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POWER AMPLIFIER PROTECTION CIRCUIT

IP.com Disclosure Number: IPCOM000007016D
Original Publication Date: 1993-Oct-01
Included in the Prior Art Database: 2002-Feb-18
Document File: 2 page(s) / 107K

Publishing Venue

Motorola

Related People

Alex Hietala: AUTHOR [+2]

Abstract

Two major goals ofany cellular telephone design are to minimize cost and to minimize current. Recently power amplifiers designed using Hetero- junction Bipolar Transistors (HBT's) have become available to aid in achieving these goals in the area of the RF power amplifier. These devices exhibit improved efficiency over standard silicon devices and appear to cost considerably less than power ampli- fier modules in use today, due to the smaller and simpler physical packaging ofthe devices.

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MOTOROLA INC. Technical Developments Volume 20 October 1993

POWER AMPLIFIER PROTECTION CIRCUIT

by Alex Hietala and Greg Black

  Two major goals ofany cellular telephone design are to minimize cost and to minimize current. Recently power amplifiers designed using Hetero- junction Bipolar Transistors (HBT's) have become available to aid in achieving these goals in the area of the RF power amplifier. These devices exhibit improved efficiency over standard silicon devices and appear to cost considerably less than power ampli- fier modules in use today, due to the smaller and simpler physical packaging ofthe devices.

  These new devices suffer from a problem which is common with several other efficient RF power amplifier device types. This problem is that the break- down of the device is such that ifthe radio is turned on while in a battery charger or while in a DHFA (Digital Hands Free Adapter) serious damage may result to the PA device. Under normal conditions of operation with a battery pack there is no problem with device breakdown.

  This problem could be overcome by operating the amplifier from a regulated supply. However, this would ruin the circuit efficiency since a large amount ofthe total output power would be dissipated in the regulator. Furthermore, it 'is usually necessary to oper- ate the power amplifier directly from the battery in order to obtain the desired output power levels.

  Therefore a circuit is needed which will track the supply voltage with very low voltage drop across the circuit until the input voltage increases to a point at which PA operation is undesirable. Above this input voltage, the circuit will hold a constant out- put voltage with any additional supply voltage drop- ping across the circuit. Since the latter condition does not occur in operation with a battery, the over- all circuit.efficiency is not significantly degraded.

Shown below is an example of one circuit which will accomplish this goal.

1KR

3.5v I.

MJD210

VC""W"l

r

fvv

1 OKR

712

(PA Loading)

64 0 Motorola, I"C~ 1993

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