1. abstract this application note gives a description of a broadband power amplifier operating over the frequency range 88 - 108 mhz and using the new stmicroelectronics rf mosfet transistor sd2932. table 1: typical acheivable performances 2. amplifier design. 2.1. input matching network. typical input gate to gate impedance of sd2932 at 100 mhz is zin = rs + jxs = 2 - 2.6 j, and can also be expressed as the combination of parallel resistance and reactance using the following formulae : rp = rs x [1 + (xs / rs)2] = 5.38 ohms xp = rp / (xs / rs) = -4.14 j ohms therefore, in order to achieve good input matching performances over the frequency range 88-108 mhz the unbalanced 50 ohms is to be transformed into an impedance with a value as close as possible to rp of 5.38 ohms. from the circuit schematic given in fig. 6 , we can see that the input matching network is based on a two section balun (1:1 balun in cascade with a 9:1 balun transformer) which will transform the unbalanced 50 ohms to a balanced 5.56 ohms (2 x 2.78 ohms / 9:1 ratio). the first section, a 5" long - 50 ohms coaxial cable and the second section, a two 3.9" long - 25 ohms flexible coaxial cables with ferrite core neoside, are connected as described: a 10 nh inductor (l1) is connected between the two gates to compensate sd2932 input parallel reactance xp. quasi-neutral region of the junction. the breakdown-voltage regime is the avalanching of carriers due to the electric field being greater than the critical electric field (approximately 1x10 5 v/cm). under these conditions an electron can be accelerated by the electric field. device 1 x sd2932 frequency 88-108 mhz vdd 50 v idq 200 ma pout 300 w gain >19 db input return loss <-11 db drain efficiency >70% july 2000 1/5 AN1229 application note for 300w fm amplifier sd2932 rf mosfet serge juhel
AN1229 - application note 2/5 0 50 100 150 200 250 300 350 frequency (mhz) -35 -30 -25 -20 -15 -10 -5 0 s11 (db) 0 50 100 150 200 250 300 350 frequency (mhz) -35 -30 -25 -20 -15 -10 s11 (db) due to elastic and inelastic scattering this electron acceleration can generate more than one carrier and thus a multiplication scheme transpires. 2.2. input matching network tuning. figure 1: input impedance of 1:1 balun in cascade with 4:1 balun figure 2: input impedance of 1:1 balun in cascade with 9:1 balun sd2932 input matching network was tuned in order to achieve the best compromise in terms of power gain (gp) and input return loss (rtl) over the frequency range 88 - 108 mhz. best results were achieved by adding a 10 pf chip capacitor (c1) between rfin and the 1nf blocking capacitor (c2). 2.3. output matching network. the output impedance of each side is a combination of the output capacitance c oss (195 pf) and the optimum load resistance which can be determined as follows : rp = (0.85 x vdd)2 / (2 x pout) = (0.85 x 50v) / (2 x 150w) = 6.02 ohms the total optimum load , seen by sd2932 (drain to drain) , is 2 x 6.02 = 12.04 ohms. therefore, a simple two section balun (1:1 balun in cascade with a 4:1 balun transformer) is used to transform the unbalanced 50 ohms to a balanced 12.5 ohms (2 x 6.25 ohms) which is very near to the total optimum load resistance.
AN1229 - application note 3/5 85 90 95 100 105 110 frequency (mhz) 60 62 64 66 68 70 72 74 76 78 80 drain efficiency (%) pout = 300w vdd = 50v idq = 200 ma 85 90 95 100 105 110 frequency (mhz) 17 18 19 20 21 22 power gain (db) pout = 300w vdd = 50v idq = 200 ma the first section, a 5" long - 50 ohms flexible coaxial cable, and the second section, two 5" long - 25 ohms flexible coaxial cables, are connected as described in figure 6. to compensate for the output capacitance c oss of sd2932 , a 40nh inductor (l2) is connected between the two drains. this lc network (l2 & c oss ) is a high pass filter with a resonance frequency calculated at 10 % below the minimum operating frequency : c oss =c oss (per side)/2 = 180 pf/2 = 90 pf frequency of resonance = 0.9x88mhz=80mhz l2 x coss (2pf)2 = 1---->l2 = 44nh. figure 3: power gain vs. frequency figure 4: drain efficiency vs. frequency
AN1229 - application note 4/5 85 90 95 100 105 110 frequency (mhz) -20 -18 -16 -14 -12 -10 -8 -6 -4 -2 0 return loss - rtl (db) pout = 300w vdd = 50v idq = 200 ma figure 5: input return loss vs. frequency 3. measured sd2932 typical performances and conclusion. figures 3, 4 & 5 show power gain, efficiency and input return loss over the frequency range 88 - 108 mhz at a constant output power of 300 watts and a drain supply voltage of 50 volts and a quiescent current of 200 ma. typical performances are as follows: table 2. finally, in this report we have demonstrated st sd2932 mosfet transistor excellent performance as a wide band 300w - 50v push-pull amplifier for fm applications. figure 6: 88-108mhz circuit schematic min max gp 19.3db 19.6db r tl -18db -11db eff 71% 73%
AN1229 - application note 5/5 table 3: 88-108mhz circuit components list 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 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://www.st.com pcb 1/32 woven fiberglass 0.0030 cu, 2 side, e r = 4.8 t1 50 ohm flexible coax cable od 0.006, 5 long t2/ t3 9:1 transformer, 25 ohm flexible coax cable od 0.1 3.9. ferrite core neoside t4 / t5 4:1 transformer, 25 ohm flexible coax cable od 0.1 5.0 long. t6 50 ohm flexible coax cable od 0.1 5.0 long. fb1 vk200 c1 10pf ceramic capacitor c2/c3/c4/c7/c8 1 nf chip capacitor c5/c6 1 nf atc chip capacitor c9 470 pf atc chip capacitor c10 100 nf chip capacitor r1 56 ohm resistor r2/r4 10 ohm chip resistor r3 10k ohm resistor r5 5.6k ohm resistor r6 10k ohm. 10 turn trim resistor r7 3.3k ohm/ 1 w resistor r8 15 ohm/ 1 w resistor d1 6.8 v zener diode l1 10 nh inductor l2 40 nh inductor l3 70 nh inductor
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