# How to explain the strange results from this circuit (op-amp : differential amplifier?)

I have a differential amplifier. Problem of "gain" of the op-amp or "how it is wired."

• I have simulated this as a voltage source controlled voltage (VofV.)
• As a generic amplifer.
• As a good old LM741C.

Very strange behavior when there is an error of "wiring." But quasi good results.

What is happening?

Precision : it is a "Dynamics-DC" analysis (so called) by microp12 Spectrum-Software ... and the results are, I think so, "numerically" correct.

EDIT : for obtaining what are "real" and correct results, one must make some other analysis on these examples, as TRANsient analysis. So, when you have a circuitry to be analysed, always try different kind of simulation. And, "in fine", ... Realize in a 'real world' by assembling it on a breadboard ... and viewing on a scope (or other toosl) the behavior. When you use "simulators" or "mathematical" tools, don't forget to verify some "answers" given. Be critical ...

• It’s an interesting question that leads to more questions. What conditions or assumptions differ between simulators and real ccts? What is the peaking factor for self-compensated OA’s in these config’s for each cct. In Differential mode. The 5 uV difference is just the error due to gain. 1/Aol*Vin. The low f phase is inverted in both cases because the current loop does not change. Commented Jul 3, 2021 at 14:22
• How can it work , makes it weird. Examine the current loop as output phase leads input instead of lagging . Commented Jul 3, 2021 at 14:53
• There is an error in computing the difference voltage across the input source of 10 uV yet defined as 1V. In the 1st plot. Why? Commented Jul 3, 2021 at 15:36
• @JRE Thanks for "correcting" my poor english. Simulation are made in DC-dynamic mode with microcap12 spectrum-soft.com/index.shtm. It is an error often found and made ... even for me. :) Commented Jul 3, 2021 at 16:08
• @Tony Stewart EE75 Some errors ... because, i think, simulators add one high resistor ( 1 Gohm) to ground for nodes when generators are found "floating". Commented Jul 3, 2021 at 16:19

It appears that you have fallen into the trap of using a simulator with positive feedback on an op-amp and gotten what appears to be a stable result. Those circuits drawn incorrectly as differential amplifiers i.e. those circuits that use positive feedback will give conditionally stable outputs that appear valid - they are not AND, whenever using a sim, you need to be sensible and not create this situation.

• There are some interesting gain/ phase effects near unity gain on each version ; normal and wrong. Commented Jul 3, 2021 at 13:54
• Yet the current loop is the same in both cases below breakpoint, so it is negative feedback in both cases as it senses differential voltage in the current loop. Thus inverting voltage in both cases except near GBW limits. That’s when it goes unstable with a phase shift >180 deg Commented Jul 3, 2021 at 14:30
• @Tony Stewart EE75 OK for stability of the circuits at right circuits. It is here a DC-dynamics analysis. Just to say that we have to be carefull with simulators. :) Commented Jul 3, 2021 at 16:22
• @Antonio51 just performing a dynamic-DC analysis in micro-cap tells you the average voltages that the nodes settled at in a very short (and predicted time). In reality, it tells you nothing about the real DC voltages that a real circuit eventually settles upon. The voltages shown in dynamic-DC analysis are the very first voltages the simulator calculates before beginning the much more computation-extensive transient analysis hence, user beware and don't plug-in circuits that are known not to be comparable to a standard linear circuit with negative feedback. Commented Jul 3, 2021 at 18:57
• @Andy aka Yes. Closed. Sorry. Commented Sep 9, 2021 at 12:01

I like the fact that Falstad’s simulator gives real results with real Op Amp’s like the 741 (saturated) yet it works with infinite GBW ideal Op Amps by default. Here I toggled the 741 inputs (normal, inverted) using select IC> edit as the plot shows normal & saturated outputs alternating. The differential input is the 120 Hz square wave with a 60 Hz sine included for Vcm.

I guess that makes it better than some other simulators.

• I don't know this simulator and what it can do. But it is interresting for animation. I don't know if this can use Laplace functions or other things such defining PWL datas. I have used microcap for 40 years (in fact, since it begins). The more complicated schematic I have simulated was a digital circuit (gates, D-FF 7474, counters 7490) on 8 pages. (it was a 8-storey elevator complete system, with all buttons/memory at each stage and intelligent up/down). Commented Sep 9, 2021 at 12:28