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PROTECTIVE DEVICE STRUCTURE FOR SIGNIFICANTLY REDUCING BREAKDOWN VOLTAGE REQUIREMENTS OF POWER DEVICES

IP.com Disclosure Number: IPCOM000006655D
Original Publication Date: 1992-Dec-01
Included in the Prior Art Database: 2002-Jan-21
Document File: 3 page(s) / 137K

Publishing Venue

Motorola

Related People

J. Staudinger: AUTHOR [+4]

Abstract

Radio frequency (RF) transistors are typically uti- lized in communication products including cellular tele- phones for amplifying RF signals to high power levels such that the information can be transmitted with an antenna. Typical equipment architectures place the last amplifier stage prior to the antenna, and thus the antenna effectively terminates the output of the transistor. This radio architecture results in the antenna loading the RF transistor's output with a rather wide range of possible impedance levels. A power amp&r specification requir- ing the amplifier to survive a defined range of imped- ance levels is termed ruggedness. This article describes a device structure which enhances the ability of the RF transistor to survive ruggedness.

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MOlOROLA INC. Technical Developments Volume 17 December 1992

PROTECTIVE DEVICE STRUCTURE FOR SIGNIFICANTLY REDUCING BREAKDOWN VOLTAGE REQUIREMENTS OF POWER DEVICES

by J. Staudinger, J. M. Golio, G. Norris, R. Davies

  Radio frequency (RF) transistors are typically uti- lized in communication products including cellular tele- phones for amplifying RF signals to high power levels such that the information can be transmitted with an antenna. Typical equipment architectures place the last amplifier stage prior to the antenna, and thus the antenna effectively terminates the output of the transistor. This radio architecture results in the antenna loading the RF transistor's output with a rather wide range of possible impedance levels. A power amp&r specification requir- ing the amplifier to survive a defined range of imped- ance levels is termed ruggedness. This article describes a device structure which enhances the ability of the RF transistor to survive ruggedness.

  Conceptually, the ruggedness sp=xification is depicted in Figure 1. The output of the RF transistor is termi- nated in a specified range of impedances. The range of impedances is usually defined in terms of a maximum VSWR (Voltage Standing Wave Ratio) or voltage reflec- tion coe&ient. Referring to Figure 1, RF input power and DC voltages are applied to the amplifier, the output reflection coe!Xcient is set to a desired magnitude and the reflection coefficient angle is varied from 0 to 360 degrees. To pass ruggedness, the RF transistor must not suffer permanent degradation.

  Extensive modeling and computer simulations have been performed to understand the failure mechanism of power devices subjected to ruggedness. These simula- tions show that under certain loading conditions, the termination resonates with the device which allows high voltage levels to develop between drain-source and gate- drain terminals. For example, Figure 2 shows the simu- lated results of a typical power MESFET subjected to a high VSWR of several phase angles. Terminal voltages remain nominal except at phase angles near 135" where exceptionally high voltage peaks develop due to a reso- nance between device and termination.

  A proposed means of limiting these voltages to acceptable levels incorporates a protective device with the RF transistor (Figure 3). This concept relies on the

Q MO1ORIIB,

Inc. 1992

protective device to sample the output wave form, and if above a prescribed level, dampen the resonance to reduce the voltage level. This protective device concept can be realized in practice using several topologies, four possible ones are shown in Figure 4. In each case, the drain voltage is sensed, and if above a prescribed...