Open-circuit time constant method

It is a special case of zero-value time constant (ZVT) method technique when reactive elements consist of only capacitors.

The OCT method provides a quick evaluation, and identifies the largest contributions to time constants as a guide to the circuit improvements.

[1] If all the poles are real and there are no zeros, this approximation is always conservative, in the sense that the inverse of the sum of the zero-value time constants is less than the actual corner frequency of the circuit.

The open circuit time constant procedure provides the linear term in jω regardless of how complex the RC network becomes.

This was originally developed and proven by calculating the co-factors of the admittance matrix by Thornton and Searle.

Also, the exact transfer function can be compared with the approximate one, that is, Of course agreement is good when the assumption τ1 >> τ2 is accurate.

The open-circuit time constant method focuses upon the corner frequency alone, but as seen above, estimates for higher poles also are possible.

Application of the open-circuit time constant method to a number of single transistor amplifier stages can be found in Pittet and Kandaswamy.

Figure 1: Simple RC circuit and auxiliary circuits to find time constants
Figure 2: Comparison of OCTC estimates with actual pole positions