## Open-loop operational amplifier

Here, an operational amplifier is shown without a feedback loop (i.e., "open loop"), in order to illustrate some of its fundamental properties. Operational amplifiers are almost never used in this way, because the open loop gain is far too high to be useful.

The differential input voltage (the voltage between the two input terminals of the op amp), Vdiff, is controlled by the slider on the left. The output voltage Vout = A*Vdiff. Because the open-loop gain A is so big, Vout is saturated unless Vdiff is very small. Note that the output voltage of an operational amplifier has definite voltage limits. In this case the limits are +14 volts and -14 volts. If you attempt to make the output voltage exceed those limits, the output will "saturate" at the limit until the input voltage is reduced.

You can investigate the effect of the open-loop gain with the slider on the right (Most op amps have an open-loop gain of 50,000 or higher).

The differential input current, Id, is the current that flows between the inputs of the op amp. It is given by Vdiff/Rd and is very small if the output is not saturated, because then Vdiff is very small and Rd is typically 1 Megohm or greater.