I'm not looking for an answer to a problem here; I'm just giving a particular instance of network theory involving opamps to illustrate an issue that is seldom dealt with. It is of interest to think about.
An opamp circuit appeared on another forum:
If this circuit is solved using the usual "Golden Rules", namely that the opamp open loop gain is infinite, v+ = v- (a consequence of infinite opamp voltage gain) and the inputs draw no current, the voltage gain Vout/Vin is determined to be 5/3 and the output voltage is .12 volts.
If the + and - inputs are reversed and the solution is carried out again using the "Golden Rule" method (without explicitly using the opamp open loop gain, A or Av), the same Vout/Vin is obtained.
In fact, if a solution is obtained with A explicitly involved, such as would be necessary for a solution with a finite A, an expression for Vout/Vin will be obtained. This expression, involving A, can be evaluated in the limit as A approaches ∞, and the same result will be obtained that the earlier "Golden Rule" method gave.
One can obtain an expression for Vout/Vin involving A for the two orientations of the + and - inputs, and the two expressions are different for finite A, but have the same limit as A approaches ∞.
Presumably, one of the orientations is not stable; there is more positive feedback than negative feedback. But, there is no indication of which is which indicated in the solution by the "Golden Rule" method; you get the same solution for both orientations of + and - inputs.
Now, in this rather simple circuit we can see by inspection which is the stable configuration, but what of a more complicated circuit, with several opamps and nested feedback loops? How do we determine stability?
An opamp circuit appeared on another forum:
If this circuit is solved using the usual "Golden Rules", namely that the opamp open loop gain is infinite, v+ = v- (a consequence of infinite opamp voltage gain) and the inputs draw no current, the voltage gain Vout/Vin is determined to be 5/3 and the output voltage is .12 volts.
If the + and - inputs are reversed and the solution is carried out again using the "Golden Rule" method (without explicitly using the opamp open loop gain, A or Av), the same Vout/Vin is obtained.
In fact, if a solution is obtained with A explicitly involved, such as would be necessary for a solution with a finite A, an expression for Vout/Vin will be obtained. This expression, involving A, can be evaluated in the limit as A approaches ∞, and the same result will be obtained that the earlier "Golden Rule" method gave.
One can obtain an expression for Vout/Vin involving A for the two orientations of the + and - inputs, and the two expressions are different for finite A, but have the same limit as A approaches ∞.
Presumably, one of the orientations is not stable; there is more positive feedback than negative feedback. But, there is no indication of which is which indicated in the solution by the "Golden Rule" method; you get the same solution for both orientations of + and - inputs.
Now, in this rather simple circuit we can see by inspection which is the stable configuration, but what of a more complicated circuit, with several opamps and nested feedback loops? How do we determine stability?
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