26.7 An Impedance-Transforming N-Path Filter Offering Passive Voltage Gain
Mohammad Khorshidian, Harish Krishnaswamy
Abstract
The four main passive circuit elements include the resistor, capacitor, inductor, and transformer. Inductors and transformers have been notoriously challenging to integrate in silicon, and they occupy a significant chip area. The limited metallization and conductive substrate result in a poor quality factor (Q) and limited self-resonance frequency. The tuning of inductor-based resonators is further challenged by the limited tuning range and Q of switched-capacitors/inductors. Recent LPTV N-path structures have demonstrated new functionalities that address some of these challenges, such as equivalent second-order LC bandpass filtering, lowpass filtering, notch filtering, and non-magnetic nonreciprocity [1-5]. Transformers face additional challenges of limited coupling coefficients, especially when a large turns ratio is desired [6], but they are critical to impedance transformation and matching networks. This paper describes an impedance-transforming N-path filter offering passive voltage gain, which realizes transformer-based coupled-resonator functionality that (i) features the traditional N-path benefits of high Q, high linearity, and LO-defined tuning, (ii) enables ideal coupling (k=1) regardless of the step-up ratio, (iii) can be cascaded with no transfer-function nonideality to achieve even higher step-up voltage gain with high out-of-band (OOB) rejection, (iv) emulates transformer behavior over a wide range of frequencies despite being single-ended (SE), and (v) has ultra-low clock-power consumption. The concepts are validated through two -0.1-to-1GHz 65nm CMOS prototypes that achieve step-up ratios (n) of 4 and 8 with k of 1, voltage gains of 15dB and 17dB at 500MHz with 27dB and 33dB OOB rejection, and buffer-de-embedded NFs of 3.4dB and 4.1dB at 500MHz, respectively. OP <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">1dB</sub> is 2dBm, and the power consumption is 1.7 to 3.6mW over 0.1 to 1GHz for both prototypes.