ST AN1224 Application note

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
frequency response because of its high f
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.

and superior gain due to the low feedback

T

October 2007 Rev 3 1/10

www.st.com

Contents AN1224

Contents

1 Circuit design . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4

1.1 Description and consideration . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4

2 Characterization results . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6

3 Conclusion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9

4 Revision history . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9

2/10

AN1224 List of figures

List of figures

Figure 1. Broadband 4:1 transformer . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4

Figure 2. Broadband power amplifier . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5

Figure 3. Layout for broadband power amplifier . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6

Figure 4. Drain current vs. gate-source voltage. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7

Figure 5. Gate-source voltage vs. case temperature. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7

Figure 6. Output power and efficiency vs. input power . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8

Figure 7. Power gain and efficiency vs. output power . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8

Figure 8. Class A safe operating area . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8

3/10