Two-Pole Compensation
Part One: Basic Two-Pole Compensator
by Dennis L. Feucht
Innovatia Laboratories
High-performance feedback amplifiers require high loop gain over a wide frequency range. Dominant-pole (single-pole) compensation reduces gain appreciably at higher frequencies because the pole must be placed at a relatively low frequency in order to decrease the loop gain to one at a desirable phase margin. The dynamic response may then be acceptable but the side-effect is that, except at low frequencies, the loop gain rolls off, and high-frequency amplifier performance suffers. The benefits of feedback are retained by keeping the loop gain high over the frequency range of interest. If loop gain is too low at higher frequencies, then distortion (or nonlinearity) is high and noise rejection low. For an ADC interface, bits of accuracy and SNR will be lost at higher frequencies, and for the audiophile, the cymbals will sound "tinny."
The two-pole compensation technique sustains high gain to a higher break frequency, where it then rolls off at -40 dB/decade (-2 slope) followed by a zero that restores the magnitude to that of dominant-pole compensation. The difference is shown in the Bode magnitude plot below.
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