Learn Three Ways You Can Create Bode Plots - Workbench Wednesdays 64

That sounds wonderful! Being able to automate your measurements is a game changer for sure!

To clarify the reason I didn't see the issue nearly as pronounced as you had was because the breadboard was doing me a favor! 

I got this capacitor for free:

Until FRA options came out, I was never happy with the function generators built into a scope, unless you could get the option for free. (I always thought the budget was better spent on either a standalone unit or just something else!)

But having the integrated source AND measurement is a fantastic measurement package.

Cool! My plan is to solder a duplicate circuit to a perfboard (with generous solder) and do the sweep/plot again when I talk about how to automate the measurement in python (for those that don’t have an FRA option on their scope.)

But your way sounds more fun. 

I tried your circuit with a TL072 (don't have a 071) and got a substantially different results, the gain peaked maybe 5-10%.

So I was intrigued to dig a little deeper. I had the bend the output pin up and solder the one leg of feedback resistor to demonstrate the effect.

Again, great episode!

And by "me," of course, I mean The Intern.

Gosh. It's almost like there was a reason I made sure to attach the TL081 model I used.   (Not sure if you used that one or another. I think I got that one from TI.)

I found just 5 pF got a similar result. (I had a Cf cap at one point too.) Enough to realize what was going on.

I knew the prolific members here would likely catch this one. But it did throw me for a few minutes.

Another great episode.

I think BE just had bad luck with the intern/murphy. The TL081 spice model doesn't include any capacitance on its input, let alone the parasitic capacitance on a breadboard.

Green: No cf, Red: Cf=4pF

On the LM358 the output and inverting input pins are adjacent so you get the breadboard row-row capacitance a helpful bonus by introducing a zero to help counteract the pole created in the feedback network from capacitance to ground. 

If the peaking problem disappears with smaller feedback resistors say 1kOhm, then the gain was likely peaking due to capacitance on the inverting input node (reducing the closed loop phase margin).

On the LM358, there was peaking with X1 probes. The TL081 peaked with both X1 and X10.

I'll give the output a little more scrutiny next time I'm in the lab. You have me curious about what I missed. I had several setups going on, so I might be mixing up scenarios.

The output waveform shouldn't dance around. I think if you look at an FFT of the output, the amplitude of your test tone will be the correct amplitude. And, you will also see and there is also something else present. I don't know if its oscillating at a lower frequency...

If you were to probe your op-amp's output with a 1x scope probe, the measured bode plot with gain peaking is entirely plausible. But, I am pretty sure you are using 10x probes.

Still, I blame the intern and great video!

*had to delete to edit*