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I have recently downloaded LTspice and when I am testing the LF351 and LM741 OP-Amp, I realised that the output voltage does not equal to the gain which should be -50. I have looked at the data sheet and the Vsat of the LF351 is around 13.5V and the LM741 is 14V. I am trying to get the design desired output which I think should be -25Vp but none of the OP-amps gave me the output. So, I am wondering why the gain is not (-R2/R1) in this case. Thank you in advance.LM741 OP-Amp LF351 OP-Amp

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  • \$\begingroup\$ try -50* 0.025=-12.5Vp or 25Vpp but G* f=50* 75kHz= 3.75MHz GBW requirement must exceed this. OA "101" error \$\endgroup\$ – Tony Stewart EE75 Jul 7 '17 at 3:36
  • \$\begingroup\$ I tried changing the frequency which has increased the output voltage. May I ask why does changing the frequency increase the output voltage or is it just a LTSpice thing? \$\endgroup\$ – ARandomGnome Jul 7 '17 at 4:09
  • \$\begingroup\$ unity gain stable OA's have a built in integrator cap to make it have a 1st order open loop response in order to have a stable closed loop response. The GBW is an individual component spec where gain and bandwidth are tradeoffs and the product is fixed. In order to make the the closed loop gain accurate the open loop gain must greatly exceed the closed loop gain. so gain of 50 is too high for this frequency of 75kHz. Better designs exist. LF351 has Wide gain bandwidth : 4MHz but not wide enough here. \$\endgroup\$ – Tony Stewart EE75 Jul 7 '17 at 4:12
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Note that the output voltage on the graph is not in phase with the input voltage, but lagging by about 90 degrees.

This means that it's not your feedback resistor controlling the gain, but a capacitance, somewhere.

LF351, and other unity gain stable op-amps, have what's called a 'dominant pole' added to them with an internal capacitor. This rolls off the open loop gain so that by the time the other internal phase shifts are getting significant, the gain has dropped below unity, so that it can be fed back for unity gain, and still be stable.

As a result of dominant pole compensation, your op-amp will act like an integrator, at surprisingly low frequencies if the gain is high enough. The figure in the data sheet that lets you know how much gain can be achieved at any frequency is the Gain Bandwidth Product (GBW).

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