One can’t be arbitrary about the place to introduce stimulation right into a circuit mannequin. It has been famous earlier than that if you wish to mannequin the achieve/section of some circuit utilizing a Bode plotter in SPICE, that you must someway present that circuit with a nominal sign stimulus.
Please see: “Simulation Hassle: Bode Plotting an Oscillator.”
Nicely, it occurred once more. This time, I wished to take a look at the accuracy of an op-amp mannequin’s gain-bandwidth properties so I attempted it two methods: first WITHOUT a slight sign stimulus which DID NOT work, after which in a second approach WITH a slight sign stimulus which DID work (Determine 1).
Determine 1 Failure and success.
Nonetheless, that profitable consequence didn’t simply occur. I had blithely thought at first that I may introduce a small stimulus sign just about anyplace I arbitrarily selected and all can be nicely.
To cite the late George Carlin: “Au contraire, mon frère.”
Once I inserted my sign stimulus as proven beneath, the Bode plotter did NOT show the op-amp info I used to be in search of (Determine 2).
Determine 2 Unsuccessful try.
The explanation why this didn’t work was that by including the stimulus sign on the op-amp’s non-inverting enter, that op-amp behaved as a non-inverting achieve stage with respect to V3. The inverting enter was compelled to observe no matter sign the non-inverting enter obtained. With R1 and R2 of equal worth, the non-inverting circuit achieve was two which comes to six.0206…dB, which the Bode plotter then introduced as 6.025 dB over the frequency vary for which the op-amp may present service.
My error was in not realizing that the tried achieve measurement can be dominated by the 2 resistors and NOT by the op-amp. One can not all the time simply arbitrarily select a stimulus level.
I gained’t (I hope) make that mistake ever once more.
John Dunn is an electronics guide, and a graduate of The Polytechnic Institute of Brooklyn (BSEE) and of New York College (MSEE).
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