Using a 741 opamp, I constructed a simple inverting amplifier circuit as in https://en.wikipedia.org/wiki/Operational_amplifier#/media/File:Op-Amp_Inverting_Amplifier.svg ; I first tried Rf = 1k and Rin = 330 Ohm. Driving the input with a 1khz sine wave with 1v peak to peak amplitude, I expected to see a gain of -3. Instead, I got a bigger output that was so dirty that I couldn't tell what the gain was. I also think that the output was even in phase with the input! (Sorry I don't remember exactly, and am away from the lab now.) Then I put on a load of 1k. Now the output was clean, but still in phase with the input and was small, maybe even smaller than the input! When I replaced Rf and the load with 10k resistors and replaced Rin with a 4.7k resistor, the result was as expected. The gain was of magnitude about 2 but 180 degrees out of phase with the input! Could someone please explain what happened when I used the smaller resistors? Does it have to do with the 741's output impedance? That would seem strange since the output resistance is only 75 Ohm. Thanks!

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    \$\begingroup\$ In future op-amp experiments, chuck out the 741 and get a modern op-amp. Just about any op-amp is "nicer" than the 741. It wants a substantial bipolar supply, and won't go anywhere near the rails. Even a jelly bean 358 will happily operate on a single 5V supply, and will to within ~1.5V of the rails, IIRC. \$\endgroup\$
    – uint128_t
    Jan 24, 2016 at 5:55
  • \$\begingroup\$ That ought to work. Were the power supplies the correct +-/12-15V? If you were trying this off +/-5V or even 0/5V you need a newer opamp. \$\endgroup\$
    – user16324
    Jan 24, 2016 at 10:50
  • \$\begingroup\$ The supplies were + and - 15V. Thanks for your input! \$\endgroup\$ Jan 24, 2016 at 22:43

1 Answer 1


The closed loop output resistance should be very low, less than 1 ohm. 75 ohms is more-or-less what you'd expect for AC output impedance.

The output resistance is only low for currents that the op-amp can supply. It will go into limiting mode if the output current exceeds a couple tens of mA roughly. It also cannot drive close to the rails with a heavy load. With a 2K load it typically can get within a couple volts of the rails, but is not guaranteed to do better than 5V, so if your supplies are +/-10V your maximum input signal with a 2K feedback resistor and 660 ohm Rin your maximum input voltage would be about +/-1.6V, and with 1K about +/-800mV. It will be less if your supplies are lower.

  • \$\begingroup\$ Thanks for replying. Are you saying that I was trying to make the 741 drive too much current with the low resistance values? \$\endgroup\$ Jan 24, 2016 at 8:29

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