Frequency Compensation

Abstract

The input, output and intermediate stages of low-voltage OpAmps, as described in the previous Chapters, all have collector outputs and hence contribute dominating poles to the transfer from input to output. If there are two or more dominating poles, the phase margin of the frequency response is negative, and overall feedback of the OpAmp is not possible. To change this, frequency compensation techniques must be applied that give the OpAmp one dominating pole frequency with a straight 6-dB/octave frequency roll-off. The technique of narrow-banding causes an unacceptable reduction of the bandwidth and is besides difficult to implement in an monolithic integrated circuit. The pole-zero cancelation technique is also unusable because the values of the poles fluctuate with the output current, which would make the cancellation very inaccurate, giving rise to pole-zero doublets and hence slow settling of the step response. The most effective way to compensate the OpAmp is by applying the technique of pole splitting[1]. Simple pole spitting, using one Miller capacitor is discussed in the first Section. The second Section deals with nested Miller compensation for OpAmps consisting of three stages. In the third Section multi-path-driven Miller compensation is introduced. This technique combines the higher bandwidth of the Miller-compensated OpAmp with the higher gain of the nested Miller-compensated OpAmp. Finally, the phenomenon of slewing is the topic of the fourth Section.