741 op-amp strange behavior #1011
Description
This circuit has a behavior where it seems that a large current is being supplied by the op-amp's inverting input, which I suspect is unphysical.
It's supposed to be a variable PWM circuit.
The 5 scopes show, in order: PWM output voltage; V and I of the capacitor vs time; V vs I of the capacitor; V of the topmost resistor vs time; and the current into the inverting input of the op-amp.
As the circuit runs, you'll see that sometimes the V of the capacitor is an approximate triangle wave, as intended, but other times it jumps unexpectedly. The peak current on the capacitor can be seen by removing voltage from the 2nd scope, so that peak current is shown; peaks can be as high as 800mA, causing very nearly a step change in the voltage of the capacitor.
We see from the 4th scope that the current is not coming through the resistor connected to the capacitor, the voltage isn't spiking up high enough to cause that (as it shouldn't). The 5th scope shows us that the current spikes are indeed coming from the inverting input of the 741.
This syndrome disappears if the max slew rate on the 741 is lowered to the default 600mV/µs. I had increased it to 13V/µs to better approximate the part on my bench, and this syndrome appeared. The cutoff for the syndrome appears to be somewhere between 1.5V/µs and 2V/µs, although the frequency with which it intermittently occurs increases as the slew rate is increased.