In op-amp inverting circuits, we can think of the op-amp output voltage as of extra voltage equal to the voltage drop across the positive element connected between the output and the inverting input (R2, in the case of an inverting amplifier). This made me present, in the dual question below, my speculation that the op-amp output acts as an element with negative impedance that neutralizes the positive impedance of the element connected between the output and the inverting input (R2, in the case of an inverting amplifier). As a result, the combination of both the (positive and negative) elements has zero total impedance:
https://www.researchgate.net/post/Does_the_op-amp_in_all_the_inverting_circuits_with_negative_feedback_behave_as_a_negative_impedance_element_negative_resistor_capacitor_etc
In op-amp non-inverting circuits (the first picture below), there is a little difference - the op-amp output voltage VOA contains within itself the input voltage VIN. So, it can be presented as a sum of two ingredients - the input voltage VIN and an additional voltage VNR (see the second picture). The extra voltage VNR is equal to the voltage drop VR2 across the positive element connected between the output and the inverting input (R2, in the case of a non-inverting amplifier). As a result, it compensates the voltage drop across the upper resistor R2, and the voltage drop across the lower resistor R1 is always equal to the input voltage... as though, VNR is a negative impedance that neutralizes the positive impedance R2.
So, my new speculation is that the op-amp output in the non-inverting circuits acts as a voltage source with negative impedance that neutralizes the positive impedance of the element connected between the output and the non-inverting input.
A typical application of this arrangement is the compensation of the line resistance (distant voltage sensing, Kelvin connection) where Rl represents R2, and RL - R1 (negative line resistance)... or virtually decrease of the op-amp output resistance where ROUT is R2, and RL - R1 (negative op-amp internal resistance). If the resistor R2 is replaced by a capacitor, diode, base-emitter junction, etc., the op-amp output will act accordingly as a negative capacitor, negative diode, negative base-emitter junction, etc... This was the idea of the funny Flash movie Strange things:
http://www.circuit-fantasia.com/collections/tools/strange-things.html
I want to thank Prof. Lutz von Wangenheim because he gave me, with constant allusions, the impetus to embody this idea in a dual question. My strong desire is to generalize it for all circuits with negative feedback...
I think you should analyze input impedance of transistor circuit inside opamp because every opamp has its own input impedance
Kittipong, the problem is that I do not see any connection between the op-amp input impedance and this idea...
I have exposed my speculation that all circuits with negative feedback possess negative impedance in the most general question below:
https://www.researchgate.net/post/Does_the_amplifier_in_negative_feedback_systems_possess_negative_impedance?
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