Cancellation of OpAmp Virtual Ground Imperfections by a Negative Conductance Applied to Improve RF Receiver Linearity

Abstract

High linearity CMOS radio receivers often exploit linear V-I conversion at RF, followed by passive down-mixing and an OpAmp-based Transimpedance Amplifier at baseband. Due to nonlinearity and finite gain in the OpAmp, virtual ground is imperfect, inducing distortion currents. This paper proposes a negative conductance concept to cancel such distortion currents. Through a simple intuitive analysis, the basic operation of the technique is explained. By mathematical analysis the optimum negative conductance value is derived and related to feedback theory. In-and out-of-band linearity, stability and Noise Figure are also analyzed. The technique is applied to linearize an RF receiver, and a prototype is implemented in 65 nm technology. Measurement results show an increase of in-band IIP 3 from 9dBm to >20dBm, and IIP2 from 51 to 61dBm, at the cost of increasing the noise figure from 6 to 7.5dB and <10% power penalty. In 1MHz bandwidth, a Spurious-Free Dynamic Range of 85dB is achieved at <27mA up to 2GHz for 1.2V supply voltage.

Design Methodology of N-Path Filters With Adjustable Frequency, Bandwidth, and Filter Shape

“…In this section, a third order Taylor approximation of V out versus V in [36], [37] is derived. It is assumed that the second order non-linearity is negligible in fully differential configuration.…”

A cross-coupled switch-RC mixer-first technique achieving +41dBm out-of-band IIP3

“…The switched resistors R M act as a high-linearity passive current commutating mixer whose baseband current is converted to an output voltage by a Trans Impedance Amplifier (TIA). This TIA produces a virtual ground, while impedance matching is provided via the switch resistance and the matching resistor RM [1]. Alternatively, impedance matching with lower noise can be achieved by omitting R M and realizing the required impedance with the TIA [3].…”

CMOS frond-end techniques for in-band full-duplex radio

“…Hence, both the main RX and the VM are based on highly linear passive mixers with series resistors into virtual ground nodes provided by transimpedance amplifiers (TIAs) [50]. Figure 4.3 shows an overview of the implemented receiver.…”

“…However, they perform such that under 27dB cancellation, the mixers will dominate the IM3 performance by a large margin. In addition, the TIAs were further linearized by a di↵erential negative conductance at their inputs [50]. While not strictly necessary for this application, it allows us to eliminate the TIA as a linearity bottleneck in measurements and study the raw linearity achieved by the mixers, even with cancellation disabled.…”

“…Figure 4.7 shows an implementation detail of one fully di↵erential VM slice for one LO phase, and one of the negative-conductance-assisted TIA's. The TIA's are implemented as high-gain, two-stage OTA's with a telescopic input stage and a push-pull output stage [50].…”