Design considerations for CMOS and GaAs OTAs: Frequency response, linearity, tuning, and common-mode feedback

Abstract

High-performance operational transconductance amplifiers (OTAs) are important in the design of high-frequency analog transconductance-C (gm-C) filters. Critical design considerations for OTAs are frequency response, linearity, tuning, output impedance, power supply rejection (PSR), and common-mode rejection (CMR). In CMOS technology, satisfactory OTA design techniques are available, except that the linear input range often is relatively small and the frequency response of the OTA is limited by the intrinsic speed of transistors. In this paper, a new approach is developed to increase the linear input range, and a trade-off between linearity and input range is discussed. A CMOS OTA with less than ±0.4% linearity error over a very large input range is given as a design example. To achieve a very high frequency response, 1μm depletion-mode GaAs MESFETs with high intrinsic speed are used to replace MOSFETs. Simple ac compensation, a new technique for output impedance enhancement, and a new tuning method for OTAs with all N-channel devices are used to design a GaAs OTA with very small parasitics and f−3dB=7 GHz. To improve PSR and CMR, fully balanced structures are used for the OTAs. Design considerations for the interaction of the operation of common-mode feedback (CMF) and tuning are discussed, and improved CMF circuits are proposed. Using the GaAs OTA and considering the frequency limitations imposed by parasitics, the design of a high order ladder filter with 300MHz cutoff frequency is presented as an application.