Methodologies of Microwave Amplifier Design

Methodologies of microwave Amplifier Design2.1 ACTIVE DEVICE SELECTIONThis chapter discusses various methodologies utilise in the design of single stage microwave amplifiers. Reaching the desired goals of gain, index loss and hinderance performance requires first selecting a suitable wide awake device (transistor) that meets these goals. The rapid advances in transistor fabrication have permitted the handed-down Si transistors to operate in the GHz re- gion. the increase for high-pitcheder absolute oftenness operation drove the innovation of new novel devices with new materials, architectures and geometries possibly the most significant difference be- tween microwave transistors and the smaller frequency ones is in the area of materials. Although low-frequency transistors are fabricated mostly from silicon, the use more dearly-won compound semiconductors like gallium arsenide (GaAs) and indium phosphide (InP) proves to be more frugal at microwave frequencies because of t heir performance advantages over silicon. The demand for higher frequencies in any case produced sophisticated material configurations like the heterojunction transistors which have no low-frequency counterparts.At low frequencies, microwave transistors can be broadly categorized into the bipolar junc- tion transistors (BJTs) and the field-effect transistors (FETs). At lower frequencies, FETs con- tains the junction FET (JFET) and the metal oxide FET (MOSFET), structural characteristics limits their high frequency operation. GaAs metal semiconductor FET pushed the frequency of operation tumesce into the GHz region. However, in the intervening decades, bipolar device caught up and now it is common to find BJTs operating at the GHz region.The selection of a suitable transistor for the required application is based on the calculateed goals of gain, noise and power loss performance. In the following sections, the GaAs HJ-FET transistor NE3210S01 from Renessa Electronics will be used to illustrate the various meth- ods for selecting the appropriate terminations used in constructing twin(a) networks for both narrowband and wideband operation.2.2 MATCHING NETWORKS TOPOLOGIESImpedance matching involves transforming one electric resistance to the other. This process is useful in traffic circles where the mismatch between the extension (ZS) and lodge (ZL) prevents supreme power transfer. Theorem states that for a level best transfer of power from source to vitiate. Load impedance (ZL) must be equal to the convoluted mix of the source impedance. Complex conjugate is interwoven impedance having the same strong part with an opposite imaginary one. For example, if the source impedance is ZS =R+jX, thusly its complex conjugate must be ZL =R-jX. For a pure resistant laden, equations (2.1) and (2.2) aided with Fig.2.1 shows that a maximum4power transfer occurs when RL=RS.VO= VSLRL+ RSRL(2.1)PO= V2S(RL+ RS)2(2.2)(a)(b) send off 2.1 (a) Pure resistive circuit w ith VS=1V and RS=1, (b) apothegm power is delivered to the load when RL=RSThe same concept can be apply to AC circuits with complex load and source. Equation.2.3 aided with figure Fig.2.2 shows that a maximum power transfer to the load occurs when XL= XS. The value of the power delivered to the load is given up by1VS2 RLPO= 2 (R+ RL)2 + (XS+ XL(2.3))2Where the resistance RS and RL and the reactants XS, XL are the real and imaginary parts of ZS and ZL. The target in applying impedance matching to make the load impedance tang like the complex conjugate of the source impedance to attain maximum power transfer to the load. This is shown in Fig. where a matching circuit is situated between points a,b shown in Fig to transfer the load impedance to the complex conjugate value of the source impedance. Since we are dealing with reactances, which are frequency dependent, the matching can occur only at single frequency. That is the frequency at whichXL= Xand, thus, cancellation or resonan ce occurs. At the surrounding frequencies, the matching becomes worse. This is the chief(prenominal) problem in broadband matching where perfect or tight fitting perfect matching along the required bandwidth is required. The methods for narraowband and wideband matching is presented after in this chapter.In Fig.2.3b, numerous topologies can be used as a matching network. The shape of the topology can vary from a simple L, or T networks to a complex ladder circuit or filter design. The concept of matching network can be explained using the two simple L-Matching topologies shown in Fig.2.4a,2.4b. Both B and X values in Fig.2.4 must be chosen to satisfy the correspond ZL=ZS*. To achieve this condition, both analytical methods, mostly with the aid of a computer, and graphic procedures, using the Smithchart, can be used.(a)(b)(c)Figure 2.2 (a) AC circuit with complex ZS and ZS, (b) For XS=j5, Maxim power is delivered to the load when XL=-j5 (c) For XS=-j5, Maxim power is delivered to the load when XL=j5For the case of RL Ro, the topology of Fig.2.4a is preferred, where B and X are given by RL 22XLB=ZoRL+ XLZoRL(2.4)R22L+ XLX= BZoRLXL1 BXL(2.5)For the condition of RLRo the topology of Fig.2.4b is used with B and X given by1 ZoRLB= oL(2.6)X= RL(ZoRL) XL(2.7)In both topologies of Fig.2.4, B and X represent either an inductor (L) or capacitor (C). The result is four simple L-matching networks as shown in Fig.2.5. (b)Figure 2.3 (a)Circuit before the matching network(b) Circit after adding the matching network.(a)(b)Figure 2.4 L-Matching topologies, a) used when RL Ro, b) used when RL Ro2.3 NARROWBAND DESIGN METHODOLOGIESAnalytical stemGraphical Solutionheel SolutionWIDEBAND DESIGN METHODOLOGIESAnalytical SolutionGraphical SolutionCAD Solution(b)(c)(d)Figure 2.5 Four basic L-matching Networks