An optimization approach to designing OTAs for low-voltage sigma-delta modulators

High performance operational transconductance amplifier (OTA) structures for high-speed, low-voltage, switched-capacitor (SC) based sigma-delta modulator applications have been designed utilizing an efficient circuit optimization strategy presented in this paper. The main idea is to use external programs together with a circuit optimizer to construct flexible optimization algorithms. Optimization goals and constraints are derived from analytical equations and application requirements. Using a 0.35 (mu)m CMOS processing technology a complementary folded cascode feedforward compensated (CFCFC) OTA with 2.5 V supply obtained 470 MHz unity-gain frequency, 350 V/(mu)s slew rate, over 74 dB gain, and over 70° phase margin using 10 pF load capacitances. The optimization strategy was verified by measuring fabricated CFCFC OTA and sigma-delta modulator. The measurement results demonstrate a good agreement with the simulations