A Fully Passive RF Front End With 13-dB Gain Exploiting Implicit Capacitive Stacking in a Bottom-Plate N-Path Filter/Mixer

A low-power interferer-robust mixer-first receiver front end that uses a novel capacitive stacking technique in a bottom-plate N-path filter/mixer is proposed. Capacitive stacking is achieved by reading out the voltage from the bottom plate of N-path capacitors instead of their top plate, which provides a $2\times $ voltage gain after downconversion. A step-up transformer is used to improve the out-of-band (OOB) linearity performance of small switches in the N-path mixer, thereby reducing the power consumption of switch drivers. This article explains the concept of implicit capacitive stacking and analyzes its transfer characteristics. A prototype chip, fabricated in 22-nm fully depleted silicon on insulator (FDSOI) technology, achieves a voltage gain of 13 dB and OOB IIP3/IIP2 of +25/+66 dBm with 5-dB noise figure while consuming only 600 $\mu \text{W}$ of power at $f_{\mathrm{ LO}}=1$ GHz. Thanks to the transformer, the prototype can operate in the input frequency range of 0.6-1.2 GHz with more than 10-dB voltage gain and 5-9-dB noise figure. Thus, it opens up the possibility of low-power software-defined radios.