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IMPROVED AMPLIFIER EFFICIENCY USING DUAL POWER CONTROL LOOPS

IP.com Disclosure Number: IPCOM000008856D
Original Publication Date: 1998-Sep-01
Included in the Prior Art Database: 2002-Jul-18
Document File: 2 page(s) / 99K

Publishing Venue

Motorola

Related People

Peter P. Walter: AUTHOR [+2]

Abstract

Power amplifier efficiency is the largest source of heat in most transmitters. Market pressure for smaller stations with lower operating cost (lower power consumption) requires better techniques for improving amplifier efficiency.

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MOTOROLA Technical Developments

IMPROVED AMPLIFIER EFFICIENCY USING DUAL POWER

CONTROL LOOPS

by Peter P. Walter and Billy John VanCannon PROBLEM SOLUTION

  Power amplifier efficiency is the largest source of heat in most transmitters. Market pressure for smaller stations with lower operating cost (lower power consumption) requires better techniques for improving amplifier efficiency.

  The voltage applied to final amplifier stage needs to be controlled but linear regulators are not a viable solution since large dissipation would just be moved from the amplifier to the controlling regula- tor. The advance of switching power supplies now offers another solution. The final amplifier stage(s) could have a separate switching power supply. This supply's output voltage would be continuously adjusted to optimize the amplifier's efficiency. Although this adjustable supply could also be used to control the amplifier output power, it is not due to several factors. First, the rise timing, decay timing and transient requirements would be difficult to ful- fill. Also, controlling the output voltage down to zero voltages as well as final amplifier stability at very low operation voltages are difficult to achieve. Finally, it is desirable to have a solution that can eas- ily be retrofitted into existing designs.

  The solution keeps the primary power control circuitry to adjust the supply. voltage to the low level first stage(s) of the amplifier. This fast circuit still determines the key, dekey and transient behavior of the amplifier. A secondary control loop is added which adjusts the supply voltage to the final amplifi-

er. During keyup, the final voltage is kept at a maxi- mum value while the primary loop stabilizes. Then, the slower secondary loop adjusts the final voltage down to a value that is just high enough to maintain the required output power. This lower voltage forces the output stage into saturation thus increasing the output stage efficiency. The actual optimum final output voltage is continuously adjusted by the slow

BACKGROUND

  It is commonly understood that the operating voltage and output impedance are the primary para- meters tha...