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My step response for LM741 at LTSpice was this: enter image description here

And for TL081: enter image description here

So as you can see the response of LM741 is linear and of TL081 is exponential, but why? My schematic is an inverting amplifier with v=10.

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3 Answers 3

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The LM741 has a slew rate of only 0.5V per us. The TL081 has 20V per us. So the LM741 ist just damn slow :-)

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The LM741 is being slew-rate limited here; op amps have a maximum slew rate, which is the maximum slope at which the output can change. The LM741 has a particularly low one (it's about the worst op amp on the market, and really shouldn't be used for anything. I don't know why it's still manufactured.).

The TL081 is faster, but approaches its final value exponentially likely because your feedback network is slowing it down somewhat; it'd help if we could see the circuit including the actual resistances used.

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  • \$\begingroup\$ Thank you for your answer! Both opamps outputs are limited by slew rate here. My circuit is a normal inverting amplifier with a 1kΩ and a 10 kΩ resistor. If my feedback network would slow it down, than both circuits would get affected and not just the one with TL081. \$\endgroup\$
    – M S
    Dec 9, 2023 at 15:39
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    \$\begingroup\$ @MS You're right, with 1k and 10k resistors you're not going to see anything slowed down by this much. I'm too exhausted from a long week at work to do any detailed analysis here, though. My answer might just be wrong. Though I will reiterate that the LM741 is very obsolete and should never be used. Please don't keep using the LM741. The TL081 is also pretty antiquated, but it at least has some half decent specs. \$\endgroup\$
    – Hearth
    Dec 9, 2023 at 15:52
  • \$\begingroup\$ I think slew rate is indeed the correct mechanism here. SPICE opamp subcircuits usually model this effect with a current source charging a capacitor. When the current source hits the limit it becomes constant and therefore the capacitor voltage rises/falls in a linear ramp. This is exactly what is shown in the simulation. \$\endgroup\$
    – Ste Kulov
    Dec 11, 2023 at 5:06
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Real world op-amps have limitations of the real world.

One such limitation is bandwidth. They do not have infinite bandwidth and it means for a relatively fast step input which has relatively high bandwidth, the op-amp output will not be a step but something limited by the bandwidth.

Different op-amps have different circuitry and different bandwidth.

Also the simulation model for the op-amps may be true or approximate or arbitrary very simple model where bandwidth is simulated just with limited slew rate so the output will be a trapezoid.

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  • \$\begingroup\$ Thank you for your answer! I actually measured the step response in the real world with an oscilloscope and got a similar result, so the models are pretty accurate. I believe that the bandwidth limitation should not make too much of a difference. \$\endgroup\$
    – M S
    Dec 9, 2023 at 15:49
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    \$\begingroup\$ Don't forget that bandwidth and slew rate are separate limitations, though. They're related, but bandwidth does not determine slew rate directly or vice versa. \$\endgroup\$
    – Hearth
    Dec 9, 2023 at 15:53

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