A new type of low noise preamplifier is de-scribed here, which is recommended for itsexceptional noise and inter-modulation char-acteristics not only for normal DX operation,but also for operation under difficult condi-tions when there are a significant number ofpowerful local stations, for example duringcompetitions. The amplifiers are designed tohave low noise, unconditional stability and ex-ceptional linearity, thanks to the use of specialultra-linear, bipolar, low noise transistors de-signed for TV signals amplifiers. Since theyare widely used, they are readily available andlow price. Construction is extremely simplewith a small number of components, very sim-ple adjustments and a high repeatability. Thishas been achieved using extensive computernon-linear and statistical optimisation. De-signs are available for all amateur bands from6m to 23cm [5].

Introduction

For three decades, MOS-FET or GaAs-FET tran-sistors have been used almost exclusively inpreamplifier designs. The reason for this is theirsuperior noise performance and amplification.What we inevitably encounter when using GaAs-FETs is a stability problem due to their condi-tional stability on VHF and UHF frequencies [1 –4]. However, with an increasing number of sta-tions using greater output powers, especially dur-ing competitions, the majority of these low noisepreamplifiers that are successfully used for DX,MS or EME activity, become overloaded. This ismanifested by a large number of inter-modula-tion products that contaminate the band, this isattributed to other stations and especially thosethat use powerful amplifiers.The problem, of course, could be in non-linearpower amplifiers due to excess input power caus-ing saturation. This generates a high level ofinter-modulation products. However, in practiceit is more frequently due to the receiver’s exces-sively high amplification and insufficient linear-ity, i.e. its input stage is overloaded causing it togenerate products that look as though they really

exist on the band.In order to understand how to cure this problem,it is necessary to know how, where and underwhat conditions it occurs. It turns out that thesource of this problem is very high amplifica-tion, a feature that the majority of amateurspraise the most and should be praised the least oreven avoided. Technically it is much more diffi-cult to achieve the two other important proper-ties of an amplifier: low noise factor and strongsignal performance, i.e. linearity. These are themost important properties for an amplifier tothose for whom decibels are not just numbersthat cover ignorance.How do we determine which amplifier is ofgood quality? In order to resolve this dilemma ameasure of an amplifier’s quality has been intro-duced which encompasses all of an amplifier’sthree characteristics: noise factor, amplificationand the output level of a signal for a determinedlevel of non-linear distortions. This measure ofquality is called the dynamic range of an ampli-fier and represents a range in which the level of asignal on an amplifier’s input can be changed,while the output signal degradation stays withindefined limits. The lower limit of this range isdetermined by the minimum allowablesignal/noise ratio of the output signal and it isdirectly determined by the amplifier’s noise fac-tor, and the upper limit is the allowable level ofnon-linear distortion. The lower limit of a dynamic range is the levelof the input signal that gives a previously deter-mined minimal signal/noise ratio (S/N) at the

Ultra Linear Low Noise VHF and UHF Preamplifiers,Dragoslav Dobricic, YU1AW

Fig 1: Mechanical layout of the preamplifier.

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output. If the lower limit value is a S/N = 0(incoming signal and following noise are equal)and if the upper limit of this range is limited bythe maximum output signal voltage, at which theamplifier, due to non-linear distortion, generatesproducts equal to the level of noise on the outputof the amplifier. Then this is the so-called SFDR(Spurious Free Dynamic Range) or a dynamicrange free from distortion, i.e. products of inter-modular distortions or IMD.Since the third order inter-modulation distortion(IMD3) is dependant on the cube of the inputsignal, i.e. with each increase or decrease of theinput signal by 1dB, the third order inter-modula-tion products increase or decrease by 3dB. It istherefore possible to calculate the maximum out-put level for different values of relations betweenproducts and the signal that is being used, or thevalue of IMD3 products, at different output sig-nal levels. Using an attenuator enables us to alsocheck whether an amplifier is overloaded, i.e.recognize whether an audible signal on our re-ceiver really exists on the band or whether it issimply the “imagination” of our overloaded re-ceiver. This enables us to dispose of overload andIMD3.Since the level of products rise faster than thebasic signal, by increasing the input signal wereach the point at which third order inter-modula-tion products, IMD3, reach the level of a usefulsignal at the output and that point is known asIP3 (Intercept Point). When the IP3 value isquoted it is necessary to state if it is referenced tothe input or output of the amplifier. These valuesnaturally differ by the value of the amplifier’samplification. Occasionally, it is stated as theTOI (Third Order Intercept). This point is oftentaken as a measure of an amplifier’s linearity andis highly convenient when comparing differentamplifiers. Knowing the value of an amplifier’sIP3 enables us to precisely calculate the value ofIMD3 products at some arbitrarily chosen outputor input signal level.If excessive amplification is used, for example ina multi stage amplifier, a danger exists where theaerial noise and the noise of the first amplifier areamplified to such an extent that they exceed thelimit of linear operation of the last transistor, at

which point the amplifier is saturated with thenoise itself without any signal.The conclusion is clear: An amplifier is worth asmuch as its dynamic range value, rather thanhow great its amplification is!

Construction

If we want to construct an amplifier with themaximum amount of SFDR we have to fulfil thefollowing conditions:

make sure that the noise factor is as low aspossible

the IP3 is as high as possible has acceptable amplification.

On the one hand, amplification should be aslarge as possible, to prevent second degree influ-ence on noise factor, and on the other hand itshould be as small as possible so that the IP3input is as high as possible, i.e. so that theamplifier should withstand the highest possibleinput signals without distortion. Compromise isessential and it usually ranges between 13–20dBamplification, depending on which parameter ismore important for us.If we want a low noise amplifier with a highdynamic range, then the choice of a correspond-ing transistor is extremely important. It is neces-sary to choose the type of transistor that besideslow noise and sufficient amplification on thegiven frequency fulfils the condition of goodlinearity, i.e. high IP3 along with unconditionalstability. Hitherto, MOS-FET and GaAs-FETtransistors did not fulfil this condition in a satis-factory manner. Specially built transistors forultra linear working, primarily for CATV fulfilthese criteria. For that reason, Siemens BFP196bipolar transistors in SMD packaging were cho-sen. The Philips transistor BFG540/X corre-sponds closely to the Siemens device, it onlyrequires slightly different base bias resistors.This Philips transistor should be used on1296MHz because it gives several decibelsgreater amplification. I should stress thatBFG540 without /X could be used, but the lay-out of pins is different, i.e. it is not pin-to-pin

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compatible with the BFP196, therefore theprinted circuit board has to be changed, which isnot recommended.Since we are talking about a broadband transistorwhose Znf and S11 values are relatively close to50Ω, the input circuit has been chosen to opti-mally match the transistor with regards to noise,while at the same time it provides some selectiv-ity at the input. By varying the circuit values acompromise is found which provides the highestselectivity with minimal degradation of the noisefactor. On lower bands where the noise factor isnot as important, the compromise was more infavour of selectivity which is more importantthan noise on these bands. The operating point ofthe transistor was also chosen as a compromisebetween minimal noise and maximum IP3. Theoutput circuit is relatively broadband and it isimplemented using a printed inductor to reducecoupling with the input and to provide high re-peatability. In order to maintain optimal outputmatching that gives minimal IMD, any matchingby trimmer capacitors or by variable inductances

is forbidden on the output. In order to achieveunconditional stability, minimal IMD, optimalamplification and minimal noise, negative feed-back is applied which cannot be changed arbi-trarily. The printed circuit board is made with the di-mensions shown in the relevant figure (Fig 2 for6m, Fig 3 for 4m, Fig 4 for 2m). Double sidedboard, type G10 or FR4 is suitable. The bottomcopper surface is an un-etched ground plane.SMD components are the 1206 type and theground connections are made using throughplated holes or with wire links through the holessoldered on both sides. The parallel resistor andcapacitor in the base bias circuit are soldered ontop of each other and not next to each other. Thetransistor collector is connected to the widertrack.The trimmer used is either of the air or PTFE foiltype, although a ceramic one can also be used ifit has a suitable capacity range. It is especiallyimportant for the higher band amplifiers that the

Fig 2: PCB layout for the 6m preamplifier. Fig 3: PCB layout for the 4m preamplifier.

Fig 4: PCB layout for the 2m preamplifier. Fig 5: Circuit diagram for the 6m preamplifier.

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trimmer capacitor has a low enough minimumcapacity.The coil is wound, as shown in the relevantcircuit diagram (Fig 5 6m, Fig 6 4m, Fig 7 2m),with silver plated copper wire, thickness “d” and“n” turns with a body diameter “D”. The coil is tobe expanded to length “l”. When the coil is fittedit has to be positioned so that the bottom isapproximately 3mm above the printed circuitboard.The box for the amplifier is made so that theprinted board is the bottom side of the box, as canbe seen in Fig 1. The easiest way to do this is tosolder 25-30mm wide copper or brass strip,0.3mm thick, around the edges of the printedboard. Connections are mounted onto the boxcreated, and a lid is made out of the same kind ofsheet metal.Once everything is carefully soldered, check forany possible mistakes such as short circuits. Thenconnect the DC supply voltage and measure thecollector current and voltage. If everything iscorrect and properly connected and the transistor

is functioning properly, the values should beclose to the ones given in the circuit diagram. Ifthe differences are within 10%, everything isOK. If the differences are greater, check thesupply voltage and then reduce the value of thebase bias resistor, which is in parallel with thecapacitor. Make it lower to raise the collectorcurrent and vice-versa. Do not change the valueof the other resistor in the base bias circuit. If thecollector voltage is not correct at the correctvalue of collector current, adjust the value of theresistor in the supply line. Such corrections areextremely rare and are necessary only if theparticular transistor used has different character-istics from the common characteristics for thattransistor type.When both collector voltage and current arewithin the expected range, connect an aerial tothe amplifier input and the receiver to the outputand adjust the trimmer capacitor for maximumreceived signal using a weaker station. Thiscompletes the final adjustment; the performance

Fig 6: Circuit diagram of 4m preamplifier. Fig 7: Circuit diagram of 2m preamplifier.

Fig 8: Power feed for preamplifier.Fig 9: IMD products from a BFP196 preampli-fier.

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will be very close to the predicted values. Withhigher band amplifiers, especially 23cm, there isa slight difference in matching for maximumamplification and for minimum noise. The ampli-fier should be set to maximum amplification andthen adjust it to a slighter lower frequency, i.e.slightly raise the trimmer capacity, until the am-plification falls by 1-2dB.Any further changes or modifications except theones stated above are absolutely not recom-mended, because the amplifier is optimised sothat it immediately reaches the required charac-teristics. Any modification would prevent thatand would produce much worse results than theexpected.The amplifier should always be mounted on theaerial pole and connected to the aerial with theshortest possible cable, using coaxial relays toswitch the aerial from receive to transmit. Itspower supply should be fed through the coaxial

cable that connects it to the receiver, using theadapter shown in Fig 8.As expected, very good noise characteristicshave been proved in practical use, which mainlysatisfy every requirement for serious DX work.Only for EME work at 432 and 1296MHz youmight try using lower noise value amplifiers, i.e.the GaAs-FET amplifiers [1-4], but in all othercases the amplifier satisfies even the most rigor-ous noise requirements. These amplifiers haveshown exceptional linearity with IP3 values farexceeding 30dBm on all bands except at1296MHz where it is 3-4dB lower.As a comparison, Figs 9 and 10 show the per-formance of this amplifier and a common ampli-fier which uses an MGF1302 GaAs-FET. Bothamplifier inputs have three signals of 7.1mV (-30dBm) to simulate “three strong stations on theband”. The graphs show what would be heardwith an ideal receiver without its own IMD.With a real receiver, because of its possibleIMD, things would look even worse! Before you

Fig 10: IMD products from an MGF1302 pream-plifier.

Fig 12: Component layout for 4m preamplifier.

Fig 11: Component layout for 6m preamplifier.

Fig 13: Component layout for 2m preamplifier.

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accuse someone of “band wasting” check with aninput attenuator whether your receiver might ac-tually be creating IMD due to a strong inputsignal! The amplifier using the BFP196 is supe-rior to the one using the MGF1302. The differ-ence in the IMD products appearing on the outputof an ideal receiver was over 30dB! Of course, inboth cases the amplifiers had approximately thesame amplification.The component layouts for the amplifiers areshown in Figs 11 - 13. The predicted perform-ance of the 6m and 2m amplifiers are shown inFigs 14 - 21. The values shown have been simu-lated on a computer, and in real life proven on asufficient number of built and measured amplifi-ers that they do not differ more than usual for thistype of construction. Strict adherence to theguidelines given here will produce amplifierswith performance very close to those shown.The final results achieved with these amplifiers

in real life conditions largely depend on the IMDcharacteristics of the receiver used. If it hasweaker characteristics, then the results may evenbe worse in respect to IMD because when sig-nals, amplified in the preamplifier, reach theinput of a bad receiver they cause overload andthe IMD in it and the results are poor. That iswhy the minimum necessary amplification isrecommended between this amplifier and thefirst mixer in the receiver or the transverter inorder to preserve as much dynamics of the wholereceiving system as possible. If IMD is apparent in the receiver it is recom-mended to put a variable attenuator between theamplifier and the receiver, define the lowest at-tenuation at which it disappears, replace it with afix attenuation of the same value and work inthat manner. This method is highly efficient be-cause the IMD products are attenuated threetimes faster than the wanted signal, so that it ispossible to weaken the products to the level

Fig 14: Amplification, input and output adjust-ment of the 6m preamplifier.

Fig 16: Stability factor and adjustment for the 6mpreamplifier.

Fig 15: Noise figure, minimum noise and stabil-ity of the 6m preamplifier.

Fig 17: Two tone test and IMD products for the6m preamplifier.

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where they are not heard whilst preserving theuseful signal with very little attenuation! Don’tbe afraid that you will not hear the desired signal,there is too much amplification as soon as IMDappears – feel free to lower it!A miniature 100-500Ω trimmer potentiometerconnected to the receiver input can be used inplace of a variable attenuator. This can be builtinto the amplifier supply adaptor box as shownon Fig 8. This represents a very practical andrather elegant solution at least on lower bands.You can also use a variable 20dB attenuator usedin CATV.

References

[1] Low noise aerial amplifier for 144 MHz,Radioamater, Dragoslav Dobricic, YU1AW,10/1998 pp 12-14 (part I) and 11/1998 pp 12-15(part II). Also: Low noise aerial amplifier for 144

MHz, KKE Lecture text, December 1998.

[2] Low Noise aerial amplifier for 144MHz,Dragoslav Dobricic, YU1AW, CQ ZRS Decem-ber 1999, pp 26-31.

[3] Low noise aerial amplifier for 432 MHz,Dragoslav Dobricic, YU1AW, Radioamater, 1and 2/2001.

[4] Low noise aerial amplifier for 432 MHz,Dragoslav Dobricic, YU1AW, CQZRS, 6/2000,pp 27-31.

[5] Web site: www.qsl.net/yu1aw/

[6] Author can be contacted on:dobricic@eunet.yu

Fig 18: Amplification, input and output adjust-ment of the 2m preamplifier.

Fig 19: Noise figure, minimum noise and stabil-ity of the 2m preamplifier.

Fig 16: Stability factor and adjustment for the 2mpreamplifier.

Fig 17: Two tone test and IMD products for the2m preamplifier.

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