2 added 25 characters in body edited Feb 15 '17 at 11:23 LvW 15.4k22 gold badges1313 silver badges3232 bronze badges niki - of course, your test circuit cannot work because you have connected an ideal ac source to the low-resistive output of the opamp (shorting the opamp output). For finding the loop gain the feedback loop must be opened at a suitable node for injecting the test signal at this node. The problem in your circuit is that we have two feedback ways (forget the term "forward" in your figure - its nonsense). Hence, opening both feedback loops (which is, theoretically necessary) will cause problems with the very large open-loop gain Aol of the opamp. Hence, we need a method which would work also for the idealized case with Aol approaching infinite. Therefore, we must treat the opamp with feedback as a finit gain amplifier with an unknown gain which is frequency-dependent and determined by ALL elements. Method: Open the loop at the left (correction: right) node of R3 and inject the test signal Vin. The ratio Vout/Vin gives you the loploop gain which must have a magnitude of slightly above unity at the zero-phase frequency. Tune this gain with R2. niki - of course, your test circuit cannot work because you have connected an ideal ac source to the low-resistive output of the opamp (shorting the opamp output). For finding the loop gain the feedback loop must be opened at a suitable node for injecting the test signal at this node. The problem in your circuit is that we have two feedback ways (forget the term "forward" in your figure - its nonsense). Hence, opening both feedback loops (which is, theoretically necessary) will cause problems with the very large open-loop gain Aol of the opamp. Hence, we need a method which would work also for the idealized case with Aol approaching infinite. Therefore, we must treat the opamp with feedback as a finit gain amplifier with an unknown gain which is frequency-dependent and determined by ALL elements. Method: Open the loop at the left node of R3 and inject the test signal Vin. The ratio Vout/Vin gives you the lop gain which must have a magnitude of slightly above unity at the zero-phase frequency. Tune this gain with R2. niki - of course, your test circuit cannot work because you have connected an ideal ac source to the low-resistive output of the opamp (shorting the opamp output). For finding the loop gain the feedback loop must be opened at a suitable node for injecting the test signal at this node. The problem in your circuit is that we have two feedback ways (forget the term "forward" in your figure - its nonsense). Hence, opening both feedback loops (which is, theoretically necessary) will cause problems with the very large open-loop gain Aol of the opamp. Hence, we need a method which would work also for the idealized case with Aol approaching infinite. Therefore, we must treat the opamp with feedback as a finit gain amplifier with an unknown gain which is frequency-dependent and determined by ALL elements. Method: Open the loop at the left (correction: right) node of R3 and inject the test signal Vin. The ratio Vout/Vin gives you the loop gain which must have a magnitude of slightly above unity at the zero-phase frequency. Tune this gain with R2. 1 answered Feb 15 '17 at 10:18 LvW 15.4k22 gold badges1313 silver badges3232 bronze badges niki - of course, your test circuit cannot work because you have connected an ideal ac source to the low-resistive output of the opamp (shorting the opamp output). For finding the loop gain the feedback loop must be opened at a suitable node for injecting the test signal at this node. The problem in your circuit is that we have two feedback ways (forget the term "forward" in your figure - its nonsense). Hence, opening both feedback loops (which is, theoretically necessary) will cause problems with the very large open-loop gain Aol of the opamp. Hence, we need a method which would work also for the idealized case with Aol approaching infinite. Therefore, we must treat the opamp with feedback as a finit gain amplifier with an unknown gain which is frequency-dependent and determined by ALL elements. Method: Open the loop at the left node of R3 and inject the test signal Vin. The ratio Vout/Vin gives you the lop gain which must have a magnitude of slightly above unity at the zero-phase frequency. Tune this gain with R2.