I have seen a number of opamps in power supply designs (SMPS) that are merely used for 'controlling' the transistors connected in flyback topologies at the primary windings of transformers. The non inverting terminal of the opamp is held to a reference voltage (controlable via a potentiometer), while the inverting terminal is connected to the final 'regulated output'. I have observed that such opamps are mostly WITHOUT any feedback - from the opamp output to the inverting terminal. Apparently such opamps are controlling the input voltages at the primary windings of transformers, on the basis of regulated output voltage that is. So, that means the opamp is subtracting the reference voltage (at its +ve end) from the regulated output voltage (at its -ve end). Without an immidiate feedback, how can an opamp behave as a differentiator?