Analyzing feedback: Properly simulating the open loop

Relative stability in a feedback system is usually characterized by its phase margin, a quantity obtained from the system 'open loop gain'. This terminology, while appropriate for ideal systems, is misleading when applied to real systems where loading and feedforward effects are lost if the loop is simply opened. This paper presents a new and complete method for finding the fully loaded loop gain for a feedback system using the driving point impedance/signal flow graph methodology. An analytical solution is presented first and then used to define properly loaded coupled circuits for computer simulation. The resulting analysis is exact limited only by the device models and CAD package.