A low-voltage and stable phase compensation technique to realize an 99 dB, 650 MHz and 1.8 V three-stage Amplifier

A 1.8-V operational 3-stage Amplifier with a 99 dB of voltage gain, a 650 MHz of the unity-gain frequency bandwidth and 54 degrees of phase margin is realized. It has two pairs of a resistor and a capacitor in series as feedback elements and a feed-forward transconductance amplifier to perform threefold phase compensation. In order to apply this threefold phase compensation scheme to realize a high gain and high frequency 3-stage amplifier with enough phase margin, the small signal equivalent circuit model with stray capacitors at the output of each stage of amplifier is newly developed and the small signal transfer function including those stray capacitors are formulated. The transfer function is factorized into poles and zeros so that their relationship and influence on the frequency characteristics can be examined. The 3-stage amplifier is actually circuit designed by using 0.18μm CMOS devices. The calculated gain and phase frequency characteristics from our equivalent circuit model with stray capacitors have agreed very well with SPICE simulation results of the actual circuit especially at high frequencies above 1 GHz.