I have a problem understanding one thing in op-amps, namely - when the cycle of stabilizing ends?

For example, when we have simple non-inverting amplifier, firstly the difference equals the amount of voltage we supply. It is amplified and through the feedback loop it goes back to the second input of the op-amp, and the difference changes.

It goes on until we reach 0 difference on both inputs. But then the output should become zero, as it can be also expressed as the product of common mode gain (0) and the average of both inputs.

If output is zero now again the difference between inverting and non-inverting terminal is not zero, so it starts to stabilize and so on.

Then when this cycle stops? How the op-amp manages to maintain any constant output if it tries to make voltage at both terminals the same, but by making it it creates output voltage that makes these voltages different.

  • \$\begingroup\$ If the circuit needs 10 volts output, and the opamp has 100,000X gain for DC, then the final input to the opamp will be 10v/100,000 = 100 microVolts. \$\endgroup\$ – analogsystemsrf Jun 9 at 2:27

An op amp does not (in steady state anyway) bring the voltage between the inputs to zero. Because of the large gain, it will bring the voltage to "small enough" to satisfy the control loop. That's usually very close to zero for signals within the useful control bandwidth of the amplifier.

So in normal use there's no oscillation because the input truly goes to zero causing the output to zero. There is a settling time due to the small signal response, and maybe some slew rate effects for large signals.

You can learn a lot more about this by studying control theory. It's not a trivial subject that we can cover in a single answer here.

  • \$\begingroup\$ Even if an op-amp were a pure integrator with infinite DC gain it would still bring the error to zero (well, if you can wait an infinite amount of time). But yes, real op-amps have finite DC gains and thus have a teeny amount of error left over. \$\endgroup\$ – TimWescott Jun 8 at 21:59
  • \$\begingroup\$ @TimWescott Absolutely- You can say that as the gain approaches infinity or an arbitrarily large number the input voltage approaches zero, or an arbitrarily small number. \$\endgroup\$ – John D Jun 9 at 2:29

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