# Couldn't get a 741 OPAMP to work as inverting amplifier with smaller (but not so small) resistors

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!

• 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. Jan 24, 2016 at 5:55
• 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.
– user16324
Jan 24, 2016 at 10:50
• The supplies were + and - 15V. Thanks for your input! Jan 24, 2016 at 22:43