October 2007

Rev 3

1/10

AN1224

Application note

Evaluation board using SD57045 LDMOS

RF transistor for FM broadcast application

Introduction

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, and biasing simpleness that lower the overall system cost and

make them attractive for high volume businesses demanding low cost RF power transistor

solutions. Thanks 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 W under 28 V supply. 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 technology 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

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.

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