I understand the basics of Operational Amplifiers, but I am curious, what would happen if the + and – input pins have the same voltage, does the output pin (x) give a high or a low voltage output?
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Best Answer
Theoretically, in a perfect op-amp the output voltage would be mid-rail. However, due to input offset voltages the output will be somewhere between the most negative rail and the most positive rail.
OP-amps have DC gains of millions so a 100uV offset at the inputs produces an open loop output level of hundreds of volts (limited by the power rails).
As a real example, the ADA4528 has an input offset voltage that is 2.5 uV maximum at ambient temperatures. It also has an open loop gain that is at least 130 dB (3.16 million). Result is potentially an output voltage in the range +/- 8V. However it's only a 5V device so most of the time the output will be clamped to the rails. For those times that it is not against the rails the internal noises inside the op-amp will be rapidly end-stopping the output against the + and - rails.