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SEMICONDUCTOR APPLICATION NOTE
NOTE: theory this application note still applicable, some products referenced discontinued.
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AN548A
Microstrip Design Techniques Amplifiers
Prepared Glenn Young
INTRODUCTION
This note uses watt amplifier design vehicle discuss microstrip design techniques. design concentrates impedance matching microstrip construction considerations. basic knowledge Smith chart techniques helpful understanding this note.1 amplifier itself, shown Figure provides watts output power band. designed 12.5 volt operation which makes useful mobile transmitting equipment. variety police, taxi, trucking utility maintenance communication systems operate this band. summary performance completed amplifier operating with 12.5 volt supply indicates
power gain bandwidth MHz. Overall efficiency 48.5% harmonics minimum below fundamental output. Sections construction device handling considerations also presented.
MICROSTRIP DESIGN CONSIDERATIONS
Microstrip design used this amplifier inherent superiority over other methods this frequency. These techniques only offer good compatibility with Motorola "stripline" package they also offer very good reproducibility. Microstrip construction more efficient than lumped constant equivalents since microstrip lines less lossy than lumped constant components.
12.5
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MICROSTRIP LINE 2N5945 MICROSTRIP LINE
2N5946 MICROSTRIP LINE
MICROSTRIP LINE
2N6136
FERRITE BEADS Microstrip lines 5.72 wide long C1,2,3 feedthru C4,5,6 Tantalum C7,8,9 Ceramic C10,11,13,15,16,17 1.5-20 Compression Trimmer ARCO C12,14 Microwave capacitor type 100-B-10-M-MS equivalent turns Closewound 3/16" I.D. 0.15 molded choke Ferroxcube 20/4B equivalent Ferrite beads Ferroxcube 65/3B equivalent
Figure Schematic Diagram Amplifier
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AN548A
2N5945 Zout 2N5946 Zout 2N6136 Zout j4.11 ohms j1.96 ohms j1.2 ohms j0.5 ohms j1.5 ohms j5.4 ohms
Microstrip board with Teflon bonded fiberglass dielectric rather than higher dielectric constant ceramics chosen ease working with that type material. substrate thickness 1/16-inch convenient since line same width transistor leads (0.225 inch) produces reasonable characteristic impedance (Zo) 40.65 ohms. value characteristic impedance calculated from:4
1.735 377h r-.0724 -.836
dielectric constant width microstrip line thickness dielectric term equal total thickness microstrip board minus thickness copper both sides. this design that term equal where
Figure Transistor Complex Input Output Impedance (Series Form) Smith chart techniques used synthesize matching networks amplifier described. complex series equivalent input output impedances taken from data sheets shown Figure There infinite number solutions required matching networks, however, once initial choice components made, only solution exists. obvious that components need kept within reasonable limits, however would seem that most critical parameter length microstrip line. Using this assumption, length line chosen starting point. input network, shown Figure will solved illustrate technique.
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1.4) 59.2 mils copper mils thick
effective width should used when conductor finite thickness.
Weff
where thickness conductor
Weff (1.4/) 59.2 227.4 mils
therefore:
.0592 .2274 1.735 2.5-.0724 40.65 227.4 -.836 59.2
AMPLIFIER DESIGN
first decision design determining type matching networks used. network shown Figure chosen because ability "map" large area complex impedances; this allows good tuning margin compensate normal variations transistor impedances other peripheral effects. side benefit this network that series tuning element provides blocking function, eliminating need coupling capacitors. synthesis matching networks utilizes large signal impedances transistors specified data sheets. These parameters should confused with small signal 2-port parameters. complete discussion large signal characterization given Motorola Application note AN-282A. impedance parameters used this note taken from respective data sheets were obtained manner described AN282A.
Microstrip Line 5.72 wide long 4.08 16.84
Figure Equivalent Circuit Input Network Before proceeding determine component values, effective wavelength desired frequency microstrip line must known. This accomplished first finding wavelength free space:
freq 0.638 meters
where propagation constant, free space mode wavelength determined:
o/(r)1/2 63.8 cm/(2.5)1/2 40.37
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propagation microstrip line pure mode, correction factor must applied last calculation.4
0.63 .1225 0.63 (2.5 (227.4/59.2).1225 1.086
AN548A
source impedance normalized 1.23 load impedance j1.5 normalized 0.032 j0.0369. load impedance plotted point Figure source impedance point arbitrary choice line length made. This electrical length
electrical length line length/ cm/43.85 0.057
(10)
Then:
(tem) (40.37) (1.086) 43.85
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This effective wavelength will used further calculations. Equation valid width height ratios 0.6:1 greater. ratios less than 0.6:1 alter (w/h) factor denominator (w/h).0297. source load impedances must normalized 40.65 characteristic impedance line plotted Smith chart. should noted that terms "source" "load" used here only reference Smith chart solution.
Point rotated constant VSWR circle 0.057 toward generator point Reactance must added parallel with impedance presented line just plotted. parallel additions more easily handled admittance form, point converted admittance rotating one-quarter wavelength same constant VSWR circle. This results point Figure constant conductance circle that point lies noted 0.23. problem move along this circle towards generator until reciprocal constant resistance circle source impedance intercepted. This circle does exist standard Smith chart must constructed.
CHART AVAILABLE ELECTRONICALLY
Figure Smith Chart Solution MOTOROLA SEMICONDUCTOR APPLICATION INFORMATIONOn This Product, More Information
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AN548A
CONSTRUCTION CONSIDERATIONS
power applications, solid emitter grounds imperative. microstrip amplifiers gain increased more than grounding both emitter leads bottom foil microstrip board wrapping strips copper foil through transistor mounting hole shown Figure
This done determining radius constant resistance circle representing real part source impedance then constructing circle equal radius with center real axis circumference tangent outer radius chart zero resistance. When this done intercept with 0.23 constant real circle seen point amount parallel susceptance needed move from point point
(Yo) (2.4 0.38) (24.6) 49.72 mmhos (11)
This parallel capacitance
BCP/2f 49.72/(2) (470 106) 16.84 (12)
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that remains finish solution determine amount reactance necessary reach source point this, first necessary transpose point which admittance, impedance. This accomplished rotating point one-quarter wavelength constant VSWR circle. This moves point point which 2.04 reactance line thus representing series reactance
(XE) (Zo) (2.04) (40.65) 82.9 ohms (13)
series capacitance with this reactance
(XCS) (470 106) (82.9) 4.08 (14)
Figure Proper Emitter Grounding Method Stability under normal operating conditions essential, however, stability should maintained over wide range supply voltage drive levels possible. amplifier stability maintained drive levels with supply voltage reduced between three five volts, designer practically certain that amplifier will remain stable under conditions load. Maintaining stability factor protecting these transistors from damage. stable amplifier that adequate heat sinking, these transistors will withstand high VSWR loads including open shorted loads without damage. major controlling factors obtaining wide range stability are: Mechanical layout: Good mechanical layout includes good emitter grounds previously described), compact layout short ground paths. Biasing: devices zero biased Class operation. relatively base chokes with ferrite beads ground side will maintain good base circuit stability. some applications, resistor series with ground side base chokes output driver stages enhance stability. Approximate values these resistors should ohms, watt driver ohms, watt output device. addition these series resistors will cause slight loss gain; (about overall). Collector supply feed method: collector supply feed system designed provide decoupling near operating frequency collector load impedance frequencies much lower than operating frequency. Heat sinking: order protect against burnout under conditions load, adequate heat-sinking must provided. heat sinking device imperative good grade thermal compound, such Dow-Corning 340, interface between device heat sink. Figure shows microstrip board layout while Figure photo completed amplifier.
This completes solution input network. interstage networks well output network solved similar fashion with following differences. case interstage networks when imaginary term source impedance other than zero, point would plotted complex conjugate source impedance. output network solution "source" output load amplifier "load" collector impedance output device. Figure gives details performance completed amplifier. porcelain dielectric chip capacitors series elements interstage networks found provide additional gain over that obtained with compression trimmers well reducing number tuning adjustments necessary.
Power Gain Bandwidth Overall Efficiency Harmonics Stability Power Output Burnout 44.5% 17.2 46.5% 48.5%
Harmonics Better Than Amplifier Stable under Conditions Drive Down volts
Damage Transistor with Load Open Shorted with 180° Phase Angle
Figure Typical Performance Specifications
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AN548A
(0.98)
0.290
(0.98)
0.290
(0.98)
(0.98)
2N5945 0.225
2N5946 0.390 dia.
2N6136
Freescale Semiconductor, Inc.
Board 1/16 Thick Teflon bonded Fiber glass Dielectric with Copper both sides.
Figure Microstrip Board Layout type crack dislodge cap. This type stress sometimes occurs adverse tolerance build-up dimensions when device mounted through microstrip board onto heat sink. Many times this type stress applied even most carefully thought designs solder build-up copper foil when device replaced. device replacement care should taken flow solder away from mounting area before stud torqued. Finally, must sure torque stud before soldering device leads. Refer Motorola Application Note AN-555 details mounting Motorola "stripline packaged transistors." Figure Photograph Amplifier
REFERENCES
Smith, "Electronic Applications Smith Chart", McGraw-Hill, 1969. Wheeler, "Transmission-Line Properties Parallel Wide Strips Conformal-Mapping Approximation" IEEE Trans. Microwave Theory Techniques Vol. MTT-12, 1964. Wheeler, "Transmission-Line Properties Parallel Strips Separated Dielectric Sheet" IEEE Trans. Microwave Theory Techniques Vol. MTT-3, March 1965. Sobol "Extending Microwave Technology Microwave Equipment" Electronics, March 1967. Microwave Engineers Technical Buyers Guide, Edition Microwave Journal, 1969.
DEVICE HANDLING CONSIDERATIONS
Although Motorola stripline package rugged assembly, some care handling should observed. most important mechanical parameter stud-torque, specified data sheet inch-pounds maximum. This data sheet specification absolute maximum should exceeded under circumstances. good limit production assembly inch-pounds reason repeated assembly/disassembly required torque should limited inch-pounds. Another major precaution observe avoid upward pressure leads near case body. Stresses this
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AN548A
NOTES
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NOTES
AN548A
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Motorola reserves right make changes without further notice products herein. Motorola makes warranty, representation guarantee regarding suitability products particular purpose, does Motorola assume liability arising application product circuit, specifically disclaims liability, including without limitation consequential incidental damages. "Typical" parameters vary different applications. operating parameters, including "Typicals" must validated each customer application customer's technical experts. Motorola does convey license under patent rights rights others. Motorola products designed, intended, authorized components systems intended surgical implant into body, other applications intended support sustain life, other application which failure Motorola product could create situation where personal injury death occur. Should Buyer purchase Motorola products such unintended unauthorized application, Buyer shall indemnify hold Motorola officers, employees, subsidiaries, affiliates, distributors harmless against claims, costs, damages, expenses, reasonable attorney fees arising directly indirectly, claim personal injury death associated with such unintended unauthorized use, even such claim alleges that Motorola negligent regarding design manufacture part. Motorola registered trademarks Motorola, Inc. Motorola, Inc. Equal Opportunity/Affirmative Action Employer. Literature Distribution Centers: USA: Motorola Literature Distribution; P.O. 20912; Phoenix, Arizona 85036. EUROPE: Motorola Ltd.; European Literature Centre; Tanners Drive, Blakelands, Milton Keynes, MK14 5BP, England. JAPAN: Nippon Motorola Ltd.; 4-32-1, Nishi-Gotanda, Shinagawa-ku, Tokyo 141, Japan. ASIA PACIFIC: Motorola Semiconductors H.K. Ltd.; Silicon Harbour Center, King Street, Industrial Estate, N.T., Hong Kong.
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