Linearized Common-Base Balanced Amplifiers Using Linvill NIC Feedback

Transcription

1 Linarizd Common-Bas Balancd Amplifirs Using Linill NIC Fdback by Chris Trask / N7ZWY Sonoran Radio Rsarch P.. Box Tmp, AZ christrask@arthlink.nt 23 Sptmr 2012 Trask: Linill Fdback Amplifir 1 23 Sptmr 2012

2 Introduction In th dsign of widband acti antnna circuitry, th intrmodulation distortion (IMD) and nois figur (NF) prformanc ar important dsign considrations. IMD prformanc is gratly dpndnt on th nonlinar haiour of bipolar transistor bas-mittr junctions and th squarlaw charactristcs of fild ffct transistors (FETs). NF prformanc is primarily a mattr of dic slction and oprating bias point as wll as th minimization of dissipati dics such as rsistors and transformr losss, as wll as th propr matching of th antnna to th amplifir. In th mattr of acti loop antnnas, th input impdanc of th amplifir nds to xcptionally low to mt th charactristically low radiation rsistanc. In this papr w will xamin an amplifir haing an input rsistanc of lss than 0.10Ω togthr with xcptional IMD and NF prformanc. Input Charactristics of th In th common-bas amplifir shown in Fig. 1, th bas-mittr oltag V is a function of th total mittr currnt I : V V BE + k T I ln q I (1) whr is th bas-mittr signal oltag, I is th transistor rrs saturation currnt, q is th lctronic charg ( x10-19 J), k is Boltzmann s constant ( x10-23 J/K), T is th tmpratur in dgrs Klin (K), and th total mittr currnt I is th sum of th mittr bias currnt I E and th mittr signal currnt i : I I + i E (2) Th mittr input rsistanc r can approximatd by: r i I ε q VBE k T (3) In th common-bas amplifir shown in Fig. 1, th signal input currnt i is shown as a linar function and th bas-mittr signal oltag is xagratd for clarity. Augmntation of th In rality, th finit nonlinar mittr input rsistanc r of Eq. 3 disturbs th mittr signal input currnt i. This can improd by rducing r by way of th application of augmntation (1, 2). Shown in basic form in Fig. 2, an inrting augmntion amplifir haing a oltag gain of A V Fig. 1 - Input Charctristics Fig. 2 - Basic Augmntation of th Trask: Linill Fdback Amplifir 2 23 Sptmr 2012

3 dtcts th mittr signal oltag, amplifis and inrts it, applying th rsult to th bas of Q 1, rsulting in a rducd mittr input rsistanc r : A + 1 r r q V (4) BE A + 1 k T I ε As th gain of th augmntation amplifir is incrasd, th mittr input rsistanc r approachs zro. Numrous ralizations of augmntation can found in th litratur citd, and th form making us of a two-winding transformr proids a good compromis of simplicity, NF and IMD prfomanc, and input rsistanc rduction. Howr, th practical ralization of widband transformrs introducs a limit to which th input rsistanc can rducd. Cascad Linarization of th To raliz a practical substantial rduction in th mittr input rsistanc r, a cascad of two common-bas amplifirs can mployd, dtcting th mittr signal oltag of th scond amplifir ( 2 ), and thn using a simpl 1:1 Fig. 4 - Balancd Using Cascad Linarization widband transformr to inrt 2 and conduct it to th bas of th first amplifir. Shown in Fig. 3, this mthod rsults in a substantial rduction in th amplifir input rsistanc: r1 r (5) hf1 2 whr r 1 is th mittr input rsistanc of transistor Q 1 and h f1 is th small signal currnt gain of transistor Q 1. This is an xcptionally low input impdanc that would typically rquir th us of acti augmntation. Fig. 3 - Cascad Linarization of th Common-Bas Amplfir Th currnts and oltags shown in Fig. 3 show that th bas oltag of Q 1 is th sam in both approachs, and in Fig. 3 this is accomplishd with a simpl 1:1 widband transformr and a scond transistor. This mthod can xtndd to push-pull amplifirs, as shown in Fig. 4. Trask: Linill Fdback Amplifir 3 23 Sptmr 2012

4 ondary winding of transformr T 2 is connctd to th bas of transistor Q 1. In this configuration, transistors Q 1 and Q 2 togthr with transformrs T 1 and T 2 constitut a Linill ngati impdanc conrtr (NIC). If all four transistors ha similar charactristics, th balancd amplifir of Fig. 5 is unconditionally stabl and has a balancd input rsistanc that is ry clos to zro. Practical Ralization of th Linill Fdback Amplifir Fig. 6 shows a practical ralization of th Linill fdback amplifir of Fig. 5. This configuration is intndd to usd with loop antnnas such as th Mobius Strip loop that has connctions to ground (3). This simplifis th coupling of th loop antnna to th amplifir as wll as th biasing of th amplifir. Fig. 5 - Linarizd Common-Bas Balancd Amplifir Using Linill NIC Fdback Linaizd Common-Bas Balancd Amplifirs Using Linill NIC Fdback Th balancd amplifir of Fig. 4 suggsts an intrsting innoation if th mittr signal oltag 2 of transistor Q 2 was conductd to th bas of transistor Q 3 and th mittr signal oltag 4 of transistor Q 4 was conductd to th bas of transistor Q 1. Howr, as th signal oltags shown in Fig. 4 illustrat, th asymmtry of th mittr signal oltags 2 and 4 prnts this from ing a practical approach whn compard to th signal oltag rquirmnts of Fig. 2 and Fig. 3, and it would rsult in a substantial rduction in IMD prformanc. Th incompatibility of ths signal oltags can rmdid by changing transistors Q 2 and Q 4 from NPN to PNP, as shown in Fig. 5. Now, th scondary winding of transformr T 1 is connctd to th bas of transistor Q 3 and th sc- Vcc is any suitabl DC supply oltag twn 8V and 15V. Transistor Q 5 proids a currnt mirror rfrnc for transistors Q 1 and Q 3, whil transistor Q 6 proids a similar currnt mirror rfrnc for transistors Q 2 and Q 4. Rsistor R 1 is slctd to st th bias currnt that proids th st compromis of IMD and NF prformanc. T 1 and T 2 ar widband 1:1 transformrs mad with bifilar twistd wir on a suitabl binocular cor such as th Fair-Rit T 3 is a 1:1:1 widband transformr mad with trifilar twistd wir on a similar binocular cor (4, 5). Closing Rmarks Th Linill fdback amplifir dscrid hrin proids an xcptionally low input impdanc that is wll suitd for us with widband acti loop antnnas. It also proids good IMD prformac by irtu of th fdback topology mployd as wll as good NF prformanc by minimizing th numr of lossy componnts. Trask: Linill Fdback Amplifir 4 23 Sptmr 2012

5 Fig. 6 - Practical Linill Fdback Amplifir C1, C2, C3-0.1uF R1 - S txt Q1, Q3, Q5-2N2222 T1, T2-1:1 Transformr (s txt) Q2, Q4, Q6-2N2907 T3-1:1:1 Transformr (s txt) Tabl 1 - Parts List Rfrncs 1. Trask, C., "Common Bas Amplifir Linarization Using Augmntation," RF Dsign, ctor 1999, pp Trask, C., s with Linarity Augmntation, US Patnt 6,271,721, 7 August Trask, C., Mastring th Art of Shildd Loop Arials, 20 March 2010 (onlin rsourc). 4. Trask, C., Dsigning Wid-band Transformrs for HF and VHF Powr Amplifirs, QEX, May/ April 2005, pp Walkr, John L.B., Danil P. Myr, Frdrick H. Raab, and Chris Trask, Classic Works in RF Enginring: Combinrs, Couplrs, Transformrs, and Magntic Amplifirs, Artch Hous, Trask: Linill Fdback Amplifir 5 23 Sptmr 2012