Driving Point Loop Gain and Return Ratio

Stability in feedback analog circuits is measured as phase margin obtained from the system loop gain function. While different functions have been used to represent loop gain, generally these functions yield similar phase margins so that the designer is usually not concerned with the particular function used. A development and comparison of two main approaches to finding loop gain, one focusing on a controlled source, Bode’s Return Ratio, the other keeping circuit variables foremost, the driving point impedance approach, leading to different ‘loop gain’ representations is presented. The source of this difference is found, concluding that the approach using a circuit focus analysis produces consistent results for phase margin while the Return Ratio may not making the former the preferred method.