1. required. a pulsed power amplifier with the following specifications: minimum peak power of 250w at 435mhz, bandwidth of 30mhz (420 to 450mhz), maximum passband flatness of 0.05db, pulse width up to 1msec., 15% duty factor and input and output (z in and z out ) impedances of 50ohms. 2. procedure. first, a transistor must be selected. then, the circuit q requirement(s) must be determined. the imaginary terms (if any) of the transistor input and output is then reduced to zero (0) at band center. then the chebyshev (low pass) matching filters required for a discrete component circuit must be computed. and last, the distributed component circuit elements are computed. note: for this design, a teflon-fiberglass board 1/32-inch thick having a dielectric constant of 2.5 will be employed. 2.1. step 1. stmicroelectronics' type sd1563 was selected for its broad bandwidth, low thermal resistance, and ruggedness. from the datasheet for a typical amplifier with 40v vcc, the impedances are given as: (and) 2.2. step 2. circuit bandwidth required to limit roll-off at band edges to 0.5db is approximately three times the 3db bandwidth (bw). then: examine the selected device thus: october 2000 1/5 AN561 application note wide band design of pulsed power uhf amplifiers z in 2 j 3.2 + = z out 1.8 j 0.7 + = 3 db 450 420 = or bandcenter f 0 450 420 435 mhz == = q max f 0 bw 0.5 db ------------------------ 435 90 -------- - 4.8 === q zin x l r ----- - 3.2 2 ------- 1.6 == = q zout x l r ----- - 0.7 1.8 ------ - 0.39 == = 0.5 db 330 = or bw bw 30 mhz 90 mhz
AN561 - application note 2/5 since q max = 4.8 > q zin >q zout , the q requirements are met. 2.3. step 3. the inductance presented by the input of the transistor is resonated out by shunting with a capacitor c z whose value is: the value of r in , now that it is pure resistance, is: capacitor c z ' required to resonate the inductance presented by the transistor output is determined to be 68.8pf and r out ' is 2.1ohms. 2.4. step 4. chebyshev design tables require the input and output impedances to be real and: inspection of chebyshev tables show a single section (n=2) filter will exhibit about 0.2db ripple; however, a two section (n=4) filter will limit ripple to less than 0.002db. the (n=4) filter will be employed. component values for the input filter compute to be: 7.12 to 50ohms c z x in 2 p f () r in 2 x in 2 + () --------------------------------------------- - = c z 3.2 2 p 435 6 10 () 2.0 2 3.2 2 + () ---------------------------------------------------------------- = c z 82 12 10 =82 pf r in r in 2 x in 2 + r in --------------------------- = r in 2.0 2 3.2 2 + 2.0 -------------------------- 7.12 ohms == w fractional = 2 f 1 f 2 () f 1 f 2 + ---------------------- = 2 450 420 () 450 420 + -------------------------------- 0.0689 == r transformational = r source r in ------------------ = 50 7.12 ---------- 7.02 == l 1 3.5 nh = c 2 35.2 pf = l 3 12.5 nh = c 4 9.7 pf = bandwidth ratio
AN561 - application note 3/5 the output matching filter is determined in the same manner where: inspection of chebyshev tables for a single section (n=2) filter shows that the ripple will exceed 0.22db. again entering the two section (n=4) table, the ripple is found to be reduced to less than 0.001db. circuit q influences the choice of filter sections, thus the maximum impedance transformation ratio (each section) is limited to: therefore, a single section (n=2) filter with a transformation ratio of 23.8 would be marginal in bandwidth. component values for the output filter compute to be: 2.1 to 50ohms the basic discrete component amplifier is shown in figure 1. figure 1: basic discrete circuit 4.5. step 5. the microstrip characteristic impedance is given as: where w = width, h = height, e r = dielectric constant. for a teflon-fiberglass board 1/32-inch thick having a dielectric constant of 2.5, the line width for 50ohms is found to be 0.09 inches. 12.5nh 3.5nh 1.5nh 9.0nh l 3 l 1 l 1 ' l 3 ' c 4 9.7pf c 2 35.2pf c z 82pf c z ' 68.6pf c 2 ' 87.9pf c 4 ' 14.4pf sd1563 ?? ???? ? ? ? ? ? ? ? w 0.0689 = r 50 2.1 ------- 23.8 == q 2 1 + 4.8 () 2 1 + 24.04 == l 1 1.5 nh = c 2 87.9 pf = l 3 9.0 nh = c 4 14.4 pf = z 0 377 h e r 12 M w 1 1.735 e r 0.0724 wh M () 0.836 + [] ------------------------------------------------------------------------------------------------ - =
AN561 - application note 4/5 the capacity of one (1) square inch of this circuit board is equal to: capacities in the order of 90pf required for this amplifier would occupy about four to five square inches of board area (a prohibitive use of real estate). discrete capacitors will be employed. the length of a microstrip inductor is approximated as: where l = length in inches and l = inductance in nh. for simplicity, a 50ohm line impedance was chosen for the inductors used in this amplifier. the line width is 0.09 inches and line lengths are as tabulated below: table 1: microstrip lines the complete distributed amplifier is shown in figure 2. figure 2: complete distributed amplifier (250w / 420-450mhz pulsed power) symbol value (nh) length (inches) l 1 3.5 0.563 l 3 12.5 1.790 l 1 ' 1.5 0.244 l 3 ' 9.0 1.360 c 2 35pf c z 40pf c 4 10pf c 4 ' 15pf c 2 ' 90pf c z ' 35pf 180nh .09 x 1.79 .09 x .56 100pf 400uf .09 x 0.24 .09 x 1.36 100pf c z 40pf c z ' 35pf 170nh 100pf l 3 l 1 l 1 ' l 3 ' +v sd1563 .. . . . . . . . . . . . . . . . . c 0.224 1.09 e r area d thickness () ----------------------------------------------------- - = 0.224 1.09 2.5 1 0.03125 ---------------------------------------------------- - =19 pf = l 11.81 l z 0 e r ----------------- =
AN561 - application note 5/5 information furnished is believed to be accurate and reliable. however, stmicroelectronics assumes no responsibility for the consequences of use of such information nor for any infringement of patents or other rights of third parties which may result from its use. no license is granted by implication or otherwise under any patent or patent rights of stmicroelectronics. specification mentioned in this publication are subject to change without notice. this publication supersedes and replaces all information previously supplied. stmicroelectronics products are not authorized for use as critical components in life support devices or systems without express written approval of stmicroelectronics. the st logo is a registered trademark of stmicroelectronics ? 2000 stmicroelectronics - printed in italy - all rights reserved stmicroelectronics group of companies australia - brazil - china - finland - france - germany - hong kong - india - italy - japan - malaysia - malta - morocco - singapore - spain - sweden - switzerland - united kingdom - u.s.a. http://w ww.st.com
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