Electrical Engineering ⇒ Topic : Output resistance RoF of Practical inverting op Amp

Output resistance RoF

The output resistance is equal to the ratio of open-circuit output voltage V₀ to the load current i_L when the load is short-circuited.

Since A, is approximately equal to -(R₂/ R₁)

                                       .............. (1)

                                   ...............  (2)

To obtain the short-circuit current I_SC, a shorting line is provided across the output terminals as shown in Figure (1).

                          Figure (a) Circuit for calculating R₀ for inverting OP Amp

From Figure (a), it can be expressed as:

                                        .......... (3)

                                                .............. (4)

Using Eq. (4) and Eq. (3) can be given as:

                       .............. (5)

The second term in Eq. (5) is very large as compared to the first term. Neglecting the first term, Eq. (5) can be obtained as:

                         ............... (6)

                [Using Eqs. (2) and (6)]          ............ (7)

                           ............. (8)