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This is an article about bilateral feedback analysis.
Could anyone explain the what is meant by this sentence?
The most important fact about this circuit is the symmetric bilateral transmission through the feedback impedance \$Z_{F}\$ albeit the forward amplifier is still unilateral.
What does "symmetric bilateral transmission" mean here?
Thank you.
What is the purpose of ZF? Answer: To provide a feedback path that produces a voltage across ZS||Zin. This feedback voltage is superimposed with a corresponding part of the "normal" signal input voltage Vi at the opamp input node.
However, this is an ideal case that happens only if there would be an output impedance Zo=0. If Zo has a finite value, a part of the input signal Vi arrives directly at the output (and NOT only via the forward amplifier Voc). This portion depends on the voltage divider Zo/(Zs+ZF+Zo). This neglects the large Zin.
That means: The feedback path works in both directions: (1) As wanted: From the output back to the input and (2) Not desired but unavoidable: Directly from the input to the output.
By the way: We face the same situation with a feedback resistor connected between the collector node and the base node of a transistor stage. This is the principle behind the so called "Miller-Effect".
It is a remaining question why this example circuit has positive feedback (perhaps just a drawing error ?).
EDIT: In equ. (2) we can introduce the following ideal values: Zin infinite, Zo=0 and the open-loop gain k as infinite. In this case, we arrive at A=-ZF/ZS . This is the classical inverting gain for an ideal opamp with negative feedback. Hence, in the shown circuit we have to exchange the non-inv. and the inv. input (drawing error).
In this context it basically means that Zf transmits signals in both directions.
Look at the amplifier model you are using - just the amplifier, take out all the surrounding components. If you force Vo from the outside, would this produce something at the amplifier input, i.e. across Zin? No.
Now take your Zf, make the same experiment... And you quickly understand that the same thing does not apply.
A unilateral system lets signals propagate in a unique direction, i.e. from one port to another, while a bilateral system allows the signal to go in both directions.
Symmetric here means that you can swap input and output and nothing changes: if you reverse Zf, nothing changes. Think of Zf as a generic two port system. For such a system you usually have two distinct transfer functions, one from port A to port B, the other from port B to port A. If the two transfer functions are identical the system is symmetrical, i.e. its ports are indistinguishable, and if you swap them nothing changes.
This is probably stressed in your textbook because the fact that Zf is not unilateral influences the closed loop gain transfer function in a non trivial way.
