I'm trying to develop a Buck Converter with a Voltage-Mode Control, using a Type III Compensator, just like the one below.
So I calculated its transfer function, and then with the correct values of poles and zeros I calculated the values of the passive components, to get the expected frequency response. My problem is: When I simulate the circuit in LTspice using an ideal and real (OP27 by Analog Devices) Op. Amp., this is the magnitude response that I get.
I'm not understanding why the gain at low frequencies is dropping so much. Why is the real circuit not behaving like the ideal one?
Vout1 is the output of ideal of the ideal one and Vout3 is the output of the real one. In the first image, Vout' is actually the input of the compensator and \$V_c\$ its output. So in the graph, it should be vc1 and vc3, but I think now it became clear.
Here are the schematics:
U1, is just a VCCS plus a parallel RC underneath. It will work for mV, GV, anywhere, flawlessly. Try using
UniverslOpamp[2,3a,3b]for better results. Those will be influenced by inappropriate biasing. \$\endgroup\$