CMOS LNA with Tunable Resonance Frequency Florentina AGAVRILOAIE 1 , Silvian SPIRIDON 1 , Claudius DAN 2 , Mircea BODEA 2 1 PhD Student, 2 Professor “POLITEHNICA” University of Bucharest, ETTI Dept., Bucharest, Romania Abstract – The tuned Low Noise Amplifier (LNA) is an attractive solution for applications asking for both low power consumption and low noise figure (NF), such as wireless sensors. This paper presents the CMOS LNA with a tunable resonance frequency analysis focused on noise-linearity-bandwidth trade-offs required for imple- menting a variable resonance frequency. To change the resonance frequency the MOS capacitor capacitance dependence on the DC gate voltage is used. By employing a MOS capacitor bank, the proposed LNA concept can cover a large frequency band, in a few programmable steps. I. INTRODUCTION The use of fully integrated tuned LNA in today’s wireless receivers has been thoroughly investigated [1-3]. The tuned amplifiers main advantage is the low NF achieved at low levels of current consump- tion. To trade-off such low figures, a large die area is envisaged for the monolithic inductors. Since multiband operation is a common requirement of wireless interoperability a tuned LNA with variable resonance frequency represents a suitable solution, in order to optimize this trade-off. Section II analyses the proposed CMOS tuned LNA concept. To avoid the design process intricacy related to deep sub-micron channel effects, mainly the degradation (a) of noise performance due to the gate-induced drain noise [4] and (b) of electron mobility under the influence of longitudinal electric field high values [5], the proposed tuned LNA concept was studied in a sub-micron CMOS process (> 130 nm). Also, Section II estimates the minimum power consumption of a wireless amplifier required to achieve a NF smaller than 3, 5 or 8 dB, while in the same time being able to switch between the desi- red frequency bands, for example, the ISM bands up to 2.5 GHz [6]. The LNA linearity specifications are not very severe, because such tuned amplifier will be used only if a high receiver gain is required. More, since one of the tuned amplifier target appli- cations is the wireless sensors circuits, which typica- lly employ a frequency modulation communication, quite a poor LNA linearity can be tolerated, [7]. The LNA’s resonant frequency is changed using the MOS capacitor capacitance dependence on the DC gate voltage, as is explained in Section III. Section IV evaluates the effects of tunable resonance freque- ncy on the amplifier parameters. Finally Section V concludes the paper by presenting an overview of the proposed solution. II. TUNED LNA ARCHITECTURE ANALYSIS Since the analysis focuses on CMOS LNAs, the designer has to choose either the common gate (CG) or the common source (CS) stage. The common collector (CC) stage is ruled out because it does not provide voltage amplification, [8]. Although, both CG and CS topologies offer the same intrinsic voltage gain, the CG stage suffers a noise penalty as its g m is locked by input impedance matching constraints, [9]. Because the CG stage input resistance is set to approximately 1/g m , the designer looses one degree of freedom, being unable to tweak anymore the circuit noise performance. Fig. 1 presents the generic CS stage LNA archi- tecture. The stage is based on the pseudo-differential pair (M1-M2) with resistive load (R L ) and inducti- vely degenerated source (L S ), two series gate induc- tors (L G ) and two capacitors (C') placed in paralleling the gate-source capacitance of the input devices. The differential structure has been chosen for its higher linearity, because it cancels out even order harmonics as opposed to a single ended stage. Fig. 1 – Tuned LNA Architecture (Biasing not shown) II.1. Input Impedance and Gain The differential input impedance of the schematic is given by:    C L g sC L L s s Z S m S G in     1 , (1) where C represents the total capacitance between the CS transistors gate and source, C = C' + C gs . The resonance occurs at the frequency ω 0 where the input impedance imaginary part cancels out:   S G L L C   1 0  (2) Hence, by making C variable the resonance frequency can be tuned, as explained in Section III. R L R L C` C` L S L S L G L G RF+ RF- M1 M2