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Basic Op Amp question Answered


If the input to the positive terminal of an op amp is 12V, the input to the negative terminal is 10V, and there is no supply voltage, the output voltage is 2V and the output current is 0. Correct?

Now, what I don't understand is what happens to the output voltage and current if I connect a supply voltage of, for example, 15V. I know that the output voltage and current both increase, but I don't know the math behind it.

Can someone explain this to me, please? 


"the output voltage is 2V and the output current is 0. Correct?"

No, if there is no supply voltage (Vcc, Vee) then there can be no output voltage at all.


5 years ago

Depends on the Op-Amp ability to saturate.
The case you describe any Op-Amp will supply full positive.

Its never a good idea to apply voltage to inputs without
power on the IC first.


For an ideal op amp, what would be the output voltage and current in the case that I described?

Maybe u know who gave the best answer ?

That is what we call a Rail_To_Rail Op-Amp.
The positive power supply is the top Rail and
the ground or negative supply is the bottom Rail.

Your ideal Op-Amp output pin is able to swing between the Rails.
When the input pins are just 0.001 volts positive the output goes to the plus Rail.
When they are -0.001 volts negative the output goes to the minus Rail.

There is no math needed to answer your question except for the current
which is limited by the ability of the battery or power supply to deliver.


To be perfectly accurate, for an ideal opamp with infinite gain ANY finite input causes the output to swing over.


There is no maths involved. The amplifier has a gain of essentially infinity. Infinity x input voltage = infinity, or the supply rail, whichever is lower.....

So if 12V goes through the positive terminal,10V goes through the negative terminal, and the op amp as a supply voltage of 15V, are you saying that the output should be 15V? And what about the current?

Current output depends on the (real) device, and the load. Of course, if we stick with playing with "ideal" devices, the possible output current might ALSO be infinite, if you short circuit the output since the o/p impedance of the ideal opamp is zero.

No real amp has a truly zero o/p impedance, but over a defined range of load it comes vanishingly close to it, if the gain is very high.