Optimal design of operational transconductance amplifiers with application for low power ΔΣ modulators

From the beginning of analog integratedcircuit design around 1960 up to present the operational amputier (op-amp) has been the most important analog building block. The operationaltransconductance amplifier (OTA) as stand-alone circuit has been introduced with the developmentof switched-capacitorcircuits in the early 1980s. For the ideal OTA the differential input voltagesteers the Output currentand so the transfer functionofan OTA stage is defined by its transconduc¬ tance and the Output load. OTAs are stabilized by their load and the OTA power consumption can be optimized for the load, whereas opamps need internal compensation for stabilization to approach load independent behavior. Because the OTAs are not subject to prior Performanceloss due to internal compensationthey can achieve superior Performancewith high impedance environments, such as switched capacitorsampled-datacircuits. High Output impedance is an important OTA requirement for many practical applications. The regulated cascode technique Sup¬ ports this requirement very well. By creation of an inner feedback loop with another amplification stage the OTAs DC-gainand thereby the Output impedance can be boosted to nearly ideal level without compromising the frequency response. With this technique the tradeoff between DC-gain and speed in amplifier design can be resolved. While the regulating amplifier delivers the required additional DCgain, the main amplifier can be optimized for speed at a given power or, vice versa, for power at a given speed. The optimization of the