High power RF-LDMOS transistors for base station applications

Figure 4. Class-AB two-tone intermodulation
distortion versus output power as a function
of drain bias.
mance of the device in the region 8 dB
to 10 dB below P 3dB a good indicator of
CDMA power capability. Moreover, the
excellent linearity of RF-LDMOS
devices in this so-called “back off”
region contributes to minimizing the
complexity of the total amplifier system
and maximizing system efficiency.
Additional considerations for linearity
performance using wide modulation
bandwidths or widely separated signals
are gain and phase flatness. These
parameters were measured over the
band of 2.11 GHz to 2.17 GHz and
found to be 0.20 dB and 0.42 degrees
respectively.
Furthermore, many W-CDMA base
stations are designed to operate with
several carriers adjacent to one another.
Intermodulation distortion caused by
two spread-spectrum signals is certain to
occur under these conditions, and therefore,
must be quantified if the device linearity
is to be characterized fully.
From a system point of view, two W-
CDMA carriers with a spacing of 10
MHz represent a worst-case situation.
Two W-CDMA carriers at f 1 = 2.1125
GHz and f 2 = 2.1225 GHz were used to
generate the data plotted in Figure 8.
The IMD of the third-order products
(IMD3) is calculated by integrating the
power in 3.84 MHz bandwidths at
2.1325 GHz (IMD3+) and 2.1025 GHz
(IMD3–), and is plotted along with efficiency
and ACPR against output power
Figure 5. Class-AB two-tone power gain versus
output power as a function of drain bias.
in Figure 9. It is clear that the IMD3,
as measured under these conditions,
tracks ACPR. Note that the drain bias
for the two carrier measurement has
been increased to 1.6 A, which is
slightly higher than the 1.3 A used to
achieve the best single carrier W-
CDMA performance. The ACPR reaches
–40 dBc at 44.6 dBm (29.0 W) at
22.9% efficiency, whereas the IMD3
generated between two adjacent multicarrier
stimuli reaches –40 dBc at 43.7
dBm (23.7 W) and 20.5% efficiency.
Choosing which metric to use and the
appropriate bias point should be application-driven.
Output power
While the average power per sector
and/or carrier in W-CDMA systems is
relatively low, the high peak-to-average
ratio of the signal requires transistors
with state-of-the-art, peak power capabilities.
Multicarrier systems require
designers to continually strive for
increased power capability from their
power amplifiers (PA’s). Delivering
transistor solutions that generate larger
and larger amounts of output power
assists designers in achieving this goal.
Output power can be defined under
several different conditions, one of
which is the previously defined P 3dB .
Figure10 shows the constant envelope
or continuous wave (CW) output power
capability of the single-ended design.
The broadband capability of the device,
along with excellent flatness, is further
shown in Figure 11 under conventional
two-tone conditions. Although GSM
base stations operate at elevated average
power levels (because of the constant
envelope nature of the signal),
CDMA and W-CDMA base stations
maintain low average power levels due
to the linearity requirements imposed
by their spread-spectrum nature.
Referring back to Figure 9, an average
power of 23.7 W (or 8.2 dB below
P 3dB ) is achieved under W-CDMA conditions
when a linearity constraint of
IMD3 +/- of –40 dBc or better is used.
Note the continuing reduction in IMD3
with decreasing power illustrating the
excellent linearity properties of the RF-
LDMOS technology. The system
designer must carefully consider the
overall system architecture, including
error correction, to determine the optimum
operating point.
For a given supply voltage, increasing
device size is the most straightforward
method used to increase a
device’s power capability. However,
when a fixed amount of power is
required, caution must be exercised
not to make the device too large or
efficiency will be degraded. An additional
challenge facing the device
designer is ensuring these very high
power devices can achieve the specified
levels of performance when
acceptable levels of circuit impedances
are presented to the I/O terminals. For
the device described in this article, the
layout design of the die incorporates
two large metal bus pads for the drain
and gate connections. These bus pads
allow advanced topology impedance
matching to be designed into the
device package. The resulting
input/output impedances achieved
across the W-CDMA band are shown
in Table 1 (Note the typical Q is 1.8 for
Z in and 1.25 for Z out .
Figure 6. Example of spectral regrowth when
driven by a CDMA signal.
Figure 7. ACPR and drain efficiency for one W-
CDMA carrier versus output power.
Figure 8. Two W-CDMA carriers showing IMD3
and spectral regrowth.
22 www.rfdesign.com March 2000