MOTOROLASemiconductor Products Inc.

AN-793Application Note

"A 15 WATT AM AIRCRAFT TRANSMITTER POWER AMPLIFIERUSING LOW COST PLASTIC TRANSISTORS"

Prepared byDavid S. HollanderRF Product Group

INTRODUCfIONThis application note describes a 15 watt carrier power,

amplitude modulated broadband amplifier covering the118-136 MHz AM aircraft band. Simplicity and low costare emphasized in this design through the use ofMotorola's common emitter TO-220 VHF power tran-sistors. The power amplifier is designed to operate from13.5 VDC. High level AM modulation is accomplished bya series modulator operating from a 27 volt supply.

CIRCUIT DESCRIPTIONThe transmitter power amplifier has three stages using

an MRF340 transistor as a pre-driver, a MRF342 as adriver, and a MRF344 as the final amplifier. All threedevices are common emitter where the mounting tab isthe emitter. The pre-driver stage is forward biased toenhance linearity and dynamic range. Amplitude modula-tion is applied fully to all three stages.

The P.A. is designed to operate with 50 ohm source andload impedances.

DESIGN CONSIDERATIONSThe design objectives are that the transmitter must be

capable of operating over the range of 118 to 136 MHzwith a minimum carrier output power of 15 watts. It mustalso be capable of being amplitude modulated greater than+85% over the frequency range, and the transmittershould be free from instability.

Other important considerations involve the interstageand the output networks. The output network is tooperate efficiently at both the carrier power of 15 wattsan(t the peak power of 60 watts while providing harmonicsuppression. The interstage networks transform the out-put impedance of the device to the input impedance of

f = 136 MHz Pearrier = 15 Watts (2 ems)% Upward Modulation = 90%

the following device during modulation of all three stages.In designing the networks, the Smith Chart is used to

obtain initial values. These values were then optimizedusing a computer aided design program.

Figure 2 shows a schematic diagram of the transmitterP.A. RF circuitry.

The pre-driver stage uses a simple 1T-section inputmatching network. Forward bias for this stage is obtainedthrough the network consisting of R I, R2, R3, DI, and isapplied to QI through L2' The quiescent current isapproximately 20ma for the MRF340.

Motorola reserves the right to make changes to any products herein to improve reliability, function or design. Motorola does not assume any liability arisingout of the application or use of any product or circuit described herein; neither does it convey any license under its patent rights nor the rights of others.

+C19I

RF ClIn 0---:)

Cl - 200 pFC2, C3 - 47 pFC4, C5, C9, C23 - .001 Disc CeramicC6, C16 - 39 pFC7 - 68 pFca, C12 - 200 pF UNELCOCl0-62pFCll -43pFC13 - 80 pF UNELCOC14 - 56 pFC15 - 75 pFC17-10pFC18 - 300 pF

C13

JJ

C19, C21, C26 - lJ,lf/50VC20, C22, C25 - 680 pF Feed throughC24 - 0.1 J,lf01 - IN4002All inductors other than molded chokes

I are wound with #18 AWG 1/8" 10Ll - 3TL2, L5, L8 - .15 J,lH molded choke

with ferrite beadL3 - 7TL4, L7, L12 - .18 J,lH molded chokeL6, Ll0 - 4TL9 - piece of wire - 3/4" long

The inter stage between QI and Q2 is designed as fol-lows. The effective collector load impedance is estimatedto be 100-j50 ohms. The input impedance, ZIN, of theMRF 342 is 1.75+j2 ohms at 136 MHz (as taken from thedata sheet). One way to match the output of the MRF340to the input of the MRF342 with a minimum of compo-nents is through the use of a 9: I transformer (1,2). Figure3 shows a Smith Chart plot of the interstage network withthe chart normalized to 50 ohms. Starting at point A, thisimpedance is rotated to point B by a shunt capacitor C8'The impedance at this point is approximately 5 to 5.5ohms. A 9: I transformer transforms this impedance toapproximately 50 ohms. Point C, the 50 ohm point, isrotated to D by series capacitor C6' Point D is thenrotated to point E, the complex conjugate of the outputimpedance of QI, by shunt inductor L3'

A different approach is used for the Q2 - Q3 interstagenetwork. The MRF342 output impedance ZOL, and theMRF344 input impedance ZIN, are taken from the datasheets. Figure 4 shows a Smith Chart plot of this networkwith the chart normalized to 50 ohms. Point A is theinput impedance of the MRF344. This impedance isrotated 'to the real axis by shunt capacitors - C12 andC13. Point B is' then ro~ated to point C by series capaci-

+13.5 V1: + (Modulated)l...C25 TC26

All Capacitors < 500 pFare Dipped Mica

C18f-o

L11 - 7TRl -820.11 114 WR2 - 39.111/4 WR3 - 82.111/4 WTl - 9:1 Unbalanced to unbalanced XFMR

#24 AWG 12 crestsper inch 3.5" long

Ql - MRF34002 - MRF34203 - MRF344

tors - C I0 and CII. This impedance is then transformedto the complex conjugate of the output impedance of theMRF342 by shunt inductor L6.

The MRF344 output matching network consists of ashunt inductor at the collector of Q3 followed by twoL·sections. L-sections were used because they provide ex-cellent harmonic suppression and good efficiency over theentire band. Figure 5 shows a Smith Chart of the outputnetwork, with the chart normalized to 50 ohms.

All impedance matching element values calculated usingthe Smith Chart and optimized with the computer pro-gram were used as starting points in building the net- .works. The final component values shown in Figure 2were derived through on-the-bench tuning and adjustmentand differ from the calculated values as the Figure 2values cover 118-136 MHz. The calculated values are onlyfor 136 MHz.

Since low cost is a key factor in the use of the TO·220devices, inexpensive components are used wherever ap-plicable. Molded chokes, dipped mica and dipped ceramiccapacitors are used throughout the circuit. One of theproblems encountered when using dipped micas at VHF istheir series lead inductance. The higher capacitance valuesapproach resonance at VHF. Selected values were

measured on an HP network analyzer at 125 MHz. Theresults are shown in Table 1.

Cnominal (PF) Cmeasured (PF)5 5

10 1025 2630 3339 4550 6475 106

100 161200 750

Note: All lead lengths kept to an absolute minimum«0.1 inch)

The data obtained shows that values below 75pf areusable. Lead length should be kept to a minimum whenusing both the dipped mica and the disc ceramiccapacitors. UNELCO capacitors are used in place of thedipped micas at the base of Q2 and Q3, since the netrequired capacitance and base current is very high.

CONSTRUCTION TECHNIQUESThe amplifier is assembled on a 2" X 5" double sided

printed circuit board. Board material is G-1O with a thick-ness of 0.062". A I: 1 photomaster of the top side of theboard is shown in Figure 6. Eyelets are placed through theboard at points marked by the letter "0". The eyelets aresoldered to both sides of the PCB to connect the topground to the bottom side ground return. Feed-thrucapacitors are mounted on the DC feed bar which is madeof G-I0 PCB. AI: 1 photomaster of the feedbar is alsoshown in Figure 6. Eyelets are placed at points marked byan "0" and feed-thru capacitors are placed at pointsmarked by an "X". The DC feedbar is soldered to themain board. The location of the critical components isshown in Figure 7. Construction details of the 9: 1 imped-ance transformer are shown in Figure 8. (1,2)

For reliable operation, the transistors must not only beheatsunk, but they must also be mounted properly foremitter RF ground return. Figure 9 shows mounting de-tails for the TO-220 package. More detailed informationon mounting is available in AN-778.(3) The entire as-sembly is mounted on a 6" extruded aluminum heatsinkusing 440 X 14 machine screws. The heatsink surfaceshould be flat and free of burrs, particularly around thetransistor mounting holes.

Valu •••Calculated at f = 136 MHz

C6 - 19 pFC8 - 350 pFL3 - 84 nh

MRF340 ZOL = 100 - j50MRF342 ZIN = 1.75 + j2

Cl0 + Cll - 130 pFC12 + C13 - 365 pFL6 - 15 nh '

MRF342 ZOL = 17-j4. 7MRF344 ZIN = 1.25 + j2.86

C14 + C15 = 100 pFC16+C17=23pFL9 = 11 nhLl0=28nhL11 = 90 nh

~oo

The transformer is wound with #24 AWG. Three differentcolors are used - red, green and blue.

@ ® Red

@) ® Green

® 0 Blue

I 3.5" I

a) Preferred Arrangementfor Non-isolated

Mounting Screw is at Semi-conductor CasePotential.4-40 Hardware is used.y 4-40 Machine Screw

II

~

"""""'"EMIN

..-::-;~EMAxt_1 Ef Modulated 50%

WaveshapeofModulating Voltage

I

6j/

Semiconductor

II=====:»

A\J

WaveshapeofModulating Voltage

Heatsink is tapped

~,o,. 440 '.rnw

MODULATIONIn an amplitude modulated waveform the amplitude of

each cycle of the modulated wave varies in accordancewith the modulating signal. Using voice modulation, theresultant waveform is not only complex, b~_tdifficult toanalyze. Therefore, when testing and analyzing the trans-mitter P.A., a simple 1 KHz sine wave is used as the modu-lating signal. When analyzing an AM waveform, one of thethings to consider is the modulation factor (M). M isusually expressed as percent modulation and is calculatedas follows:

M = Emax - Emin X 100%Emax + Emin

Figure 10 shows amphtude modulation waveforms.

The above formula is valid only when the modulationprocess is symmetrical and little distortion is present. Ifsignificant assymmetry is present then up modulation anddown modulation must be analyzed separately.

M = Emax - Ec X 100% For positive peak modulationEc

Ec - Emin .M = Ec X 100% For negative peak modulation

When a carrier is modulated by a pure sine wave, twosidebands are generated at the carrier frequency plus andminus the modulating frequency. The power level of the

sidebands is dependent upon the percentage of modula-tion. At 100% modulation, the total power contained inthe sidebands is one-half the carrier power or one-fourthin each sideband. For modulation levels ofless than 100%,the total power is:

1PSB=-m2pC2

where m = mod'ulation faCtorPc = carrier power

Collector modulation is a commonly used method formodulating a solid state transmitter. Using this method,the modulating voltage is applied to a collector through atransformer. The secondary winding of the transformermust be capable of handling the DC current required bythe transistor and have low DC resistance, so as not tocause a significant voltage drop. The voltage drop willreduce the voltage applied to the collector of the stagebeing modulated.

Another form of collector modulation uses a seriesmodulator. This type of modulator is used to evaluate thetransmitter in this application note,( 4) A series modulatoruses audio power transistors instead of a transformersecondary. A schematic diagram of the series modulator isshown in Figure 11. 27 volts is applied to the modulatorand the quiescent DC voltage applied to the transmitter isset by the 1OKn potentiometer.

1. +27 V

10 jJF1AudioInput To

CircuitUnder1Test

118 MHz 127 MHz 136 MHz

Pin (mW) 14.0 15.0 18.0

Pcarrier (W) 15.0 15.0 15.0

Total Current lAde) 2.2 2.0 2.5

PowerSupply Voltage (Vdc) 13.5 13.5 13.5

Upward Modulation (%) 89.0 88.0 90.0

Harmonic Rejection (dB) (Relative to PeakPower)

21 55.0 55.0 52.031 58.0 58.0 57.0

Load Mismatch Capableof Operating into 3:1 Load VSWR.

75

60

.•~~ 45~0

0..~~S~ 300;.~0

0..

15

II If-- f = 136 MHz

Pin=18mW

.7V

/./

./V./

./ V

~ V-vcc. Supply Voltage (Volts)

Pout is initially set at the carrier power of15 watts at 13.5 Vdc, then the supplyvotlage is varied from 0 to 27 Vdc keepingPin constant. Th is demonstrates the peakpower output capability of the transmitter P.A.

TEST SET-UPWhen adjusting a broadband RF power amplifier, it is

advantageous to have a swept test station. Using a sweptset-up, one can observe the following:

I) The effect of varying individual component values2) Bandwidth3) Instability4) Input VSWR bandwidth.

A Wavetek 1002 Sweep Generator driving a MotorolaMHW590 module is recommended as a drive source.Figure 12 shows a block diagram of the swept set-up usedto test and evaluate the amplifier.

Pwr. Supplyand

Modulator

DirectionalCoupler

REFERENCES

I. Ruthroff, "Some Broadband Transformers," PROC.IRE Vol. 47, No.8, August, 1959

2. Granberg, H. "Broadband Transformers and PowerCombining Techniques for RF," Motorola AN-749

3. Roehr, Bil1, "Mounting Techniques for Power Semi-conductors," Motorola AN-778

4. "A 13-W Broadband AM Aircraft Transmitter,"Motorola AN·507

5. Terman, Frederick E., "Electronic and Radio Engi-neering," McGraw-Hil1

® MOTOROLA Semiconductor Products Inc.