sufficient precision and meaure the output voltage. Output voltage by current gives the output impedance. Can refer application notes by device manufacturer available in internet for more precise and practical solutions.
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