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I am new to Operational Amplifiers and I am trying to simulate a differential amplifier with a gain of 1.2, here it is:

enter image description here

As you can see I am using input voltages higher than VCC and according to datasheet the Common Mode Voltage Range is VCC-1.5V so the Operational Amplifier should work "ok" only with input voltages less than 28.5V am I right?

But on this simulation I am getting a correct output: (40-32)*1.2 = 9.6V. So I can put higher voltages than Vcc or simulation is wrong?

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  • \$\begingroup\$ I tried this, I have same unexpected results (it works) with input voltages 100V above Vcc (input directly at op amp, not before divider). Im too lazy to try this on breadboard. I give up. Deleting my answer, so put link to file in your question. \$\endgroup\$ – Kamil Apr 22 '14 at 18:52
  • \$\begingroup\$ What does the simulation say is the voltage at pins 2 and 3 of the op-amp? \$\endgroup\$ – The Photon Apr 22 '14 at 19:07
  • \$\begingroup\$ I have 21.8V on each pin. Anyway seems that Multisim is simulating extremely wrong because I cant get output voltages up to 500V with a 30V Vcc \$\endgroup\$ – Andres Apr 22 '14 at 19:08
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    \$\begingroup\$ It is often the case that device models try to get it right within the normal operating range and don't try to model device malfunction or failure. \$\endgroup\$ – Joe Hass Apr 22 '14 at 19:13
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I have tried this with higher voltages and I think, that multisim seems to work correctly when you are not exceeding devices maximum ratings. If you put 100V at the input of op-amp powered from 30V - do not expect any correct results.

In real life op-amp with input voltage above maximum from datasheet would blow up - I dont remember internal structure of LM324, but there are probably ESD protection diodes. They blow up first in that scenario.

My conclusion is - check voltages at input and output pins and make sure they not exceed maximum ratings :)

Also, notice that in your circuit you have voltage dividers at the op-amp inputs and they divide your input voltage by 120ohm/(100ohm+120ohm) = ~0.54.

V2 voltage is divided by R2 and R2, so you have ~21,818V at noninverting input

V1 voltage is divided by R1 and R4, so yoy have ~17,454V at inverting input

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On the one hand, you are only applying 21.8 V to the input pins of the op-amp, so you are not violating the common mode input specs.

On the other hand, simulators are not necessarily good at detecting unusual conditions. Overvoltages, overcurrents, or thermal runaway conditions are often not modeled accurately (or at all) by SPICE-like simulators.

Whether your circuit works in real life might depend, for example, on how the three voltages (V1, V2, Vcc) are ramped up when you turn the circuit on, or how they ramp down when you turn the circuit off.

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