A Loop-Breaking Method for the Analysis and Simulation of Feedback Amplifiers

Two-circuit models for feedback subcircuits are developed from a modification to the two-port method of feedback amplifier analysis and are applied in a method to break the feedback loop. These models are terminally equivalent to linear and nonlinear subcircuits, have physically-disconnected topologies, and employ dependent sources to control lateral signal transfer with a switch-like on or off manner. With feedback subcircuits replaced by these models in a SPICE simulation, it is possible to keep the amplifier biased at its closed-loop dc operating point while it operates in an open-loop or a closed-loop ac configuration. Small-signal analysis of the amplifier’s equivalent circuit yields exact expressions for open-loop and closed-loop response functions, and the loop-gain.