Opamp Output Impedance

One of the practical op-amp limitations is that there is a finite output impedance. For the 741 it is about 75 ohms but can be as high as several thousand ohms for some low power op-amps. The effective output impedance is further lowered by the use of negative feedback, so the focus becomes not one of the number of ohms looking into the output, but what limitations are placed on the output current.

Limiting the output current also limits the allowable output voltage swing: the lower the load resistance, the lower the allowable voltage amplitude. For a load >2K, the 741 can swing to within about 2 volts of the supply.

This is about all that is permitted by common mode limits for a 15 volt supply so the output impedance is not a serious limitation. For lower load impedances, however, the safe output voltage swing will be progressively curtailed. Op-amps with MOS transistor outputs(CA3130 and CA3160) can swing all the way to positive and negative supply voltages.