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LINEAR REGULATED SUPPLY TO DISCRETE GaAs MESFET POWER AMPLIFIER

IP.com Disclosure Number: IPCOM000007113D
Original Publication Date: 1994-Feb-01
Included in the Prior Art Database: 2002-Feb-26
Document File: 3 page(s) / 123K

Publishing Venue

Motorola

Related People

Bill Alberth: AUTHOR [+3]

Abstract

We used a linear regulator to supply a fixed volt- age to the power amplifier final stage in a portable cellular phone. This resulted in longer talk times with Lithium Ion batteries, and reduces the power dissipation requirements of portable cellular phone power amplifier final device packages. Adding a lin- ear regulator to the radio adds 5 to 10 minutes of talk time when a Lithium Ion battery is used.

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MO7VROLA Technical Developments Volume.21 February1994

LINEAR REGULATED SUPPLY TO DISCRETE GaAs MESFET POWER AMPLIFIER

by Bill Alberth, Terrie Frane, and Don Millar

ABSTRACT

  We used a linear regulator to supply a fixed volt- age to the power amplifier final stage in a portable cellular phone. This resulted in longer talk times with Lithium Ion batteries, and reduces the power dissipation requirements of portable cellular phone power amplifier final device packages. Adding a lin- ear regulator to the radio adds 5 to 10 minutes of talk time when a Lithium Ion battery is used.

the battery voltage is equal to or less than 6V
3. The pass device must be able to handle the worst case heat dissipation.

  One circuit that accomplishes these goals is shown in Figure 2. The feedback around the op amp causes the circuit to control the resistance of the power MOSFET to hold the output voltage con- stant. For low battery conditions, the MOSFET is turned full on and very little voltage drop is realized between the battery and the PA final device. Figure 3 shows current drain versus battery voltage with and without regulation,

BACKGROUND

  Previously all cellular portable phones were pow- ered by NiCad or NiMH batteries which spend most of their useful life near 6.0 V Lithium Ion batteries will soon be introduced with cellular products. The voltage ofa Lithium battery pack varies from 5 V to
8.2 V during its useful life (Figure 1).

  Our next generation phone uses a discrete GaAs Power amplifier optimized at 6.0 V (the phone must work with LiIon or NiC?ad/NiMH battery packs). We discovered that the current drain of the PA increased by up to 30 mA as the battery voltage varied from 6.0 V to 8.3 V In order to achieve maxi- mum talk time with a LiIon battery we desired the PA current drain to be flat with voltage.

SWITCHING SUPPLY

  A switching supply would allow additional cur- rent drain savings at higher battery voltages, but has several drawbacks:
1. Switching supplies are more expensive and require more real estate than this linear regulator (the inductors must be capable of handling the power amplifier current without saturating).
2. The MOSFET linear regulator can realize a lower drop out voltage than a switching supply.

HEAT DISSIPATI...