Output Resistance

We mentioned above that the output resistance of a 741 op-amp is approximately 40$\Omega$. Feedback acts to reduce the output resistance of the circuit to virtually zero. To find the output resistance of a circuit, the output of the circuit can be thought of as a voltage source in series with an output resistor. If no load resistor is attached to the output, the output voltage will be equal to the source voltage (This source voltage is often refered to as the open circuit output voltage, why?). However, once current is drawn from the output, the output voltage will be different from the source voltage. This is described by the following equation:

 

V_out= V_open-I_outR_out (13)

where V_open is the measured op-amp output voltage when no load resistor R_L is attached to the output (I_out=0 ). By determining I_out for various load resistors, equation(1.13) can be used to determine the output resistance R_out of the circuit.