Abstract: Power amplifiers used in base stations require biasing for proper RF performance. This article explains the two classes of biasing that are prevalent in the RF industry, analyzes their characteristics, and shows implementations with existing ICs.
The power device of choice for base-station amplifiers today is the lateral DMOS (LDMOS) MOSFET. This article uses it to illustrate biasing techniques.
RF Classes and Biasing
LDMOS amplifiers used in RF circuits exhibit varying
degrees of nonlinearity, depending on the DC-bias level
upon which the input RF waveshape rides. That is, while
maintaining a constant RF gating signal, the output
current's (Iout) harmonic content varies as the DC bias at
the gate of an LDMOS device (Figure 1) changes. The
harmonic content of the LDMOS amplifier's current is
important because, in the RF load, it creates power interference
with the local bandwidth (in-band interference) or
with adjacent bandwidths (out-of-band interference).
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.
Biasing Requirements and LDMOS Behavior
Biasing requires managing the DC content in the LDMOS
current across temperature and supply variation. The
ultimate objective is to ensure that the amplifier RF gain,
as well as its distortion levels, varies within limits consistent
with requirements. In this respect, proper biasing can
assist linearization techniques to minimize distortion.
The equation governing LDMOS's gain is Iout = K (Vgs -
Vth)2, where K is a constant reflecting gain due to electron
mobility and Vth is the FET's threshold. Both K and Vth
are temperature dependent. In Figure 2, LDMOS characteristics
are shown across temperature. In class AB,
designers tend to operate the bias to the left of the
crossover region where the gain has a positive temperature
coefficient. In class A, operation occurs to the right of the
crossover region.
Figure 2. LDMOS characteristics are shown across temperature.
Controlling Class-A and Class-AB Bias with
the DS1847
Figure 3 shows a DS1847 dual, temperature-controlled
variable resistor controlling the gate of an LDMOS
amplifier. The DS1847's internal temperature sensor
provides a temperature reading to its look-up tables. These
look-up tables adjust the IC's two 256-position variable
resistors so the amplifier's gate receives the proper bias
voltage. The user programs the look-up tables to generate
a constant LDMOS-amplifier output current. Refer to
Figure 2 (or to manufacturer-specific data curves) for
LDMOS characteristics. By using the two resistors to
attenuate the reference voltage, a temperature-insensitive
voltage is maintained.
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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