analogrf wrote on Apr 14^th, 2011, 8:33am:


It can occur that a particular pole or zero is associated with a particular node or can be approximately associated with a particular node.  However, in general, a specific pole or zero is not associated with a specific node.  It is possible to derive transfer functions in symbolic factorized pole-zero form which allows the poles and zeros to be identified in terms of the circuit elements.  The derivation is performed primarily with pencil and paper.

analogrf wrote on Apr 14^th, 2011, 11:04am:


The PZ simulation is always performed on a small signal model.  This small signal model can be one created by the simulator from the DC operating point of a circuit with non-zero DC biasing.  Or it can be the small signal equivalent circuit entered into the simulator by the user.  In either case, if the user manually derives a transfer function, then the user will find that , in general, the poles and zeros of that transfer function do not have a one-to-one correspondence to individual nodes of the circuit.  The poles and zeros are, in general, functions of any of the elements in the circuit.  

A pole or zero in radians per second is equal to the reciprocal of a time constant.  For example, a simple RC time constant.  The R can be between a pair of nodes in the circuit and the C can be between a different pair of nodes.  In this  example, then, their are 4 nodes which correspond to the pole or zero.  And therefore, I think the question of tracking which node is associated with a pole or zero can only be answered by explaining that the question contains an incorrect assumption.

There are situations in which both the R and the C are connected between a node and AC ground, and there are situations where this is an appropriate approximation, but, it is not the general case.