A Robust Feedforward Compensation Scheme for Multistage Operational Transconductance Amplifiers With No Miller Capacitors

A multistage operational transconductance amplifier with a feedforward compensation scheme which does not use Miller capacitors is introduced. The compensation scheme uses the positive phase shift of left-half-plane (LHP) zeroes caused by the feedforward path to cancel the negative phase shift of poles to achieve a good phase margin. A two-stage path increases further the low frequency gain while a feedforward single-stage amplifier makes the circuit faster. The amplifier bandwidth is not compromised by the absence of the traditional pole-splitting effect of Miller compensation, resulting in a high-gain wide-band amplifier. The capacitors of a capacitive amplifier using the proposed techniques can be varied more than a decade without significant settling time degradation. Experimental results for a prototype fabricated in AMI 0.5m CMOS process show dc gain of around 90 dB and a 1% settling time of 15 ns for a load capacitor of 12 pF. The power supply used is 1.25 V.