**Equivalent Circuit of Practical Op Amp:**

The Circuit which represents op-amp parameters in terms of physical components, for the analysis purpose is called equivalent circuit of an op-amp. The Equivalent Circuit of Practical Op Amp is shown in the Fig. 2.8.

The Equivalent Circuit of Practical Op Amp shows the op-amp parameters like input resistance, output resistance, the open loop voltage gain in terms of circuit components like R_{in}, R_{o} etc. The op-amp amplifies the difference between the two input voltages.

V_{o} = A_{OL} V_{d} = A_{OL }(V_{1} – V_{2})

where

- A
_{OL }= Large signal open loop voltage gain. - V
_{d }= Difference voltage V_{1}– V_{2} - V
_{1 }= Noninverting input voltage with respect to ground - V
_{2 }= Inverting input voltage with respect to ground - R
_{i }= Input resistance of op-amp - R
_{o }= Output resistance of op-amp

The output voltage is directly proportional to the difference voltage V_{d}.

It is to be noted that the op-amp amplifies difference voltage and not the individual input voltages. Thus the output polarity gets decided by the polarity of the difference voltage V_{d}.

The voltage source A_{OL} V_{d }is the Thevenin’s equivalent voltage source while R_{o} is the Thevenin’s equivalent resistance looking back into the output terminals.

The equivalent circuit plays an important role in analyzing various op-amp applications as well as in studying the effects of feedback on the performance of op-amp.