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 fT 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.

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

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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

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