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Opamp Behaviour

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petesmc

New Member
If i have a standard 741 opamp, on +15 and -15 rails, and i created an inverting amplifier circuit with gain of 1, what would happen if the non-inverting input was brought to something like 16V, or more extreme, 50V?

Thanks
 

TheOne

New Member
Not advised, as the spec quote +15 and -15 max input voltage and exceeding this will damage the device. Operating on lower supply voltages will pin the allowed input voltages to that operating limits.
 

petesmc

New Member
OK thanks. Just that i'm doing a simulation in spice with a 741, and i'm finding that i can bring the input up to 30V, gain 1, to get an output of 30V, and strangely, i can go up even higher, then the opposite happens.

I always beleive though that it should not go above 15V at all, actually, should reach only around 14V.
 

TheOne

New Member
Software models are far more forgiving!
 

JimB

Super Moderator
Most Helpful Member
Simulated world - Garbage in, Garbage out.

Real World - Garbage in, Smoke out.

JimB
 

audioguru

Well-Known Member
Most Helpful Member
Heh, heh, heh, heh.
Those simulators should go back to school!
Or maybe they are just stupid and will never learn.

Wait a minute!
30V in, 30V out. Sure, when the opamp melts its input and output together. The sim is smart afterall.

If you have an inverting opamp with a gain of 1 or less, you can safely apply 44V to its input resistor when using 15V dual supplies. The voltage divider of its input and feedback resistors will limit the voltage at the opamp's inverting input to 15V or less. But don't overload its output, let it swing freely to near the rails.
 

Oznog

Active Member
petesmc said:
OK thanks. Just that i'm doing a simulation in spice with a 741, and i'm finding that i can bring the input up to 30V, gain 1, to get an output of 30V, and strangely, i can go up even higher, then the opposite happens.

I always beleive though that it should not go above 15V at all, actually, should reach only around 14V.

741 is not a "rail to rail" device in either input or output. For a +/- 15V supply, it can track only +/- 12v inputs and still remain a functional amplifier. The output can drive only +/- 12v for light loads and +/- 10v for heavier ones.

While not recalling the 741 specifically, I have seen specs for op amps where the output will give the opposite response than expected when the input voltage exceeds its limits (I think the +/- rails are the limits).
 
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