AN1224 STMicroelectronics, AN1224 Datasheet

AN1224

Manufacturer Part Number

AN1224

Description

LDMOS RF POWER TRANSISTORS FOR FM BROADBOARD APPLICATION

Manufacturer

STMicroelectronics

Datasheet

1.AN1224.pdf (7 pages)

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July 2000

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

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

108

Frequency (MHz)

LDMOS RF POWER TRANSISTORS FOR FM

10.8-j7.60

10.6-j8.36

10.5-j9.87

BROADBOARD APPLICATION

Z input

APPLICATION NOTE

7.5-j0.15

7.8-j0.34

8.1-j0.61

Z output

AN1224

Ahmed Mimouni

1/7

and

Related parts for AN1224

AN1224 Summary of contents

Page 1

... 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 July 2000 APPLICATION NOTE BROADBOARD APPLICATION Z input 10.8-j7.60 10.6-j8.36 10.5-j9.87 AN1224 Ahmed Mimouni and T Z output 7.5-j0.15 7.8-j0.34 8.1-j0.61 1/7 ...

Page 2

... AN1224 - APPLICATION NOTE 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 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 ...

Page 3

... C4,C6, C10 10000pF chip capacitor R2 1.2 k C5,C12 10uF, 50V electrolytic capacitor R3 240 C9, C11 1200pF chip capacitor C8 33pF chip capacitor C7 25-115pF variable cap-Arco trimmer L2,L6 4:1 transformers, 10.7", 25 Board 30mils, 2 ounces of copper, Figure 2: Broad Band Power Amplifier transmission line resistor / 40W resistor . = 2.55 r AN1224 - APPLICATION NOTE 3/7 ...

Page 4

... AN1224 - APPLICATION NOTE Figure 3: Layout for Broad Band Power Amplifier 3. CHARACTERIZATION RESULTS. Absolute maximum ratings (T CASE Table 3. Symbol Parameter Drain-Source Voltage V (BR)DSS V Drain-Gate Voltage (RGS = 1 M DGR Gate-Source Voltage Drain Current D P Power Dissipation (@ TC=70 ºC) DISS T Operating Junction Temperature JMax ...

Page 5

... Drain Current vs. Gate-Source Voltage 2.5 3 3.5 4 VGS, GATE-SOURCE VOLTAGE (VOLTS) Gate-Source Voltage vs Case Temperature 0 25 Tcase, CASE TEMPERATURE (°C) Output Power & Efficiency vs Input Power Freq=95 MHz Idq=250 mA Vdd=28V 0.2 0.3 0.4 Input Power (W) AN1224 - APPLICATION NOTE 4.5 5 Id=3A Id=2A Id=1.5A Id=1A Id=250mA 50 75 Pout 70 Eff 0.5 0.6 5/7 ...

Page 6

... AN1224 - APPLICATION NOTE Figure 7: Power Gain and Efficiency vs. Output Power Figure 8: Class A Safe Operating Area 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 ...

Page 7

... STMicroelectronics - Printed in Italy - All rights reserved Australia - Brazil - China - Finland - France - Germany - Hong Kong - India - Italy - Japan - Malaysia - Malta - Morocco - Singapore - Spain - Sweden - Switzerland - United Kingdom - U.S.A. The ST logo is a trademark of STMicroelectronics STMicroelectronics GROUP OF COMPANIES http://www.st.com AN1224 - APPLICATION NOTE 7/7 ...

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