cut the feed forward in the miller compensation with voltage buffer

Posted on Feb 6, 2014

An internally compensated operational amplifier is described which has been fabricated using n-channel Al-gate MOS technology. Only enhancement mode devices are used, and the circuit has been designed so that its performance is insensitive to process parameters. A block diagram of the operational amplifier is shown in Fig. 1. The input stage is a

cut the feed forward in the miller compensation with voltage buffer
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source-coupled differential amplifier. The differential output signal of this stage is applied to a differential-to-single-ended converter, which develops a single-ended version of that signal. The converter drives a cascode stage, around which feedback is applied through a source follower and a capacitor, to implement a Miller compensation scheme. Another source follower, fed from the cascode stage, drives the output stage. The bias point interdependence of the stages is such that their quiescent voltages track one another, so that all stages except the last two remain Frequency compensation of the circuit is accomplished through the capacitance C, shown in Fig. 1, which is Miller multiplied by the cascode stage and used to create a dominant pole in conjunction with the output resistance of the differential-to-single-ended converter. The compensation capacitor C is not connected directly between the input and output of the cascode stage because for an inverting gain stage with simple pole splitting compensation, the compensation capacitor, in addition to introducing a dominant pole, also introduces a right half plane zero corresponding to ‰z = gm/C, where gm is the transconductance of the stage and C the compensation capacitance. The right-half plane zero is clearly undesirable, since it degrades the phase of the complete amplifier by 90 ° at high frequencies, while at the same time eliminating the effect of the dominant...

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