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

Abstract

<?Pub Dtl=""?> 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 <formula formulatype="inline" xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex Notation="TeX">$_{3}$</tex> </formula> from 9 dBm to <formula formulatype="inline" xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex Notation="TeX">${&gt;}$</tex></formula> 20 dBm, and IIP2 from 51 to 61 dBm, at the cost of increasing the noise figure from 6 to 7.5 dB and <formula formulatype="inline" xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex Notation="TeX">${&lt;}$</tex></formula> 10% power penalty. In 1 MHz bandwidth, a Spurious-Free Dynamic Range of 85 dB is achieved at <formula formulatype="inline" xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex Notation="TeX">${&lt;}$</tex></formula> 27 mA up to 2 GHz for 1.2 V supply voltage.