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Here’s an example where an error amplifier has been added, resulting in a high DC gain. There needs to be a pole at the origin to make sure that the gain rolls off at higher frequencies. The engineer is going to implement is what is called a “type two” compensation. This means a pole at the origin, to knock down the gain and then a zero at the double pole frequency to help offset the double pole. At the double pole frequency the compensation has a single zero, but one more zero is required for stability. The ESR of the output capacitor to act as the second zero to offset the double pole. In this example, type two compensation works very well to create a stable system. This is because the ESR of the output capacitor is large enough to allow us to have proper compensation at the right frequency. In a sampling system, there are Nyquist poles that are not necessarily modeled in the equation shown earlier. The phase including the Nyquist poles is shown above as the sold red line and the dotted line is what the control loop might look like if it had an amplifier feeding in to the inductor and capacitor instead of a switching sampling system.
