I. Deep negative feedback
An operational amplifier whose output voltage
where A is the open-loop gain of the op-amp.
For the negative feedback circuit shown above, with a feedback factor of F, its negative phase input voltage
(1) and (2) can be combined to obtain
Where 1+AF is called the feedback depth, when 1+AF is much greater than 1, the circuit is said to be in a deep negative feedback state.
For the ideal op-amp, its open-loop gain A is close to infinity, therefore, the feedback depth 1 + AF is much greater than 1, the circuit is in a deep negative feedback state, at this time
Second, the virtual short
can be obtained from (4), in the op-amp in the condition of deep negative feedback, the
A comparison of (2) and (5) yields the following relationship between the voltage at the in-phase input of the op-amp and the voltage at the inverted input.
This is the virtual short. That is, in the ideal case, the two input potentials of the op-amp are approximately equal.
Third, the virtual break
The virtual break is due to the infinite input resistance of the ideal op-amp, resulting in its two input currents being approximately zero, as if it were disconnected.
IV. Simulation Analysis
From the simulation results of the circuit above, we can see that the ideal op-amp’s two input currents are approximately zero, so the voltage at the in-phase input is approximately the DC supply voltage, and under the condition of deep negative feedback, the potential at the two inputs of the op-amp is approximately equal, both about 1mV.
