Negative image amplifier technique for performance enhancement of ultra wideband LNA

Received Dec 27, 2017 Revised Aug 10, 2018 Accepted Aug 31, 2018 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.


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 pHEMT and HEMT transistors provides promising results at microwave frequencies. A 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 [1][2][3][4][5][6].
Recently InGaAs ba sed 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 minimize 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][11][12][13][14][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. By using the ideal negative lumped elements for input and output matching of LNA design has achieved gain more than 21dB, Noise Figure (NF) less than 2dB and return losses less than -10dB throughout band of 3-10GHz. The efficient technique is proposed in this article to convert negative valued lumped elements in to practically realizable microstrip line structure. This paper is organised 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.

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 realisation. Finally, it presents results which includes stability, noise figure, gain, input and output return. Figure 1 shows the complete architecture of the design UWB LNA. The Avago technologies ATF-36163 is a low-noise Psuedomorphic High Electron Mobility Transistor (PHEMT) has low noise-resistance, which reduces the sensitivity of noise performance. In Figure 1 netlist (ID=S1 and ID=S2) are s-parameters based model of ATF-36163 which is used for small signal analysis of the complete LNA design. 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.

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 [17][18][19][20].

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). For the lossless transmission line telegrapher's equation, the propagation constant is purely imaginary = is as follow: where, wavenumber = 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 shown in Figure 2 is prepared by using series short stub (MLIN) and open stub (MLEF and MLSC). In case of shorted load ( = 0), the input impedance will be purely imaginary and depends upon periodic variation of the wave number as shown in (2).
In case of open load ( = ∞), the input impedance will be again purely imaginary and depends upon periodic variation of the wave number as shown in (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: 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 as shown in Figure 2. 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 Table 1.  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 x10 07 s/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]. Microstrip line has excellent performance in the wideband characteristics and their size restricts for the utilization up to 10GHz. The combination of the lumped and microstrip line will give optimistic performance of the LNA. 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 10GHz 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. 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 4736µm and average shunt open stubs length (with and without end effect) is 3784µm.   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 3 rd order Low pass filter microstrip line geometry for optimum bandwidth requirement. For the closed form expressions Z0 and Ɛre when conductor thickness t=0 is given by: where = 120 Ω 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 is electrical length separtion between multiple sections. 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. Figure 6 shows the negative valued lumped elements used initially for output matching in the UWB LNA design.

S-parameter comparison of input matching
Figures 5. S11 parameter comparison between negative value elements and distributed elements structure of the input matching block   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.  Figure 8. S11 parameter comparison between negative value elements and distributed elements structure of the input matching block

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 microtrip 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. 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 flicker noise, whereas the reason for increase of NF at higher frequencies is drop in gain. The noise sources and thermal noise voltages due to Rg and Rs respectively, Here, ω: angular frequency; Cgs: gate to source capacitance; Gm: transconductance; Rs: ohmic contact; Rg: metalization resistor and Ri: instrinsic resisance. 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 in performance. If the gain flatness is required over UWB, an additional multiband gain peak in the following stages can be added to compensate gain drop in few sag. This case can be applied to this work 3-10 GHz UWB LNA. 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 3-10GHz bandwidth all calculation are based on the s-parameters. In this consequence, the designed UWB LNA is made as unconditional stability with the help of microstrip line at the sources of the pHEMT.   The presented UBW LNA for 3-10GHz is summarized in the Table 2. The measured 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.

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: a. Compressed and squeezed down to a smaller size, total board outline is 17mm x 10.5mm b. Sufficient gain is achieved with unconditional stability. c. Good noise figure less than 2 dB over complete 3-10GHz band d. Good bandwidth 3-10 GHz, which is ultra-wideband with return loss of -8dB.

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 23dB gain and low noise figure of less than 2dB with return loss less -8dB for UWB 3-10GHz. 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.