A common tool to measure the performance of switching and linear regulators as well as many different type of control systems, is to measure the loop gain of a system. Every closed-loop system has a characteristic called loop gain. In this analysis of voltage regulator topologies, loop gain will be defined as the magnitude of the voltage gain that the feedback signal experiences as it travels through the loop. A transformer is used to inject an AC signal into the feedback path between points “A” and “B”. Using this transformer, a small-signal sine wave is used to “modulate” the feedback signal. The AC voltages at “A” and “B” are measured and used to calculate loop gain. The loop gain is defined as the ratio of the two voltages: Loop Gain = VA / VB. It is important to note that the signal starting at the VB point has a phase shift introduced into it as it travels through the loop (eventually arriving at the VA point). The amount of phase shift is critical in determining stability.