Keywords: base station, rf power amplifier, DS1847, DS1848
The power device of choice for base-station amplifiers today is the lateral DMOS (LDMOS) MOSFET. This article uses it to illustrate biasing techniques.
Figure 1. LDMOS device gating is shown with an uncontrolled DC bias.
The best linearity occurs when the output current tracks the input voltage-a 360° conduction angle. Operating the MOSFET in this manner (i.e., class-A operation) creates less distortion than when biasing it in any other way. From a power-dissipation perspective, however, class-A operation is least desirable because it consumes the most DC current.
At high RF power, given a nominal power-supply voltage of 28V, the DC power dissipated in the amplifier is prohibitive. For this reason, RF engineers use class-AB biasing in the last stage of an amplifier chain, while they favor class-A operation in the preceding stages where power dissipation is smaller by orders of magnitude. In class-AB stages, the output current does not track the input voltage entirely, and thus the amplifier's conduction angle is lower than 360°.
Distortion of the RF signal in class AB is more significant than in class A. The spectrum of this distortion is wider and more densely populated than that of class A. However class-AB power dissipation is lower because the average current into the amplifier is lower. In short, the basis for choosing a given class of commercial RF amplifiers is a tradeoff between linearity and efficiency.
Figure 2. LDMOS characteristics are shown across temperature.
Figure 3. DS1847 dual, temperature-controlled variable resistor controls the gate of an LDMOS amplifier.
A similar article appeared in the November 2003 issue of Wireless Design & Development.
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