Browse Prior Art Database

SELF TUNING NARROWBAND CLASS C AMPLIFIER

IP.com Disclosure Number: IPCOM000006292D
Original Publication Date: 1991-Dec-01
Included in the Prior Art Database: 2001-Dec-20
Document File: 1 page(s) / 50K

Publishing Venue

Motorola

Related People

Scott Olson: AUTHOR

Abstract

A narrowband class C ampliier may contain an ele- ment which must be manually tuned to maximize the gain at a particular operating frequency. A feedback mech- anism is presented which automatically tunes a class C amplitier for maximum gain.

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MOTOROLA INC. Technical Developments Volume 14 December 1991

SELF TUNING NARROWBAND CLASS C AMPLIFIER

by Scott Olson

   A narrowband class C ampliier may contain an ele- ment which must be manually tuned to maximize the gain at a particular operating frequency. A feedback mech- anism is presented which automatically tunes a class C amplitier for maximum gain.

  The dc collector voltage of a grounded emitter narrowband class C ampliier decreases as the signal through the amplifier increases. A feedback circuit was constructed to tune the output of one stage for a mini- mum dc voltage at the collector of the next stage. Referring to Figure 2, the class C amplifier Ql was previously tuned manually via Ll; a varactor diode was placed at the output of the device to alternately tune the stage electrically. A graph of Vcollector vs. Vcontrol is shown in Figure 1. The control circuit

sweeps Vcontrol until it finds the minimum voltage at Vcollector.

  U2 detects the slope of the changing Vcollector. If Vcollector in increasing, the noninverting input lags the invetting input thereby forcing the output low. Conversely when Vcollector is decreasing, the output of U2 goes high. When the output of U2 is low, Q3 is off, Cl charges, and Vcontrol increases. When the output of U2 is high, Q3 turns on, Cl discharges, and Vcontrol decreases. Since Rl is tied to B+, which is always on, Cl will ini- tially be charged. Upon transmit, 'Ik B+ goes high, tum- ing on Q3, which causes Vcontrol to...