type 3 error amplifier

Type III error amplifier starts with a pole at 0 Hz (for good regulation), a double zero
below the loop resonance (this decreases the phase lag of the amplifier, which is needed
to compensate for the large phase lag associated with the LC resonance), and a double
pole above the loop resonance to ensure low gain at high frequencies. The low frequency
gain is controlled mainly by R1 and C5. A gain of 26 dB at 100 Hz was
obtained by selecting 5600 pF for C5, and 10 kohms for R1. The double zero was
placed below the measured resonant frequency. One zero is determined by R3 and
C5, which placed it at 1.4 kHz. Since C5 is already specified, R3 was chosen to be
22.1 kohms. The second zero, also at 1.4 kHz, is determined mostly by R1 and C3.
Since R1 is already specified, C3 was chosen to be 0.01 uF. Theory suggests that the
double pole should be in the same ratio to the crossover frequency (the frequency at
which the loop gain is 0 dB) as the crossover frequency to the double zero. One pole
was placed at 16.6 kHz by setting R2 to 499 ohms (R2 and C3 determine this pole),
while the second was placed at 32 kHz by the choice of 470 pF for C4 (although C5
has some effect, this pole is mainly determined by the combination of C4 and R3).
These final values for these components were chosen, partially by trial and error, to
obtain an acceptable Bode plot.