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1 rf application reports a 30 w a tt , 800 mhz amplifier design prepared by: alan w ood semiconductor product sector introduction simplicity and compactness mark the design of this 30 watt amplifier des igned for the 800 mhz mobile communications band. the amplifier uses the internally matched mrf844 transistor in a common base class c configuration providing a minimum of 5.0 db gain over a fixed tuned bandwidth of 800 to 870 mhz at 12.5 volts. lower manufacturing costs are of prime concern to land mobile equipment suppliers and single-board, fixed tuned transmit- ter amplifier designs are becoming increasingly common. t wo versions are therefore presented, one using glass teflon laminate and the second using less expensive g-10 board. (figure 1). circuit description the circuit is designed to be driven from a 50 ohm source and be terminated in a nominal 50 ohm load. both input and output matching networks are similar in design and consist of two element short-step chebyshev transmission line transformations fabricated as microstrip lines (reference 1). mini-underwood mica capacitors are used at the input and output of the transistor transforming the complex inductive impedance to an essentially non-reactive real impedance over most of the band. a minimum of additional components provide the dc biasing and rf decoupling. refer to figure 2 for a schematic diagram of the amplifier . a. circuit using glass t eflon laminate b. circuit using g10 board figure 1. t wo v ersions of mrf844 broadband circuit mot orola semiconductor order this document by EB105/d
? motorola, inc. 1993 EB105 http://
2 rf application reports b1, b2 e ferroxcube bead 56-590-65/3b c1 e 15 pf mini-underwood mica c2 e 12 pf mini-underwood mica c3, c4 e 18pf mini-underwood mica c5 e 91 pf mini-underwood mica c6 e 1000 pf unelco mica c7 e 1.0 m f electrolytic c8 e 36 pf mini-underwood mica l1, l2 e 4 t urns, #20 a wg enameled wire 0.15 , id z1 z4 e microstrip; see photomasters board material e see t ext '
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figure 2. circuit schematic of 30 w att 806 870 mhz amplifier design of microstrip circuits using a g-10 board material is complicated by several factors. this is discussed in detail in reference 2. the main points to be considered are, the lack of control over the dilectric constant in the manufacturing process; a greater tolerance in the dilectric thickness than in the cas e of higher quality subs trates intended for microstrip applications, and changes in relative dilectric constant with frequency . despite these apparent disadvan- tages, g-10 board can be used successfully if the ultimate in bandwidth is not sought. frequency dependence of the relative dilectric constant was determined by charac teriz ing a nominal 25 ohm microstrip line over a wide range of frequencies using an automatic network analyser . compensation for the coaxial to microstrip transitions was established using a computer optimized model (reference 3). figure 3 is a graph of the relative dilectric constant versus frequency determined for the laminate used by this method. it should be noted that dif ferences in epoxy composition could af fect both the low frequency dilectric constant and its frequency dependence. construction procedures both amplifiers were mounted o n 0 . 5 , thick copper blocks, 2.25 , by 2 , in the case of the g-10 board design and 3 , by 2 , for the glass teflon board. the blocks were slotted to a depth of 0.130 , to enable mounting the transistor leads level with the top of the circuit board. thermal compound was used between the transistor flange and the mounting block to ensure low thermal resistance. with the block held in contact with a larger heatsink this configuration proved adequate for test purposes. in a production design, the transistor would normally be thermally connected to the case of the transmitter . however , care should be taken to operate the device under all conditions within the power dissipation limits shown on the data sheet. as with any circuit designed to work at uhf frequencies, good grounding is essential for best performance and stability . copper foil was wrapped around the board adjacent to the transistor mounting to connect the underside ground plane to the transistor common leads. additional copper foil was wrapped around the board to connect the 1000 pf unelco capacitor pad to the lower ground plane. #"&) 3 # %'%# $% % # e
figure 3. relative dilectric constant (g-10) versus frequency
3 rf application reports positioning of the emitter and collector shunt capacitors is critic al to the res ulting amplifier performanc e. the capacitors should be mounted as close to the transistor case as possible. minor tuning of the circuit can be achieved by lateral movement of these components. larger tuning adjustments can be incorporated by replacing part of the fixed shunt capacitance by a variable trimmer . both circuits use 28 mil dilectric 2 ounce copper clad laminate. refer to figure 6 for a 1:1 photomaster of the circuit boards. figure 4a. t ypical performance in broadband circuit figure 4c. output power versus supply voltage figure 4b. output power versus input power % #) + h ! &%!&% ! (#( %%$ 021
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performance da t a similar performance was measured for the same part soldered in either circuit. t ypical performance curves for this broadband design are shown in figures 4a, 4b, and 4c for the glass t eflon design and figures 5a, 5b, and 5c for the g-10 based circuit. circuit losses in the g-10 board were less than expected and were certainly minimized by the short fractional wavelength transmission lines employed. figure 5a. t ypical performance in broadband circuit figure 5b. output power versus input power figure 5c. output power versus supply voltage % #) + h ! &%!&% ! (#( %%$ 021
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4 rf application reports note: not to scale b. photomaster using g-10 board a. photomaster using glass t eflon laminate # # #&% ( ( # % figure 4. t wo photomaster v ersions of mrf844 broadband circuit references 1. g. l. mattheai. short-step impedance t ransformers. ieee t ransactions on microwave theory and t echniques. v ol. mtt -14 no. 8 august 1966. 2. glenn y oung. uhf microstrip amplifiers utilizing g-10 epoxy glass laminate. motorola application note an-578. 3. m. l. majewski. modeling and characterization of microstrip to co-axial t ransistions. ieee t ransactions on microwave theory and t echniques. vol. mtt-29 no. 8 august 1981.
5 rf application reports motorola reserves the right to make changes without further notice to any products herein. motorola makes no warranty , representation o r guarantee regarding the suitability of its products for any particular purpose, nor does motorola assume any liability arising out of the applicati on or use of any product or circuit, and specifically disclaims any and all liability , including without limitation consequential or incidental damages. at ypicalo parameters can and do vary in dif ferent applications. all operating parameters, including at ypicalso must be validated for each customer application by customer ' s tec hnical experts. motorola does not convey any license under its patent rights nor the rights of others. motorola products are not designed, intended, or auth orized for use as components in systems intended for surgical implant into the body , or other applications intended to support or sustain life, or for any othe r application in which the failure of the motorola product could create a situation where personal injury or death may occur . should buyer purchase or use motorola products for any such unintended or unauthorized application, buyer shall indemnify and hold motorola and its of ficers, employees, subsidiaries, af filiates, and distributors harmless against all claims, costs, damages, and expenses, and reasonable attorney fees arising out of, directly or indirectly , any clai m of personal injury or death associated with such unintended or unauthorized use, even if such claim alleges that motorola was negligent regarding the desig n or manufacture of the part. motorola and are registered trademarks of motorola, inc. motorola, inc. is an equal opportunity/af firmative action employer .

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