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RF POWER LEVELLING SCHEME WITH ADJUSTABLE CHARACTERISTICS OVER VOLTAGE

IP.com Disclosure Number: IPCOM000005831D
Original Publication Date: 1990-Mar-01
Included in the Prior Art Database: 2001-Nov-09
Document File: 1 page(s) / 74K

Publishing Venue

Motorola

Related People

Chan Sek Loon: AUTHOR [+4]

Abstract

The power output of a RF power amplifier may be adjusted by controlling the base bias of the power amplifier tran- sistor. By varying this base bias via a control circuit the amplifier is made to exhibit various characteristics over supply voltage variations leading to power levelling control as well as protection of the RF power amplifier under high line conditions.

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MOTOROLA Technical Developments Volume 10 March 1990

RF POWER LEVELLING SCHEME WITH ADJUSTABLE CHARACTERISTICS OVER VOLTAGE

by Chan Sek Loon, Teo Pek Bing, Peter J. Green and Koo Choong Yew

   The power output of a RF power amplifier may be adjusted by controlling the base bias of the power amplifier tran- sistor. By varying this base bias via a control circuit the amplifier is made to exhibit various characteristics over supply voltage variations leading to power levelling control as well as protection of the RF power amplifier under high line conditions.

   The basic power amplifier (Fig 1) is designed such that variations in base bias will control the output of the amplifier. This is accomplished by initially selecting appropriate input drive and designing the input circuits such that varying the base bias resistor, Rb. will vary the conduction angle and hence the output. The base dc voltage and current through the base resistor may then be measured for use in estimating design values for the control circuit shown in Fig 2.

   In this circuit (Fig 2), the base is biased via diodes, Dl, D2, resistor, Rl, and choke Ll. The value of Rl is selected to give approximately the same base voltage as before. The bias current for the diodes and the base is supplied from another transistor, Q2. This transistor and its associated bias components, R2, R3, R4, R5, R6 and Zener diode, CR1 forms a current source whose current is made a function of supply voltage. The high output impedance of the current source provides the high resistance required for good adaptive bias operation of the circuit. At a fixed supply voltage, variations in RF input drive to the power amplifier causes changes in the bias voltage that tends to counteract the tendency of increase RF drive in in...