0
\$\begingroup\$

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!

\$\endgroup\$
3
  • 1
    \$\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

1
\$\begingroup\$

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.

\$\endgroup\$
1
  • \$\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

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service and acknowledge you have read our privacy policy.

Not the answer you're looking for? Browse other questions tagged or ask your own question.