Analysis and Design of N-Path Band-Pass Filters With Negative Base Band Resistance

Abstract

This paper reviews the possibility of adding an active circuit that implements a small signal negative resistor, to the baseband portion of the N-path filter circuit, in order to compensate for losses that are caused due to harmonic products and other parasitic effects. By adding an active circuit inside the baseband part insertion loss can be eliminated reciprocally. Interestingly, in the case of two-port configuration, in addition to the improvement in insertion loss, a lower noise figure can be theoretically achieved as well. The introduction of the negative resistance is analyzed using linear periodically time-variant theory and linear time invariant approximation. The theoretical analysis is verified in simulation and measurement. The circuit implementation consists of a two-port N-path filter, implemented in a 65-nm CMOS process, with a PMOS cross-coupled pair serving as the negative differential resistor. We achieve $\sim 3.5~dB$ insertion loss improvement at the expense of $0.64~mW$ power addition, at the frequency range of 0.75–2 GHz.