A Broadband HF Amplifier Using Low-Cost

1 40 March 1999 MBy Mike Kossor, WA2 EBY1 Notes appear on page ) from 160 through 10 meters. To thebasic Amplifier , I added an RF-sensed TRrelay and a set of low-pass filters designed tosuppress harmonic output and comply withFCC requirements. The Amplifier is built ondouble-sided PC board and requires no tun-ing. Another PC board contains the low-passfilters. Power-supply requirements are 28 Vdc at 5 A, although the Amplifier performswell at V of these amplifiers have been builtand exhibit similar performance. Al has beenusing his Amplifier on each of the HF bands,logging well over 500 contacts in 18 reports indicate a noticeable improve-ment in readability (about two S units on av-erage) over his 5 W rig.

2 No indications of in-stability, CW key clicks or distortion on SSBhave been reported. To make it easy for you toduplicate this project, PC boards and parts kitsare available, all at a cost of about $100!9An Overview of MOSFETsMOSFETs operate very differentlyfrom bipolar transistors. MOSFETs arevoltage-controlled devices and exhibit a veryhigh input impedance at dc, whereas bipolartransistors are current-controlled devices andhave a relatively low input impedance. Bias-ing a MOSFET for linear operation only re-quires applying a fixed voltage to its gate viaa resistor.

3 With MOSFETs, no special bias orfeedback circuitry is required to maintain thebias point over temperature as is requiredwith bipolar transistors to prevent With MOSFETs, the gate-thresh-old voltage increases with increased draincurrent. This works to turn off the device,especially at elevated temperatures astransconductance decreases and RDS(on)(static drain-to-source on resistance) in-creases. These built-in self-regulating ac-tions prevent MOSFETs from being affectedFigure 1 Jim Wyckoff, AA3X, 1 W In, 30 W Out With Power MOSFETs at 80 M, Hintsand Kinks, QST, Jan 1993, pp articles have been writtenencouraging experimenters to usepower MOSFETs to build HF RFamplifiers.

4 That s because powerMOSFETs popular in the design of switch-ing power supplies cost as little as $1 each,whereas RF MOSFET prices start at about$35 each!Over the years, I tucked away several ofthese articles, waiting for an opportunity toexperiment with them. That opportunity camewhen I received a call from Al, W2 OBJ. Alwanted a Low-Cost linear Amplifier to use withhis 5 W QRP transmitter when band condi-tions got poor. Ideally, the Amplifier wouldgenerate at least 25 W on all the HF s inquiry renewed my interest in the topicand provided the motivation I needed to getmy project provided me with an extensive list ofRF- Amplifier construction articles that usepower These articles provideduseful information about MOSFETs and gen-eral guidelines for working with them, in-cluding biasing, parasitic-oscillation sup-pression, Broadband impedance-matchingtechniques and typical Amplifier performancedata.

5 It was clear from the performance datathat Al s desire to get 25 W output frompower MOSFETs on to 30 MHz was go-ing to be a challenge! The RF output power ofmost of the amplifiers described in the ar-ticles drops off to 10 W or less as frequencyincreases just to 14 Idea BrewsAfter hundreds of hours of experimenta-tion, I came up with a design that exceeds ouroriginal objective: One watt of input powerproduces over 40 W of output (after harmonicPar t 1 With only 1 W ofdrive, you ll get over40 W out from 160through 10 meters!A BroadbandHF AmplifierUsing Low-CostPower MOSFETsMarch 199941C1-C8 F chip (140-CC502Z104M)C9 47 pF chip (140-CC502N470J)C10 100 F, 35 V (140-HTRL35V100)C11, C13 15 F, 35 V (140 MLR35V10)C12 1 F, 50 V ( )C14 F, 35 V tantalum( )C15 F chip (140-CC502B103K)C16, C17 F chip(140-CC502B102K)D1 1N4733A, V, 1 W Zener diode(583-1N4733A)D4 1N4004A(583-1N4004A)D2, D3 1N4148 (583-1N4148)D5 1N4744A, 15 V, 1 W Zener diode(583-1N4744A)J1, J2 SO-239 UHF connector(523-81-120)Figure 2 Schematic of the MOSFET all-band HF Amplifier .)

6 Unless otherwise specified, resistors are 1/4 W, 5% tolerancecarbon-composition or film units. Equivalent parts can be substituted. Part numbers in parentheses are Mouser (Mouser Electronics,968 N Main St, Mansfield, TX 76063; tel 800-346-6873, 817-483-4422, fax 817-483-0931; sales@ ; ); see Note 12 V DPDT, 960 coil, mA(431-OVR-SH-212L)L1, L2 91/2 turns #24 enameled wire,closely wound IDL3 31/2 turns #24 enameled wire, closelywound IDQ1, Q2 IRF510 power MOSFET(570-IRF510)Q3 2N3904 (610-2N3904)R1, R2 10 k trim pot (323-5000-10K)R3, R4 27 , 1/2 W (293-27)R6 1 k chip (263-1K)R7 k chip ( )R8 130 , 1 W (281-130); for 7 dB pad(5 W in, 1 W out)R9 43 , 2 W (282-43).

7 For 7 dB pad(5 W in, 1 W out)R10 130 , 3 W (283-130); for 7 dB pad(5 W in, 1 W out)R8, R10 300 , 1/2 W (273-300); for3 dB pad (2 W in, 1 W out)R9 18 , 1 W (281-18); for 3 dB pad(2 W in, 1 W out)R11 k , 1/2 W ( )T1 10 bifilar turns #24 enameled wire onan FT-50-43 10 bifilar turns #22 enameled wire ontwo stacked FT-50-43 Pri 2 turns, sec 3 turns #20 Teflon-covered wire on BN-43-3312 balun : Aluminum enclosure 8 6 inches(HWD) (537-TF-783), two TO-220mounting kits (534-4724), heat-sinkcompound (577-1977), Amplifier PC board(see Note 9), heat sink (AAVID [Mouser532-244609B02]; see text), about two feetof RG-58 coax, #24 enameled wire and#20 Teflon-insulated 1999by thermal runaway.

8 MOSFETs do not re-quire negative feedback to suppress low-fre-quency gain as is often required with bipolarRF transistors. Bipolar transistor gain in-creases as frequency decreases. Very highgain at dc and low frequencies can causeunwanted, low-frequency oscillation to oc-cur in bipolar transistor RF amplifiers unlessnegative feedback is employed to prevent oscillation can damage bipo-lar transistors by causing excess power dissi-pation, leading to thermal LimitationsOf course, MOSFETs do have their limi-tations. The high gate impedance and thedevice structure make them susceptible toelectrostatic discharge (ESD) damage.

9 Someeasily applied precautions prevent this: Usea soldering iron with grounded tip; use a wriststrap connected to ground through a 1 M resistor to bleed off excess body chargewhile handling MOSFETs and do all work onan anti-static mat connected to ground via a1 M sensitivity of a MOSFET s gate tostatic and high-voltage spikes also makes itvulnerable to damage resulting from parasiticoscillation. This undesired self-oscillationcould result in excessive gate-to-source volt-age that permanently damages the MOSFET sgate insulation. Another MOSFET limitationis gate capacitance.

10 This parameter limits thefrequency at which a MOSFET can operateeffectively as an RF Amplifier . I recommendreviewing the referents of Notes 1, 2 and 3 ifyou are interested in more detailed informa-tion about MOSFET RF AmplifiersOf the several power MOSFET amplifiersI built to check their performance, the oneproviding the best performance is the push-pull design described by Jim Wyckoff, AA3X,in QST (see Note 3). I used IRF510 powerMOSFETs rather than the IRF511s performance of this power MOSFET Amplifier design is summarized in Figure 1;its basic design is very similar to anotheramplifier described in the referent of Note 4,written 10 years earlier.