ECEN 5014, Spring 2013 Special Topics: Active Microwave Circuits and MMICs Zoya Popovic, University of Colorado, Boulder

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1 ECEN 5014, Sprin 2013 Special Topic: Active Microwave Circuit and MMIC Zoya Popovic, Univerity of Colorado, Boulder LECTURE 2: INTRODUCTION TO MICROWAVE AMPLIFIERS L2.1. AMPLIFIER BLOCK DIAGRAM The eneral block diara of a inle-tae icrowave aplifier i iven in Fi.L2.1. The active device can be a FET or bipolar device, and thi will deterine the type of biain. In a FET aplifier, the ate i biaed neatively w.r.t. the ource, and the drain poitively. The ource i uually rounded, both RF and DC-wie. The neative ate bia uually need to be turned on firt to avoid burnin the tranitor, often referred to a bia equencin. In contrat, bipolar tranitor do not require bia equencin, o they can eaily be biaed with a inle DC upply. The bia i upplied throuh a biain circuit that need to preent a hih ipedance to all preent RF inal o a not to preent an additional (uually not well characterized) load. Thi i relatively traihtforward in a narrowband aplifier dein, but becoe a challene for the broadband cae. RF capacitor are needed to block the DC inal to the RF input and output. Uually the ource (or eitter) terinal of the active device i connected to RF (and often DC) round. In the cae of icrotrip, one or ore etalized via hole are ued for roundin, and they preent an equivalent inductance between the terinal and round. You will exaine the effect of thi inductance in your project. In the cae of coplanar waveuide (CPW) circuit, the connection i ore traihtforward and the paraitic reactance can be inial. 50 input port Input atchin network RF hih ipedance Input bia (ate or bae) RF hih ipedance Output bia (drain or collector) Blockin capacitor Output atchin network RF out 50 output port RF in Blockin capacitor Groundin connection (ource or eitter) Fiure L2.1. General circuit diara of a icrowave aplifier. Matchin circuit at the input and output deterine ultiately the perforance of the aplifier. The followin type of aplifier reult fro different atchin circuit: - all inal ain-atched aplifier input and output are atched for bet return lo - low-noie aplifier input i atched to a pecial ipedance that cancel oe of the noie oriinatin fro the aplifier input and output. Output port i atched for ood return lo. 1

2 - hih-power aplifier output i atched for lare-inal axiu power delivery to the load, and input i atched to axiize ain - hih-efficiency aplifier everal option exit dependin on other requireent - broadband aplifier there are everal architecture that enable broadband operation, uually they involve ore than one active device. It i difficult to provide broadband atchin to a inle-tae aplifier without introducin ubtantial lo. In all the above dein, the firt tep i to enure that the aplifier will be table, i.e. that it will not be an ocillator. It i relatively traihtforward to dein table all-inal aplifier, ince the tability criteria can be forulated in ter of tranitor S-paraeter. In aturated aplifier (power aplifier), all-inal tranitor paraeter are not valid, and tability i ore difficult to predict durin the dein. An aplifier ha ain at the expene of input DC power. In a previou lecture, we entioned that a MESFET need a poitive drain-to-ource DC voltae and a neative (but not too neative) ate-to-ource voltae. Thi ean that in eneral the input and output of the aplifier need to be connected to a bia upply, which hould not chane the input and output reflection coefficient. Deinin ood biain circuit i half of aplifier dein, and the followin are critical dein paraeter: 1) the bia network need to be inviible to the rf wave, i.e. a cloe to an open circuit a poible. The reaon i that we cannot afford any of the rf power to be lot in the biain circuit and power upply. 2) the dc bia need to be iolated fro the rf circuit, i.e. we do not want the dc voltae to be preent at the rf input (e.. we iht be dealin with a 2-tae aplifier, and the previou tae require a different voltae). 3) finally, the dc bia circuit hould behave properly over the entire frequency rane where the device ha ain, o a not to caue intabilitie. In order to atify the firt criterion, the dc bia line need to be inductor. It i difficult to ake an inductor at icrowave frequencie due to paraitic capacitance. Another option i that the bia line have the characteritic of a low-pa filter (review baic low-pa filter if needed). In order to atify the econd requireent, a dc blockin capacitor need to be added to the circuit, and need to be taken into account in the dein. Capacitor are not ideal hort at icrowave frequencie (they have paraitic. The bia circuit can be interated with the aplifier, or alternatively, an external biain circuitry can be ued. External bia circuit are often referred to a Bia Tee, a block diara i hown in Fiure L2.2. Thee device are expenive if they cover a broad bandwidth, and uually have current liitation. The reaon i the inductor in the DC path, which need to be ade of thin wire o a not to have appreciable paraitic capacitance. Coercial bia Tee are fairly lare and have typically SMA connector at the two RF port. Often, biain circuit are part of aplifier dein, and oe exaple are hown in Fiure L2.3. 2

3 DC in RF choke RF in Blockin capacitor DC in RF out Fiure L2.2.. A bia-tee equivalent circuit. The DC biain circuit hould be taken into account when analyzin tability, i.e. it i part of the input and output network. Even thouh it i deined to preent a hih ipedance to the RF inal at the dein frequency (convince yourelf why thi i o), it i not a real open circuit. For exaple, at a frequency other than the dein frequency, the quarter-wave horted line i not a quarter-wavelenth lon, and therefore i not an open circuit to the RF inal. There i alo oe lo in the blockin capacitor the DC blockin capacitor ha lead inductance and oe reitance, and it will not be perfectly atched to the input RF 50-oh line. In the rounded capacitor ipleentation (rihthand ide of Fiure L2.3), the capacitor and via hole have inductance that i uually not well characterized, and thi alo deterine the quality of the open circuit preented to the RF inal. Ferrite RF choke open radial tub Ferrite RF choke Ferrite RF choke Grounded luped capacitor hih-ipedance line hih-ipedance line open radial tub hih-ipedance line DC blockin RF capacitor DC blockin RF capacitor DC blockin RF capacitor Fiure L2.3. Exaple of icrotrip biain circuit. The ferrite choke i an inductor at lower frequencie and i effective at chokin frequencie up to a few hundred MHz. Thi i iportant, ince the bia line can be ood antenna for broadcat inal. At icrowave frequencie, however, the ferrite i jut a lare reitor (the aterial i very loy), o the RF current will be very attenuated and will not reflect back into the circuit. However, the power i lot and any power flow into the ferrite line hould be iniized. A difficult proble i a broadband bia line. In principle, a ood inductor with everal hundred nh inductance would olve the proble, but icrowave inductor typically do not work above a few GHz due to paraitic capacitance. If lo i not an iue, however, the Q factor of the 3

4 inductor can be reduced by addin reitor or ferrite and very broadband bia network can be ade. A very broadband aplifier fro Ailent (0-40GHz) need very broadband bia line, a hown in Fiure L2.4. The tiny cone-haped inductor with ferrite loadin are fro Piconic, but Coilcraft ake the a well. The idea i that the Q i reatly reduced at hih frequencie, o the reonance due to the paraitic capacitance i not relevant. Thee broadband inductor are conical, o effectively they look like a continuou et of erie inductor with proreively hiher inductance value and, correpondinly, proreively lower elf-reonance frequencie. Effectively, thi i a ditributed erie of bandpa filter, hown in Fi.L2.4 at the botto. You can ake a broadband bia line uin everal erie inductor in thi fahion. RF L 1 L 2 >L 1 L 3 >L 2 DC C 1 C 2 >C 1 C 3 >C 2 Fiure L2.4. Top: Photoraph of iniature conical coil with ferrite loadin ( Botto: dicrete erie inductor which in the continuou liit behave like the conical inductor. If a ferrite i added, it increae inductance at the lower frequencie, and add lo at the hiher frequencie. The ot coonly ued active device at icrowave frequencie are the MESFET, HEMT and HBT. FET are ued ore coonly than BJT. We will later tudy briefly how thee device work. For now, it i iportant to know: - in what for you can buy the device; - what pecification are iven; - how to ue the pecification to dein a all-inal aplifier. 4

5 L2.2. ACTIVE DEVICE PARAMETERS When you buy a tranitor, it can coe in a packae or in chip for. You will alo et eaured S-paraeter at a few different bia point for a certain frequency rane, in a tandard file ize called 2p. Thee S-paraeter are eaured uually with the ource terinal rounded and the drain and ate lookin into 50, o they are two-port paraeter. The 21 paraeter correpond to the ain of the device in coon-ource confiuration. The aplitude and the phae of all four paraeter are iven at any dicrete frequencie and can be interpolated. Another way to repreent the tranitor i with an equivalent circuit, like you have probably done in your circuit clae. The idea behind equivalent circuit i to odel the device over a rane of frequencie with invariant paraeter. Let u bein with the iplet linear (all-inal) equivalent circuit, hown in Fi.L2.5 for a FET. In thi cae, it i ipler to ue adittance paraeter, iven by I=YV, ince ot of the eleent are in hunt. You can find converion forula between Z, Y, S and ABCD (T) paraeter in ot book (e.. Pozar). (a) (b) Fi.L2.5. (a) Cro-ection of a FET. (b) Hih-frequency approxiate intrinic equivalent circuit. At very low frequencie, ay below 1GHz, the capacitance and inductance aociated with the MESFET are quite all, and we can aue they are neliible. The ae i true for the reitive loe. The reult i a iple low-frequency odel (ae a in Fi.2.5b, but without the capacitor). Thi odel ha an infinite input ipedance and cannot be atched at the input. What i the order of anitude of the eleent of thi circuit? Let u look at the TriQuint TGF2960 device S-paraeter that you are iven for Project 1 at the lowet 100MHz frequency with V DS = 8V, I DS =100A: S 11 = 0.677dB S 21 = 20dB S 12 = -41.9dB S 22 = -5.06dB

6 If we wih to find the value of the eleent in the equivalent circuit, we would firt olve for the S-paraeter of the equivalent circuit in ter of the unknown eleent, and then et the expreion equal to the known S-paraeter, thu ettin a yte of equation. Findin the expreion for the S-paraeter of the equivalent circuit can be quite coplicated, and uually the Y-paraeter are found and then converted to S-paraeter. For the low-frequency odel fro Fi.L2.5b (with no capacitor), the Y-paraeter are and the S-paraeter are obtained a follow: 0 0 d S d d d. Since you are iven the S-paraeter, you can find the conductance value; note that they are noralized to 1/50=0.02S. Conider next the RFMD FPD HEMT device. At 5V and 300A, the lowet available frequency S-paraeter value are at 50MHz: Goin throuh the ae approxiation a for the MESFET above, we et the paraeter calculated fro the low frequency eaured data to be If you look at the eaured data at f=600mhz for the ae bia point, however, thee are the value: A d V 0.875S 28.5S Referrin back to the dicuion about the low-frequency tranitor odel, you can ee that the iple equivalent circuit fro before cannot be ued. There are capacitance that are already 6

7 quite pronounced at 600MHz. The reaon i that the MESFET device fro the previou odel ha ain up to uch hiher frequencie (12GHz) and can ive at ot a hundred W of power, while the RFMD HEMT i a 1W device for lower frequency operation. At hiher frequencie, capacitance need to be included in the odel. The circuit in Fi.L2.5b i called the intrinic equivalent circuit becaue additional paraitic fro the packae are not included. The depletion capacitance of the Schottky barrier ate i repreented by C and C d. Uually C i uch larer. The reaon i that the poitive voltae on the drain caue the depletion reion on the drain ide to be wider than on the ource ide. Alo, the eparation between the drain and ate contact i uually about 1 larer than that between the ource and ate. The capacitance between the ource and drain i priarily throuh the ubtrate, and i not neliible becaue of the hih dielectric contant of GaA of 13. The reitance of the ate i inificant becaue the ate contact i lon and thin, and a typical value i L G R G R D L D R S L S Fi.L2.6. External paraitic repreentin lead inductance and contact reitance added to the intrinic circuit. The uual fiure of erit for the tranitor i the voltae ain AV / d. Since both conductance are proportional to the ate width, the voltae ain doe not depend on the width. Thi i iportant in MMIC, where there i coplete control over ate width, but ate lenth are fixed by the fabrication proce. The ate lenth deterine the axiu operatin frequency of the device (directly, the RC tie contant). An experientally obtained forula i f ax Hz, L where L i the ate lenth in eter. Several cutoff frequencie are coonly ued. ft i the cutoff frequency when the hort-circuited current ain of the device drop to unity. Thi paraeter i often ued, but never eaured, ince a icrowave tranitor tend to ocillate with a hort-circuit load. If the input current for the hih-frequency equivalent circuit i I in then we have 7

8 I I f in out I I out T in V i d 2C jc V 1 T C The two ot iportant paraeter for the hih-frequency perforance are therefore and C - lare and all C reult in a hih cut-off frequency. A typical procedure ued to calculate the cutoff frequency i to derive the hort-circuit ain fro the eaured -paraeter, and extrapolate thi curve to the value of the ain equal to 0dB. Thi ive a iplified 6dB/octave repone, althouh the actual one i obviouly ore coplicated (we ued only approxiate forula). Fi.L2.7. Calculatin the cutoff unity current ain frequency. Thi cannot be eaured, becaue the tranitor will ocillate when the drain i horted, due to the feedback capacitance. There are a nuber of paraitic when the device i ounted in the circuit. To deterine the contact reitance, we perfor three DC eaureent with forward-biaed ate: S rounded (R a ), D rounded (R b ), both S and D rounded (R c ) and ue the forula iven in Fi.L2.6 to find the reitance value. Thee value depend on bia, and are not quite correct at RF (kin effect), but are adequate a a tartin point. Followin thi, calculate the difference between Y- paraeter with intrinic eleent plu reitance, and eaured Y-paraeter. Fro the difference between the two, find initial ue for inductance. Recoended readin related to thi lecture: Microwave Tranitor Aplifier, Gonzale, 1984 edition, Chapter 1 Fundaental of RF and Microwave Tranitor Aplifier, Bahl, Chapter 1 and 2 8