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A voltage regulator for GaAs FETs.

Introduction

A new voltage regulator has been developed to assist in biasing of GaAs power transistors. The model RFP-1147 voltage regulator accepts a +15 V input voltage and provides -5 V for the GaAs field-effect transistor's (FET) gate, and a +3 to +12 V adjustable output for the drain. The regulator sequences the two voltages such that the positive drain voltage is not available until the negative gate voltage is on, thus protecting the GaAs FET from current overload. The regulator is integrated into a thermally efficient, low cost package.
TABLE I


SPECIFICATIONS


Input voltage (V DC)                +15


Output voltage


Positive (V DC)             +3 to +12 adjustable
Negative (V DC)                   -5 fixed


Output current


Positive (A)                         6
Negative (mA)                       100


Base plate
Temperature ([degrees] C)        -25 to +75


Features

The RFP-1147 voltage regulator offers a one-step, easy-to-use solution to GaAs FET biasing problems. The regulator is packaged in a small drop-in housing with four solder-in tabs for mounting to a microstrip circuit board. The drain voltage is adjustable by adding a 10 K [ohms] potentiometer to one of the four leads, allowing the regulator to be used to bias a large array of GaAs FETs requiring various drain voltages. The regulator is capable of delivering 6 A of positive output current for biasing of a single large power FET or multiple medium/low power FETs. The regulator is also self-protected from both thermal and current overload.

Performance

The model RFP-1147 regulator utilizes a linear voltage regulator for the positive output and a charge pump inverter in conjunction with a -5 V linear regulator for the negative output. General specifications are listed in Table 1.

GaAs FETs operate similarly to N-channel junction FETs where [I.sub.dss] occurs near [V.sub.gs] = 0 V and [I.sub.d] (off) occurs near [V.sub.gs] = -5 V. To avoid damage to the GaAs FET due to current overload, the gate voltage must be sequenced on before the drain voltage is applied. To achieve the proper voltage sequencing, a metal-oxide semiconductor (MOS) FET switch is used in series with the positive output voltage. This switch remains off until the -5 V output is on. If the -5 V output is inadvertently shorted to ground, the positive output also is turned off. This is essentially a fail-safe method of protecting the GaAs FET from the situation where a drain voltage is present with no gate voltage.

Packaging

This voltage regulator is housed in a 1.75[inches] x 0.75[inches] package, as shown in Figure 1, suitable for connecting to microstrip PC boards via silver solder tabs. The regulator is conduction cooled and designed to be top mounted to a suitable heatsink. Semiconductor chips are mounted to a beryllium oxide substrate, connected via wire bonding and protected with a plastic cover. Beryllium oxide is used as a substrate due to its low thermal resistance, thus keeping the semiconductor junction temperatures low. The substrate is bonded to a metal base, which has a thermal coefficient of expansion closely matched to that of the beryllium oxide. This prevents possible cracking of the beryllium oxide due to thermal cycling of the regulator. A thin layer of thermal grease is recommended between the regulator and the mounting surface.

Applications

The model RFP-1147 voltage regulator is designed to be used as a partner to today's power GaAs FETs, which are widely used in the amplification of microwave signals. It replaces larger discrete regulators and requires only one positive input voltage. The regulator is also useful in power analog and digital circuits where a regulated high current positive and low current negative voltage are required.

RF Power Components Inc., Bohemia, NY (516) 563-5050.
COPYRIGHT 1995 Horizon House Publications, Inc.
No portion of this article can be reproduced without the express written permission from the copyright holder.
Copyright 1995 Gale, Cengage Learning. All rights reserved.

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Title Annotation:field-effect transistors
Publication:Microwave Journal
Date:May 1, 1995
Words:614
Previous Article:Broadband surface-mounted devices.
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