Shown here are the two different error-amp circuits. One (above top) uses an open loop transconductance amplifier (Gm amplifier), and the other (above lower) uses an Op-Amp with local feedback. The Gm amplifier drives a current into a filter network Z(s), generating a frequency compensated voltage (VC) used to control the modulator stage. Note that the gain from VOUT to VC is a function of the feedback ratio. Therefore, if the designer uses this type of stage, the regulator loop gain will increase as VOUT is made smaller. Also note that gain is directly affected by changes in the error amp’s gain. The Op-Amp circuit does not do this. Instead, it has local feedback that establishes a virtual ground at the inverting input. As a result, the AC gain is defined only by Z2(s)/Z1(s), and is not a function of the op-amp’s open loop gain or the feedback ratio of RF1 and RF2. The feedback ratio of RF1 and RF2 defines only the DC gain. Most of the DC/DC controllers use a true op-amp.