The purpose of this article is to describe howto assemble several VHF power amplifiers,examples are given for the 50 and 144 MHzbands. Two types of amplifiers will be consid-ered, amplifiers with one and with two transis-tors. Assembly instructions are given as wellas explanations of some general issues withregard to the construction of VHF power am-plifiers.

Introduction

The first step is to choose a transistor, this ulti-mately dictates what will be achieved. The qual-ity of an amplifier is defined by several param-eters that define its quality. An amplifier can bedeemed to be a high quality amplifier if it has thefollowing characteristics:

High efficiency level High power amplification Good suppression of harmonics and low

level of IMD products Good electric and thermal stability Simple design and adaptability

Some of these are contradictory and often have tobe solved by various compromises, while othersare compatible and by creating the first we auto-matically create the other. High efficiency is themost important, maximising this ensures that themajority of the other parameters approach theiroptimum. Beside an arbitrarily chosen workingpoint or class in which the transistor will operate,the matching as well as the circuit losses influ-ence the efficiency. For highest efficiency it isnecessary to:

Strictly respect AC and DC current and volt-age recommendations by the manufacturerfor the class of operation.

Match the transistor to the output conditionsrecommended by the manufacturer formaximum efficiency. Also the losses in thematching circuits should be minimal.

Keep the temperature of the transistors un-der the maximum allowed temperature,which is achieved by adequate cooling.

By fulfilling these conditions a high degree ofefficiency is obtained, as well as highest output

power, relatively low operating temperaturesand slight harmonic and IMD distortions. The second condition, high power amplification,is achieved by matching the input to ensure sta-ble operation. Losses in the matching circuitsdirectly influence the power amplification. An-other extremely important condition that has tobe fulfilled for high amplification is extremelygood grounding of the emitter or source in FETtransistors. These should be extremely shorttherefore of extremely low resistance!Stability of the amplifier depends on the transis-tor itself, but also in a large measure on otherfactors such as: matching, mechanical construc-tion and separation of electric paths in the inputand output circuits. This will eliminate unwantedfeedback between input and output. The mostimportant issue for the stability of any amplifierconcerns the low frequency performance wherethe transistors have enormous gain. It is impor-tant to make sure that the base and collector“see” several dozens ohms. If a collector or base“sees” a short circuit via a large capacitor oropen circuit via a high inductance at low fre-quencies, an amplifier oscillates very easily.The third condition, good suppression of higherharmonics and low level of generated inter-modular products (IMD) also depends on manyfactors. We will list just a few:

choice of the optimal value of efficiency(Q) of output circuit

linearity of a transistor’s chosen operatingpoint

output matching small output resistance of the base bias cir-

cuit value of excitation power

By correct design, the unwanted secondary prod-ucts generated by the amplifier could be reducedto a minimum. Output filtering can reduce theseeven further. Filtering cannot reduce some of theodd order inter-modulation products becausethey are very close to the operating frequency.They can only be influenced by correct design ofthe amplifier, making it linear generates a verysmall level of these unwanted products. Evenharmonics and even series of inter-modulationproducts can be efficiently decreased by push-

VHF Transistor Power Amplifiers, Dragoslav Dobricic,YU1AW

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pull amplifier design. This is an advantage incomparison with amplifiers operating with twoparallel transistors.For amplifiers operating in linear classes (A andAB) the correct choice of the base bias circuit isalso very important and it provides good linearityof the input signal. The stability of collectorvoltage also has an influence, it should be stabi-lised for amplifiers up to 20V, and for voltagesabove these it is sufficient if it has good regula-tion. For the linear operation of an amplifier, thelevel of excitation power is also crucial. It should

not be too high, because it “pushes” the transis-tor into non-linear operation.Thermal stability is provided by the correctchoice of a heatsink and more importantly, bycorrectly mounting the transistor. In order tocompensate the thermal movement of the operat-ing point it is necessary to provide thermal feed-back that will, by monitoring the temperature ofthe transistor, this will change the base bias tomaintain the DC operating point stability.All these conditions that have to be fulfilled if an

Fig 1: Circuit dia-gram of the onetransistor poweramplifier.

Fig 2: PCB layoutfor the one transis-tor power ampli-fier.

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amplifier can be described as a high quality am-plifier, they are only discussed briefly here. Theyare elaborated in literature [1] and [2].

One transistor poweramplifiers

The circuit of the one transistor power amplifieris shown in Fig 1. A PCB layout is shown in Fig2 and the component layout in Fig 3. The PCBmay have to be varied slightly depending in theactual RF transistor chosen. Table1 shows thecomponent values for various transistors and Ta-ble 2 shows the expected performance. A circuitconsisting of C1, C2 and L1 performs inputmatching. The Q of this circuit is about 10-15,

which is the optimal value in this case. Base biasis fed via an RF choke, which is a VK 200 ferritebead with six holes. The output circuit thatmatches the output of the transistor to 50Ω isperformed by L2, C4 and C3. All capacitors inthe matching circuits are ARCO, ceramic-micatrimmers of corresponding capacity. The valuesof the matching circuit elements are shown inTable 1. The number printed on the trimmergives the range of certain trimmers (Fig 4). Cer-tain types of trimmers and the correspondingrange of capacity are given in Table 3. Thevoltage values, currents, power and efficiencyfor each separate type of transistor are illustratedin Table 2. The collector supply is fed via an RFchoke which instead of a ferrite is open wound.The diameter of this choke is 5mm, it is12mmlong, and has 8 turns of 0.8mm wire. After the

Fig 3: Componentlayout fro the onetransistor poweramplifier.

Table 1: Components required for various transistors.Transistor Freq.

MHzC1pF

C2pF

C3pF

C4pF

L1 L2

Ind.nH

Dmm

Lmm

nturns

dmm

Ind.nH

Dmm

Lmm

nturns

dmm

BLW76 50 36 200 21 46 44 6.5 6 3 1.2 157 12 10 4 1.5BLX15 50 56 363 20 37 23 6 4 2 1.2 195 15 12 4 1.5MRF317 50 40 210 24 57 47 10 10 2.6 2 140 15 15 3.7 22xBLW84 145 12 33 5 3.3 15 6 3 1.5 1.2 113 10 10 4 1.52xMRF245 145 21 50 21 33 7.5 7 3 1 1.5 14 10 4 1 22xMRF247 145 21 50 21 33 7.5 7 3 1 1.5 14 10 4 1 2

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choke, a fairly small value capacitor decouplesVHF frequencies to the ground. The 100nF ca-pacitor decouples low frequencies via a 15Ω re-sistor to ground. The choke in parallel with theresistor is a VK 200 ferrite or a similar one thatcan carry the high currents that flow through thetransistor.A BD135 transistor connected as an emitter fol-lower with extremely low output resistance car-ries out base bias, this ensures a stable operatingpoint. Thermal monitoring and compensation ofthe operation point is done by diode, D1, posi-tioned so that it has physical and thermal contactwith the transistor (not with the heatsink!). Thisensures stable operation over a large temperaturerange. The other diode, D2, monitors the ambienttemperature. Both diodes are of the 1N4007 typeor similar. Transistor BD135 is mounted on thesame heatsink as the RF transistor. Adjusting theoperating point is performed by 5KΩ trimmer

potentiometers. Beside electrolytic (or tantalum)capacitors all the others are disc ceramic or ofsome similar VHF quality.

Two transistor amplifiers

As can be seen in the circuit diagram show inFig 5 there are two identical amplifiers con-nected by phasing transformers, they operate in asymmetrical anti-phase or push-pull connection.A PCB layout is shown in Fig 6 and the compo-nent layout in Fig 7. The PCB may have to bevaried slightly depending in the actual RF tran-sistor chosen. Table 1 shows the component val-ues for various transistors and Table 2 shows theexpected performance. Circuits for supplying thebase and collector of the transistors are the sameas the one transistor amplifiers. Even the inputand output matching circuits are similar. Theonly novelties are two pieces of 50Ω coaxialcable 95mm long that transform the asymmetricinput and output of the amplifier onto the sym-metric connection of two transistors. Teflon ca-ble several millimetres thick, RG-142, type orsimilar should be used. The coaxial cable can bewound in a coil, or even better on some small

Table 2: Transistor parameters.Transistor Frequency

MHzVC

Volts

IC0

mA

ICmax

APdriveW

PoutW

Efficiency%

BLX15 50 50 50 6.5 15 150 65BLW76 50 28 50 8 6 90 60MRF317 50/145 28 10 6.5 12 100 60MRF238 145 13.8 20 4 3 30 552xMRF245 145 13.5 2x30 2x18 35 200 502xBLW84 145 28 2x30 2x2 6 60 552xMRF247 145 13.5 2x30 2x18 35 200 50

Fig 4: Picture ofan ARCO trimercapacitor.

Table 3: Types of ARC trimmer.Type of ARCO trimmer Range404 7pF – 60pF423 7pF – 156pF426 37pF – 250pF462 5pF – 80pF

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Fig 6: PCB layoutfor the two transis-tor power ampli-fier.

Fig 5: Circuit diagram of the two transistor power amplifier.

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ferrite toroid or onto a larger bead. Ferrite withtwo small holes can be used for this purpose.Using two such ferrites, two windings could bewound. If you do not have these, you couldthread two small ferrite beads with one hole eachonto the cable so that there is one bead on eachend, but it is even better if you thread severalbeads and place them along the cable length. Thiswill improve the symmetry of the transformer,especially at low frequencies, which can be sig-nificant for a stable amplifier. It is very importantthat the entire amplifier has to be totally sym-metrical with regards to the mechanical layout ofcomponents and electrical parameters (values of

elements, currents and voltages, etc.), so thatmore power, amplification, efficiency and sup-pression of even harmonics can be achieved.While adjusting, it is very important to maintainthe capacitances of C1 and C3 in both amplifiersthat same.

Mechanical construction

The whole amplifier has to be built on a rela-tively small piece of a single layer FR4 printedcircuit board (Fig 6). Emitter leads are solderedas short as possible onto the ground of the board.The transistor should be mounted onto a largeheatsink using thermal paste as illustrated in Fig8. The surface of the board should be small toenable the fins of the cooler to be as close to thetransistor as possible. To provide better coolingit is important to make sure that the hole drilledfor mounting the transistor onto the heatsink isbig enough to allow a screw to pass through it.The transistors should be mounted with theirentire surface on to the heatsink. The surface ofthe heatsink where the transistors are mountedhas to be smooth. Input and output connectorscan be fixed onto the board but also on to the

Fig 7: Component layout for the two transistor power amplifier.

Fig 8: Mounting transistor on heatsink.

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heatsink or eventually the box and connectedwith coaxial cable to the board with its groundssoldered on both ends. The coils, L1 and L2, inthe matching circuits should be mounted so thattheir axes are at a right angle, to decrease interac-tive coupling.Once everything is connected, as illustrated inFig 7, check once again to ensure that there areno mistakes and short circuits to the ground, andadjust the potentiometers for maximum resist-ance. Connect the supply to one transistor andadjust the collector current to the value fromTable 2. The same procedure should be carriedout with the second transistor. Even more impor-tant than the exact value of quiescent current isthat they are identical in both transistors! Thenconnect both transistors to the supply and con-nect a 50Ω dummy load to the output via awattmeter or SWR meter. If you do not have adummy load, a good aerial with low SWR can beused. Supply minimal excitation and by measur-ing the output power alternately adjust trimmersuntil maximum output power is achieved. Repeatthe adjustment several times, gradually increas-ing the excitation power. Finally, with full excita-tion, which should not exceed the permitted out-put power, adjust all trimmers to the highestoutput power. At the same time the transistor’scurrent should be measured so as not to exceedthe maximum allowed value. If input trimmers,C1 or C2, need to be at maximum or minimumcapacity during adjusting, it is necessary to

change the length of the cable between the ex-citer and the amplifier itself. The optimal lengthof the cable should be determined experimen-tally to obtain adjustment with approximatelythe same values of C1 and C2. This experimen-tally determined cable should always be usedwhen operating the amplifier. A change of ex-citer could occasionally require a new length ofcable to be determined. In push-pull amplifiers itis extremely important to perform adjustment sothat the corresponding trimmers on each transis-tor are adjusted simultaneously to ensure thatthey have approximately the same capacity dur-ing adjustment. By maintaining symmetry dur-ing adjustments, extremely dangerous situationsare avoided which can cause the amplifier toself-oscillate.For transistors with a higher than 20V powersupply, a non-stabilized supply can be used, butit is constructed so that it has very good voltageregulation as illustrated in Fig 9. It should bewell specified with good quality electrolytic ca-pacitors. The transformer should be slightly overspecified. To avoid blown fuses caused by thecharging current of electrolytic capacitors, it isnecessary to build in a delayed switching device.It is performed simply by a 220V relay con-nected as shown in the circuit. At the momentwhen power is switched on, the transformer isconnected to a power supply via a resistor thatlimits high charging currents. When the capaci-tors are charged and transients in the transformersettle down, the current through the resistor de-creases, the voltage on the primary increases andthe relay that bridges the resistor with its con-tacts is switched on. It is also possible to userelays that switch on via some electronic timerafter a couple of dozen seconds. Although thisappears to be a more elegant solution, it is a farworse solution, due to two reasons: first, in thecase of a very short interruption of supply volt-age the timer has not been reset, voltage isswitched on without delay; and second, in thecase of a fault that causes high current consump-tion, when the relay would not be switched on,the entire system would protect itself, whereasthe timer would switch on the relay and subse-quently full power.

Fig 9: Circuit diagram of power supply.

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Transistors with 12-18V collector voltage mustbe supplied with stabilized voltage. In this case itis compulsory to build in a thyristor device forover voltage protection. This device must con-nect the voltage via a thyristor to the ground andthereby induce a forced blown fuse if the voltageexceeds some preset value. This prevents the RFtransistor from blowing up. Any type of relayprotection is not recommended in such cases as itsimply cannot react quickly enough and by thetime the relay is switched, the transistor is al-ready blown up!

ConclusionIn this article the necessary conditions are listedfor the construction of transistor power amplifi-ers for the VHF range. On the basis of all that hasbeen stated above, it can be concluded that theproposed amplifiers have a design to providenearly optimal results with regards to perform-ance, maximum simplicity of construction andadjustment. In practice good agreement with theexpected results has been demonstarted, whichjustifies the proposed solutions.

Credits

I thank all my colleagues who helped me in theconstruction of numerous amplifiers that I havedesigned throughout the years. Some of themhave not been published since some use rare anddifficult to obtain transistors. However, all theones that I considered relatively easy to con-struct have been collected and published here.

References

[1] Transistorised power amplifier for 144MHz,Dragoslav Dobricic, YU1AW, Radioamater,2/1988 pp 34-37, Radioamater, 3/1988 pp 66-68.

[2] Internet address: www.qsl.net/yu1aw/

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