Presenting Systematic Design for UWB Low Noise Amplifier Circuits

Yadollah Rezazadeh, Parviz Amiri, Parisa Momen Roodaki & Maryam Baghban kondori 1 Shahid Rajaee Teacher Training University, Electrical and Computer Engineering Department, Lavizan, Babaee Highway, Tehran, Iran 2 Amirkabir University of Technology, Department of Electrical Engineering, Hafez Avenue, Tehran, Iran 3 Space Research Institute (SRI) of Iranian Space Agency (ISA), Sa’adatabad, Tehran, Iran


Introduction
Low noise amplifier is the most important component of the receiver, due to the fact that it can determine the total noise level of the receiver system.Designing this part includes many constraints which make total design challenging.Some of these considerations are low noise level, sufficient gain without contributing much noise, input and output matching improvement and low power consumption.Also telecommunication technology improvement and increasing data transferring rate, require new designs working in wide and high frequency bands which make design more complicated.So it seems that designing a systematic approach which comply all of these considerations in wide and high arbitrary band of frequency is useful.
One of the major applications of wide frequency bands is Ultra-Wideband (UWB).It may be used to refer to any radio technology having bandwidth exceeding the lesser of 500 MHz or 20% of the arithmetic center frequency, according to Federal Communications Commission (FCC).This band was first used by military of America but because of attractive advantages and technology improvement, FCC authorized the unlicensed use of UWB in 3.1-10.6GHz.
The purpose of this paper is presenting a systematic design for LNAs which are used in any frequency bands of ultra wide band, based on mathematical relations.
Any low noise amplifier has three parts which must be designed carefully.The first and important part of LNA is input impedance matching.Design theory and the relevant equations for input matching are given in section (2).Second is designing the transistor core of the amplifier for amplifying received signal, this part is designed in section (3) and (4).After input impedance matching and amplifier designing, output impedance matching has an important role in LNA design.This part is considered in section (5).At the end the accuracy of this method will be shown by simulation results for 2 selected bands in section (6).

Input Impedance Matching
Input and output of LNA must possess a stable input and output impedance for about 50 Ω over the frequency range of interest.There are a lot of ways for input impedance matching.One of the simplest methods is using simple resistor, but as you know thermal noise of resistor will increase noise level and power dissipation directly.Using shunt feedback resistor is another way of input matching which could reach good noise performance and suitable gain with using multi stages.One of the most attractive methods is inductive degeneration which was first introduced for narrow band amplifiers.Using band pass filter instead of low pass will increase band width of this method, which our design approach is based on this point (Lee et al., 2005).Proper design could provide the central fre

LNA Structure and Transistors Size
In order to isolate input and output of the amplifier and providing suitable gain, without occupying a lot of space on the chip, we choose cascode structure for the core of the amplifier.As it is shown in Figure 2, the band pass filter must be stuck to the cascode stage.So gate-source capacitance of first transistor could have the same function as C 1 in band pass filter.Now we must choose the size of transistors.As we define C 1 equal to C gs , by using C gs formula the width of transistor will be found (Razavi, 1998).
For take of simplicity, the size of transistors in cascode structure must be equal.

Evaluating Ls, Lg and Biasing Voltages
According to Figure 2, the input impedance of the first transistor (M1) must be calculated.It is shown in Equation ( 8).
) 1 ( It is clear that imaginary part of input impedance must be zero in resonance frequency but real part of it must be equal to 50 Ω.
Simplifying real part of this impedance and paying attention to formulas, we could find good values for Ls and V eff (which effective voltage is Vgs-V th0 ).
Choosing low values for Ls will increase effective voltage which directly increases biasing voltages and power consumption.By choosing a good value for L s , V eff will be identified.Since L 1 in band pass filter is equal to L s +L g in Figure 2 and it must resonance with C gs in resonance frequency, L g value will be found.) V bias1 and V bias2 and V dd will recognize.

Output Impedance Matching
A simple way for output matching is using source follower stage (Roodaki et al., 2008).Output impedance of this stage is simply defined by 1/g ms where g ms a gate-source trans-conductance of the source follower.According to value of g ms and current formulas, current bias will choose simply.
Figure 2 shows the overall schematic of LNA.We assume L RFC equal to 2nH.Since we addressed this method as a systematic approach, it must operate in all arbitrary bands.For testing this approach in other bands one of the most usage bands in UWB was selected.3-5 GHz simulation results are shown in Figure 3(b).As you see S11 is again less than -10 dB and S22 is les than -15 dB.Noise level of LNA in this band is less than 0.8 dB and shows very good noise performance.S21 is about 12 dB in average.

Conclusion
Based on the theoretical and mathematical analysis, a systematic procedure for design of UWB CMOS LNA was presented.The designed LNA for arbitrary band of frequency achieves up to maximum 10 dB power gain with a suppressed NF less than 2 dB and provides good input and output matching less than -10 dB.The designed LNA consumes only 8.5 mw with 0.85 v supply voltage. Figure

Considering
Figur