Looking deeper at control loop theory for switching regulators, the above plot shows the open loop response of a switching regulator. The error amplifier has been removed, and simply perturbing the control signal into the modulator, the modulator has some gain. In this case it’s a little bit more than 20 dB, and included is the response of the LC filter on the output. There is a double pole at the LC double pole frequency, and then there’s an ESR zero. For a switching regulator 20dB of gain is not enough to provide good line and load regulation, so an error amplifier needs to be added to increase DC gain.