1. abstract ldmos technology allows the manufacturing of high efficiency and high gain amplifiers for fm transmitters. ldmos has proven advantages against bipolar devices in terms of higher gain, efficiency, linearity, biasing simpleness that lowers the overall system cost and makes them attractive for high volume businesses demanding low cost rf power transistor solutions. due to these advantages, ldmos rf power transistors are the proven mainstay in the power amplifier business of the cellular base station today. the device used for the present characterization, sd57045, an stmicroelectronics product, is a lateral current, double diffused mos transistor that delivers 45 watts and 28 volts. it is unmatched from dc to 1 ghz making it eligible for a variety of applications, especially for high performance, low cost fm driver applications. this application note documents the feasibility of a low cost 900 mhz cellular device as a commercial fm driver. the key advantages of ldmos technologies are improved thermal resistance and reduced source output inductance. the wire-bonded connections to the external circuitry (dmos config.) are no longer required because the source at the chip surface is connected to the substrate by the diffusion of a highly doped p-type region. consequently, ldmos has excellent high frequency response because of its high f t and superior gain due to the low feedback capacitance and reduced source inductance. an additional advantage of the ldmos structure is that beryllium oxide (beo), a toxic electrical insulator required to isolate the drain with dmos transistors, is no longer needed. hence, not only the thermal resistance is improved, but package cost and environmental impact are significantly reduced. finally, in an ldmos, the parasitic bipolar has been nullified guaranteeing good ruggedness, efficiency and high current handling capability. 2. circuit design: description and consideration. input and output impedances for the sd57045 are shown in the table below: table 1. with respect to these impedances, two 4:1 transmission line auto transformers were designed using a 25 ohm, 1/8 wavelength, semi rigid coaxial cable. to achieve this transformation across the band, a capacitor was added to the low impedance port of each transformer to cancel the leakage inductance. the frequency response is shown in figure 1. simple l-sections were utilized to make the final transformation frequency (mhz) z input z output 88 10.8-j7.60 7.5-j0.15 95 10.6-j8.36 7.8-j0.34 108 10.5-j9.87 8.1-j0.61 july 2000 1/7 AN1224 application note ldmos rf power transistors for fm broadboard application ahmed mimouni
AN1224 - application note 2/7 s11 0db -30db -60db 80mhz 90mhz 100mhz 110mhz from the low impedance port of the transformers (12.5 ohms) to the measured impedances of the device (see table 1). this design uses printed series inductors on a 30 mil glass teflon board. the gain of any power fet is extremely high from dc throughout the low hf frequency band. a feedback network is necessary to suppress the low frequency gain, as well as give a nominal amount of gain at the frequency of interest. this feedback also helps to increase the input impedance. since ldmos has such a high gain at low frequencies, a low value, high power, flange mount resistor must be comprised in the design. the capacitor in the feedback path (c3) provides negative feedback at low frequencies. this component was designed to be self-resonant. far below the fm band, at 100 mhz, the capacitor looks slightly inductive, reducing the amount of feedback in the band of interest. figure 1: broad band 4:1 transformer unbalanced transformers offer an efficient matching method from 50 w to low impedance. besides, auto transformers have a zero impedance point over a broad bandwidth, offering an ideal dc feeding point to the gate and drain circuits. in order to prevent high frequency oscillations, a bypass capacitor is used at the zero impedance point of the transformer. capacitor value must be selected so that its self resonant frequency is above the frequency of interest. depending on the application, additional low frequency bypass capacitors isolated with lossy elements (ferrite beads) may be required to prevent power supply noise affecting gate and drain circuits.
AN1224 - application note 3/7 circuit schematic is given in figure 2, and layout in figure 3 with component values in table 2. table 2. figure 2: broad band power amplifier l1,l3, l4, l7 50 w transmission line c1,c13 1000 pf chip capacitor c2 39000pf chip capacitor c3 36pf chip capacitor r1 1 k w resistor c4,c6, c10 10000pf chip capacitor r2 1.2 k w resistor c5,c12 10uf, 50v electrolytic capacitor r3 240 w / 40w resistor c9, c11 1200pf chip capacitor c8 33pf chip capacitor c7 25-115pf variable cap-arco trimmer l2,l6 4:1 transformers, 10.7", 25 w . board 30mils, 2 ounces of copper, e r = 2.55
AN1224 - application note 4/7 figure 3: layout for broad band power amplifier 3. characterization results. absolute maximum ratings (t case = 25oc) table 3. thermal data symbol parameter value unit v (br)dss drain-source voltage 65 v v dgr drain-gate voltage (rgs = 1 m w ) 65 v v gs gate-source voltage +/-20 v i d drain current 5a p diss power dissipation (@ tc=70 oc) 93 w t jmax operating junction temperature 200 oc t stg storage temperature -65 to 200 oc r th(j-c) junction-case thermal resistance 1.4 c/w
AN1224 - application note 5/7 0.1 0.2 0.3 0.4 0.5 0.6 input power (w) 12 24 36 48 60 output power (w) 40 50 60 70 efficiency (%) output power & effici ency vs input power pout eff freq=95 mhz idq=250 ma vdd=28v 2.5 3 3.5 4 4.5 5 vgs, gate-source voltage (volts) 0 0.5 1 1.5 2 2.5 3 3.5 4 id, drain current (a) drain current vs. gate-source voltage -25 0 25 50 75 tcase, case temperature (c) 0.96 0.98 1 1.02 1.04 vgs, gate-source voltage (normalized) gate-source voltage vs case temperature id=250ma id=1.5a id=2a id=3a id=1a figure 4: drain current vs. gate-source voltage figure 5: gate-source voltage versus case temperature figure 6: output power and effeciency versus input power
AN1224 - application note 6/7 1 10 100 vds, drain-source voltage (volts) 1 10 id, drain current (a) class a safe operating region tc= 70c tc=100c tj=200c 15 30 45 60 pout (w) 18 20 22 24 gain (db) 40 50 60 70 efficiency(%) power gain & efficiency vs output power gain eff freq=95 mhz idq=250 ma vdd=28v figure 7: power gain and efficiency vs. output power figure 8: class a safe operating area 4. conclusion in this application note we have demonstrated the feasibility of a low cost, 900 mhz cellular device as a commercial fm driver. one can conclude that st ldmos technology offers viable solutions for power amplifiers at frequencies covering the high hf throughout the high uhf bands. more information about these devices can be found at http://www.st.com.
AN1224 - application note 7/7 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
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