A fast-settling CMOS op amp for SC circuits with 90-dB DC gain

A technique that combines the high-frequency behavior of a single-stage op amp with the high DC gain of a multistage design is presented. This technique is based on the concept that a very high DC gain can be achieved in combination with any unity-gain frequency achievable by a (folded-) cascode design. Bode-plot measurements for an op amp realized in a 1.6- mu m process show a DC gain of 90 dB and a unity-gain frequency of 116 MHz (16-pF load). Settling measurements with a feedback factor of 1/3 show a fast single-pole settling behavior corresponding to a closed-loop bandwidth of 18 MHz (35-pF load) and a settling accuracy better than 0.03%. This technique does not cause any loss in output voltage swing. At a supply voltage of 5.0 V an output swing of about 4.2 V is achieved without loss in DC gain. The above advantages are achieved with a 30% increase in chip area and a 15% increase in power consumption.<
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