Negative Lumped Element Matching Technique for Performance Enhancement of Ultra-Wideband LNA

Kishor G Sawarkar,Kushal Tuckley

1 Research Scholar at Department of Electronics and Telecommunication,MCT's RGIT,University of Mumbai,Mumbai 400054,India

2 Chairman R&D AGV System,Adjunct Professor at IITB Mumbai,400076,India

Abstract:The paper aims at designing of two stage cascaded ultra-wideband (UWB) low noise amplifier (LNA) by using negative image amplifier technique.The objective of this article is to show the performance improvement using negative image amplifier technique and realization of negative valued lumped elements into microstrip line geometry.The innovative technique to realize the negative lumped elements are carried out by using Richard's Transformation and transmission line calculation.The AWR microwave office tool is used to obtain characteristics of UWB LNA design with hybrid microwave integrated circuit (HMIC) technology.The 2-stage cascaded LNA design using negative image amplifier technique achieves average gain of 23dB gain and low noise Figure of less than 2dB with return loss less than -8dB for UWB 3-10GHz.The Proper bias circuit is extracted using DC characteristics of transistor at biasing point 2V,20mA and discussed in detail with LNA layout.The negative image matching technique is applied for both input and output matching network.This work will be useful for all low power UWB wireless receiver applications.

Keywords:LNA; ultrawide band; pHEMT; matching network; HMIC

I.INTRODUCTION

In today's world LNA design innovation is developing rapidly in the field of electronics and telecommunication services.Since,the interest for fast advancing in multimedia communication has moved towards research a greater demand in the field of RF and Microwave engineering.In most of the LNA design techniques Pseudomorphic High Electron Mobility Transistors (pHEMT) and High Electron Mobility Transistors (HEMT) transistors provides promising results at microwave frequencies.An LNA plays the vital role in overall performance of RF receiver.It is the first component in any RF receiver.The general topology of LNA consists of three stages:input matching network,the amplifier itself and output matching network [6].

Recently InGaAs based pHEMT have exhibited better linearity,high survivability,less noisy and better performance than GaAs FET's [7][8].InGaAs based pHEMTs offering less microwave noise Figure with a very less Power Consumption and high transducer gain at low voltage and current.The Two critical factors in this regard are high gain and minimum gate current.Since gate current is a component contributing to shot noise,decreased leakage gate current improves the noise performance of the HEMTs [9].Several amplifier architectures are considered which includes distributed amplifier,lumped feedback amplifier,cascode and cascade topology [10-15].For UWB,the distributed amplifier requires more number of transistors than the lumped feedback which consume more power.The distributed amplifier likewise languishes low efficiency when designed for high gain and normally higher noise performance.In this paper two stage cascaded UWB LNA is designed without compromising noise Figure,gain and return loss.

Stephen A.Maas [19],in their application note has described negative-image modeling design for LNA.Designed LNA using real-time tuning and advanced genetic optimizer in AWR microwave office tool.Kim et.al [20],designed power amplifier using negative-image amplifier model which incorporates optimization techniques for circuit performance and negative-image device models.Also,derived mirror images of the optimized negative-image matching circuits.B.Guo [21],designed wideband LNA with highlight of excellent linearity,gain and noise figure.

This paper is organized into several sections.Section 2 discusses design,simulation and analysis of UWB LNA.The section 3 describes significant measurements and simulated results of the presented LNA.The section 4 emphasizes novelty and innovation of the article and section 5 comprises the conclusive remark of the designed work.

This paper proposes a novel procedure of acknowledgment of the negative valued lumped elements by utilization of the microstrip line parasitic effects.

II.UWB LNA DESIGN SIMULATION AND ANALYSIS

This section explains the design of the UWB LNA design with the necessary steps involved in the design and simulation process.Moreover,it consists of following subsections as complete schematic,layout,biasing topology,input and output negative value element matching circuit with realization.Finally,it presents results which includes stability,noise Figure,gain,input and output return loss.

Figure 1 shows the complete architecture of the design UWB LNA.The Avago technologies ATF-36163 is a low-noise pHEMT has low noise-resistance,which reduces the sensitivity of noise performance.In the figure 1 netlist (ID=S1 and ID=S2) are s-parameters based model for ATF-36163 is used for small signal analysis of the complete LNA design.The figure 1 includes many sub circuits namely “Input Dis Block”,“Output dis network”,“Drain Bias” and “Gate Bias” which will be discussed in further subsections with schematic and layout.The sources are connected to ground through microstrip lines because the source degeneration is useful to insure stability and help the noise match [16].The capacitor (ID=C1) is used for intermediate matching between two transistors and also to provide decoupling effects.The microstrip lines are used between every element throughout the design with dielectric constant Ɛr=9.8,height H=381 µm,conductor thickness T=17µm,metal bulk resistivity normalized to gold Rho=0.7 and Loss tangent of dielectric Tand=0.002.The initial design is carried out using negative lumped elements at both input and output matching of the design shows good LNA performance than the positive lumped elements in the circuit.So,the realization of the negative elements is discussed in the following sub sections.

2.1 Biasing network

In this design,the drain and gate bias circuit are designed by using pair of inductor and bypass capacitors for pHEMT.The proper optimization of the lumped elements,microstrip lines with layout formation have ensured the LNA shows very low noise figure.This biasing network provides sufficient gain low noise figure and stable operation.Several experimentations are carried for the wideband LNA biasing circuits in the earlier published work [20].

2.2 Realization of negative valued lumped elements

This section explains the methodology to convert negative valued elements into practically realizable distributed elements.There are many methods to convert negative elements in to practically realizable circuit.Here,by using Richard's transform negative lumped elements are expressed in term of equivalent electrical length (βl).

Fig.1.Circuit schematics of two stage UWB LNA.

For the lossless transmission line telegrapher's equation,the propagation constant is purely imaginaryγβ=jis as follow:

While calculating β,the wavelength inside the transmission line with respect to microstrip line dielectric material plays the vital role.In this realization method microstrip line geometry (Figure 3) is prepared by using series short stub (MLIN) and open stub (MLEF and MLSC).

In case of shorted load (ZL=0),the input impedance will be purely imaginary and depends upon periodic variation of the wave number as shown in equation 2

In case of open load (ZL=∞),the input impedance will be again purely imaginary and depends upon periodic variation of the wave number as shown in equation 3.

By the making use of above concepts,For the known value of the lumped elements L and C,the electrical length can be calculated by the equations (4),(5) using Richard's transformation.From the basic equation,denoting the L and C as negative values,solving using by trigonometric techniques:

Mathematical calculations are carried out for three different frequencies in order to find physical lengths of the distributed elements to the input matching network shown in the figure 2.

The figure 2 shows the schematic of the negative valued lumped elements for input matching of the LNA.The circuit is simulated initially by this network which ended with good performance of the LNA.To convert negative elements into distributed microstrip line calculation are made and depicted in the table 1.The table 1 shows the calculated values of physical length and width of the microstrip lines whose Ɛr=9.8,loss tangent Tand=0.002,conductivity 5.88*1007s/m,characteristics impedance Z0=50 Ω,height (H)=381 µm and conductor thickness (T)=17 µm.Since microstrip transmission line structure has metallization placed on top of a substrate and exposed to air,“effective” permittivity is a complex relationship between the substrate properties and air properties.To derive the frequency dependent (Keff) is done by using procedure suggested by Eikichi Yamashita,Mahmoud El Sabbagh and H.A.Atwater [17].

Table I.Tabulated frequency versus physical characteristics of negative valued elements.

Fig.2.Negative valued lumped elements for input matching network.

Fig.3.Schematic of the distributed elements of the input matching network.

Microstrip line has excellent performance in the wideband characteristics and their size restricts for the utilization up to 10 GHz.The combination of the lumped and microstrip line will give optimistic performance of the LNA.The figure 3 shows the microstrip geometry with ‘Short standard' (MLIN),‘open standard' (MLEF) and ‘Shorted standard without end effect'(MLSC).The offset length of MLIN is arranged to build nearly for ideal reflection coefficients till 10 GHz with polynomial coefficients of the negative capacitor.The length of the MLEF exhibits capacitances fringes and characterized by capacitance coefficients.The length of the MLSC exhibits the performance like negative inductor whose offset is analyzed without end effects.

The figure 3 shows the schematic of the distributed microstrip line prepared based on the values from the table 1.Analysis is done with different frequencies for input matching circuit to calculate physical characteristics of the microstrip lines.The geometry structure is design to fulfill negative capacitor as linear series microstrip line and negative inductance as open microstrip line with end effect or shorted without end effects.All distributed elements like microstrip line exhibits frequency dependency and their characteristics introduce parasitic ripple effects in the wideband frequencies.It needs to construct geometric microstrip line structures in a such way that their frequency variation with S-parameters (especially S11(ω)) conforms to a desirable function of ω.The design of the microstrip line geometry to fulfill desired S-parameter performances is similar to filter realization.The determination of the filter pass band throughout the wideband characteristics is depend on the order of the filter.So,in this realization it is considered up to third order whose effective length is equal to 4736 µm and average shunt open stubs length (with and without end effect) is 3784 µm.

Figure 4 shows the 2D layout of the schematic shown in figure 3 i.e.input matching network.For the consideration of the layout point of view all width of the microstrip line is kept constant of 300 µm and corresponding lengths are optimized for required s-parameter characteristics.

For ultra-wideband matching,a higher Q is needed or a higher order filter must be employed.Note that for higher Q,the bandwidth gets narrower,requiring the filter to be tuned to compensate for mismatch and process variation.So,in this design experimentation is carried out up to 3rdorder Low pass filter microstrip line geometry for optimum bandwidth requirement.

For the closed form expressions Z0and Ɛrewhen conductor thickness t=0 is given by

Whereηπ=120Ω and

The microstrip line negative capacitance (-C) and negative inductor (-L) is given per unit length with respect to c (speed of light).

The bandwidth can be enhanced by using multiple sections of impedance transformed microstrip lines.It is important to consider the product of quality factor (Q) and Bandwidth (BW).

For the ultrawide band matching,the reflection coefficient of the microstrip line geometry is given the series of sum of the partial reflection coefficient arising at the each discontinuous ‘L' sections of the layout shown in the figure 4.

Where,θis electrical length separation between multiple sections.

Fig.4.2D layout of the distributed elements of the input matching network with reference to figure 3.

Fig.5.S11 parameter comparison between negative value elements and distributed elements structure of the input matching block.

Fig.6.Negative valued lumped elements for output matching network.

Fig.7.2D layout of the distributed elements with reference to figure 6.

2.3 S-parameter comparison of input matching

The figure 5 shows the S11 characteristics of negative value lumped elements and microstrip line realization done using Richard's transform.The left axis represents the S11 characteristics of the negative lumped elements whose curve is linearly decreasing with respect to frequency.The right axis represents S11 characteristics microstrip line structure which is having minor variations with respect to frequency.Variations can be minimized by increasing the order of the microstrip line geometry.

Similar technique is adopted to realize the microstrip line geometry from the negative valued elements of output matching network.The figure 6 shows the negative valued lumped elements used initially for output matching in the UWB LNA design.

To compress the circuit and to avoid the complexity of the circuit,drain bias sub circuit is added inside the distributed element block of the output matching circuit which is clearly shown in the figure 7.Same technique is applied here which were used in analysis of input matching block.

2.4 S-parameter comparison of output matching

The figure 8 shows the S11 characteristics of negative value lumped elements and microstrip line realization done of the output network using Richard's transform.The left axis represents the S11 characteristics of the negative lumped elements whose curve is linearly decreasing with respect to frequency.The right axis represents S11 characteristics microstrip line structure which is having minor variations with respect to frequency.Variations can be minimized by increasing the order of the microstrip line geometry.

Fig.8.Noise Figure and NFmin characteristics of the complete UWB LNA.

Fig.9.S11 parameter comparision between negative value elements and distributed structure of output matching network.

Fig.10.Return loss characteristics of the UWB LNA.

III.RESULTS AND ANALYSIS

The presented UWB LNA is designed by using ATF 36163 pHEMT transistors with negative image amplifier technique is measured with parameters like return loss,gain,noise Figure and stability check.The following graphs with rectangular display type simulated results using AWR microwave tool are discussed in detail.

Figure 9 shows the simulated input return loss (S11) and output return loss (S22) of the complete UWB LNA by adopting the negative image amplifier technique.As experimented,the LNA is well-matched over the frequency range of interest.The circuit using negative lumped elements has achieved return losses less than -10dB without comprising other parameters of the LNA which was not achieved by using positive value lumped elements in the circuit.After the realization with microstrip line the input and output return loss values are less than -8 dB throughout band.The return losses are deteriorating because of actual microstrip line technology cannot reach the same effects of negative valued lumped elements.To overcome this limitation,several experimentations are required to be carried out in conversion of negative elements into practically realizable circuits.

The figure 10 shows that the noise figure (NF) is below 2 dB from 3 to 10 GHz and NFmin is less than 1.6 dB from 3 to 10 GHz.Another advantageous property of the negative image amplifier technique is that the gain,NF and return loss is obtained simultaneously.Indeed,the simulated NF almost equals the simulated NFmin of the complete LNA over a bandwidth from 4 to 10GHz.The deviation between NF and NFmin is at lower frequencies occurred because of minor impedance mismatch at the input port.At low frequencies increase of NF is because of flicker noise,whereas the reason for increase of NF at higher frequencies is due to drop in gain.

The noise sources and thermal noise voltages due toRgandRsrespectively,

Here,ω:angular frequency;Cgs:gate to source capacitance;Gm:transconductance;Rs:ohmic contact; Rg:metallization resistor and Ri:intrinsic resistance.

Fig.11.Gain performance of the UWB LNA.

Fig.12.Stability test using K and B.

Fig.13.Two-dimensional layout of the complete UWB LNA which be fabricable.

The figure 11 shows the gain characteristics of the designed UWB LNA.As for gain is concern,a sufficient high gain is required over complete UWB to withstand the effects noise Figure of the upcoming stages in the receiver system.The transducer gain (S21) of this LNA is the resultant of two stage cascaded transistors is having variation performance i.e.between 20 to 25 dB.If the gain flatness is required over UWB,an additional cascaded stage can be added to compensate gain and restricting to 22 dB using open stub microstrip line at every grounding point.By reducing the total gain flatness can be achieved.

In order to make sure that designed UWB LNA is unconditionally stable,a rollet's factor (K) test and the auxiliary factor (B) are being plotted [18,19].The K-factor is depicted in left sided Y-axis and secondary stability factor in right sided Y axis in the figure 12,and the values of the K-factor are greater than 1,B greater than zero over the entire 3-10 GHz bandwidth all calculation are based on the s-parameters.In this consequence,the designed UWB LNA is made as unconditionally stable with the help of microstrip line at the sources of the pHEMT.

The figure 13 displays the complete layout of UWB LNA on HMIC technology with all sub circuits i.e.Input and output matching blocks,drain and gate bias circuits,intermediate capacitors and transistors.Among the PCB layout,the red color rectangle component is the transistor,blue color lines all are microstrip lines and the rest drain and gate bias circuit compose of inductors and capacitors which shown soldier pads.The presented UWB LNA for 3-10 GHz is summarized in the table 2.The simulated performances of the presented LNA in comparison with those of recently published LNAs.It is noted that the gain,bandwidth,noise Figure and input return loss is compared clearly which shows the importance of this work.

IV.INNOVATION IN THE PROPOSED WORK

Though the negative image amplifier technique results in the performance improvement,yet it imposes the use of negative valued components which are not realizable.This technique presents a novel method of tackling the issue of the negative valued components.The conversion of negative reactance into realizable component is done using microstrip line structure.The complete strategy leads to realizable design of LNA circuitry.This approach provides following remarkable achievements of negative image amplifier technique over conventional matching circuit:

·Compressed and squeezed down to a smaller size,total board outline is 17mm x10.5mm

·Sufficient gain is achieved with unconditional stability.

·Good noise Figure less than 2 dB over complete 3-10 GHz band

·Good bandwidth 3-10 GHz,which is ultra-wideband with return loss of -8 dB

V.CONCLUSION

This paper proposes novel procedure of acknowledgment of the negative valued lumped elements by utilization of the microstrip line parasitic effects.Whose effects is clearly explained with supporting mathematical analysis and simulation results.Bandwidth characterizing for the LNA is very complex,this work has met with several conflicting measurements like return loss,NF,gain and stability based on the S-parameter analysis.This work presented the two-stage cascaded LNA which exhibits average gain of 23 dB gain and low noise figure of less than 2 dB with return loss less -8 dB for UWB 3-10 GHz.The designed LNA furnished least NF alongside with good gain when contrasted with recent works.Consequently,the coveted focus of the design was achieved with a specific end goal to be incorporated as a first stage of the receiver's circuitry.

Table II.Performance comparison with recently published articles on LNAs.

ACKNOWLEDGEMENTS

This work is upheld and maintained by MCT's Rajiv Gandhi Institute of Technology,Mumbai.Moreover,authors might need to express appreciation towards Dr.Udhav Bhosle and Dr.Sanjay Bokade,RGIT for basic support and comfort for this investigation work.The authors may need to one of a kind an obligation of appreciation is all together for Dr.Pramod K.B.,Mr.Manjunatha Reddy H.V Technical Manager,AWR Group,National Instruments.