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RF2123: New High Power, High Efficiency HBT GSM Power Amplifier
RF Micro Devices introduces a new power amplifier for
GSM applications based on revolutionary HBT (Het-
erojunction Bipolar Transistor) technology. This power
amplifier operates from a single 4.8V or 6V power sup-
ply without the need for a negative voltage. The power
output at 4.8V is 35dBm, and at 6V the PA provides
36dBm! The overall efficiency is as high as 62 percent.
On-board power control is included, as is power down.
The part is packaged in an industry standard 16-lead
SOIC with 4 fused, wide leads.
efficiency is extremely important. Sixty percent total
efficiency for a two-stage, 30dB gain GSM power
amplifier IC is ideal for maximizing talk time – a key
performance advantage at the competitive system
level.
•
Small Package Size. As cellular phone sizes shrink,
the available real estate for RF components shrinks
as well. Traditional power amplifier designs become
difficult to implement in the required area; thus the
SOIC packaged, integrated amplifier approach is
extremely beneficial. The RF2123 takes the place of
a large discrete implementation, or a MESFET IC
implementation with additional components such as
a negative voltage generator and a supply-side
switch. The unique “Quad BatWing” package pio-
neered by RF Micro Devices allows superior heat-
sinking and electrical grounding. This allows over 4-
W of output power to be transmitted in GSM mode
with no special heat slug or non-standard packag-
ing.
No Negative Voltage. HBT is a unique technology,
allowing performance better than GaAs MESFETs,
yet allowing biasing similar to Silicon Bipolar from a
single positive voltage. This eliminates one of the
primary disadvantages with GaAs MESFETs – the
requirement for a negative voltage. For a system
de-signer to implement negative voltage with suffi-
cient current to drive a MESFET gate, some kind of
switching regulator or “charge pump” must be used.
This can be expensive and cumbersome. If the
charge pump is implemented on-chip, excessive
low-frequency noise, additional leakage current,
and additional external components minimize the
benefit.
HBT provides an elegant solution to the high-effi-
ciency power amplifier. With no need for additional
components, the part provides an overall smaller,
more efficient, and lower cost solution.
No supply-side switch. The RF2123 HBT Power
Amplifier provides a single pin for power down. This
function powers down the part with less than 0.5V
on the control pin, and provides full power with 4V
on the control. In power-down mode, less than 1mA
of total current is consumed, allowing very long
stand-by times for the phone.
With the maturation of digital cellular systems in
Europe, Japan, and North America, next generation
handsets must offer advantages to the consumer.
These generally take the form of lower cost, smaller
size, and longer battery life.
One component which has traditionally been either
expensive, large, power-hungry, or any combination of
these has been the RF power amplifier. This critical
component governs much of the battery life, size, ease
of implementation, and manufacturability of the phone.
For 5-cell applications between 5.3V and 6.0V, this
function can now be performed using HBT (Hetero-
junction Bipolar Transistor) technology from RF Micro
Devices. The new RF2123 GSM power amplifier pro-
vides all of these advantages to the handset designer,
amplifying a +6dBm input signal to over +35dBm out-
put, at up to 60 percent efficiency. Moreover, no nega-
tive voltage is required, either internal or externally
generated. Additionally, an on-board analog gain con-
trol provides greater than 60dB of power control from
1V to 4V control voltage. When the control is reduced
to <0.5V, the part is shut off, drawing less than 1mA.
•
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TECHNICAL NOTES
AND ARTICLES
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The HBT Power Amplifier drives several key features of
the phone operation and design. Some are advantages
to the end customer, such as talk time and overall
phone size. Others are related to the ease of design
and manufacturing the phone, such as single voltage
supply, on-board power-down, and on-board power
control. These advantages are discussed below:
•
Talk time. The current consumption of the transmit-
ter is dominated by the power amplifier. For battery
operated applications, the power-added (or total)
•
Copyright 1997-2000 RF Micro Devices, Inc.
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•
To utilize a GaAs MESFET power amplifier, the sys-
tem designer must insert a switch into the bias sup-
ply line to the part for shutdown. The gate cannot be
used to switch the MESFET on and off due to the
high gate-source capacitance. On-board charge
pumps must consume milliamps of current during
power-down. This supply-side switch must be capa-
ble of supporting very high currents, and tends to be
very ex-pensive as a result. A MOSFET switch will
cost on the order of $0.50 to $0.75, which is a sub-
stantial portion of the overall power amplifier cost.
The loss through the switch also reduces the volt-
age available on the drain of the MESFET PA, thus
requiring more current to achieve the same output
power.
•
Gain Control. Using the same pin as is used for
power down, the gain is controlled over 60dB with a
1V to 4V control range. A typical power control
characteristic is shown in Figure 1.
High Reliability. At RF Micro Devices, we have
logged over 1.5 million device hours under RF
stress accelerated lifetest since 1993. Junction tem-
peratures of 250°C are used to accelerate the test.
In addition, TRW has been running accelerated DC
lifetest, which correlate with RF stress results. The
reliability is much improved over the initial HBT
technologies available 8-10 years ago. Today, over
40 million hours MTBF at 125°C junction tempera-
tures is achieved.
•
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TECHNICAL NOTES
AND ARTICLES
Figure 1. Output power vs. Vpc from 0 to 4V. Note that the power control range is >60dB with
the maximum output power at 36.3dBm.
The RF2123 is one of a family of power amplifiers from
RF Micro Devices based upon HBT technology for both
linear and constant-envelope applications. This tech-
nology, provided by TRW, is a proven technology origi-
nally developed for military and space applications.
Based upon a Gallium Arsenide/Aluminum Gallium
Arsenide (GaAs/AlGaAs) heterostructure, the power
and efficiency performance is the highest of any com-
mercially available integrated solution. Being a bipolar
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structure, the part can operate from a single positive
voltage supply without adding components – extremely
important in a battery operated system such as a cellu-
lar phone.
The critical geometries in an HBT transistor are vertical
structures, not lateral. The emitter, base, and collector
are stacked vertically by semiconductor layer growth,
using MBE (Molecular Beam Epitaxy). This is a very
accurate and repeatable growth process. Since each
layer is placed over the entire wafer at once, no photo-
Copyright 1997-2000 RF Micro Devices, Inc.
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Vcc1
1
16 BiasR
GND
2
15 GND
14 GND
13 RFout
12 RFout
11 GND
10 GND
Vpc
GND
3
GND1
4
RFin
5
GND
6
BIAS
GND
7
Vcc1
8
9
Figure 2. Block Diagram and Pinout for RF2123.
lithography is required for this process; thus, mask
alignment and optical resolution is not an issue. Also,
this means wafers can be prepared and stock-piled,
eliminating this step from the critical path of product
manufacturing.
Once the layers are completed, then the lithography
begins. Since all the critical geometries are already de-
fined, the minimum feature size is currently 2mm. This
is much more manufacturable than the 0.5-1.0mm gate
geometries typically required by GaAs MESFETs.
We feel the TRW HBT process is the most reliable
commercially available HBT process in the world. As a
military subcontractor, TRW has qualified the process
for many of their military programs. Additionally TRW,
as a space equipment manufacturer, has qualified the
HBT process for Class S space applications. This level
of ruggedness is absolutely needed for spacecraft,
since it is somewhat difficult to repair a failed compo-
nent in space, but is also demanded by the commercial
marketplace today. RF Micro Devices and TRW have
both been diligently testing the HBT process and prod-
ucts to determine the ruggedness and failure rates.
The MTBF is currently defined to be 4x107 hours at a
junction temperature of 125°C. Additional information
is available on the reliability of HBTs, and may be
obtained with the application information package on
the RF2123.
In order to meet the continuing demand for GaAs HBT,
RF Micro Devices has licensed the proprietary TRW
HBT process for commercial wireless applications and
is building a high-volume wafer fab in Greensboro, NC,
to provide the industry with these power amplifiers and
other high-performance ICs. This wafer fab will be the
largest GaAs HBT wafer fab in the world, and will con-
tinue to enforce our dominant position as the leading
supplier of GaAs HBT circuits.
! "
#
The block diagram for the RF2123 is shown in Figure
2. The part is a two-stage device with 30dB gain at full
power. The drive level required to fully saturate the out-
put is +6dBm. Bias control is provided through a single
pin interface, and the final stage ground is achieved
through the large pins on both sides of the package.
First stage ground is brought out through a separate
ground pin for isolation from the output. These grounds
should be connected directly with vias to the PCB
ground plane. The output is brought out through the
two output pins, and combined off-chip to form the out-
put line.
The amplifier operates in Class AB bias mode. The
final stage is “deep AB”, meaning the quiescent current
is extremely low. As the RF drive is increased, the final
stage self-biases, causing the bias point to shift up
and, at full power, draws about 1A. The optimum load
for the output stage is ~3.5W. This is the load at the
output collector, and is created by the series induc-
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TECHNICAL NOTES
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Copyright 1997-2000 RF Micro Devices, Inc.
TA0012
tance formed by the output bond wires, leads, and
microstrip, and two shunt capacitors external to the
part. With this match, a 50Ω terminal impedance is
achieved. The input is matched to 50Ω with just a
blocking capacitor needed. The input is DC coupled;
thus, a blocking cap must be inserted in series.
Vcc1 provides supply voltage to the first stage, as well
as provides some control over the operating band.
Essentially, the bias is fed to this pin through a short
transmission line. A tuning capacitor external to Pin 1
sets the frequency of the gain peak. A resistor in paral-
lel with the bias choke for the first stage helps to stabi-
lize and dequeue the circuit.
The RFout pins provide the output power. Pins 12 and
13 should be combined externally. Bias for the final
stage is fed to this output line, and the feed must be
capable of supporting the 1-1.5A of current required.
The HBT breakdown voltage is >20V, so nominally at
6V there should be no issue with overvoltage. Under
extreme conditions, however, which can occur in a cel-
lular handset environment, the supply voltage could be
as high as 8.5V to 9.5V. These conditions may corre-
spond to operation in a battery charger, especially with
the battery removed, which “unloads” the supply cir-
cuit. To add to this worst-case scenario, the RF drive
may be at full power during transmit, and the output
VSWR could be extremely high, corresponding to a
broken or removed antenna. Under all of the above
conditions, the peak RF voltages could well exceed 2X
the supply voltage, forcing the device into breakdown.
The RF2123 includes overvoltage protection diodes at
the output, which begin clipping the waveform peaks at
~15V. This protects the device’s output from breaking
down under these worst-case conditions, and provides
a rugged, robust component for the system designer.
High current conditions are also potentially dangerous
to any RF device. High currents lead to high channel
temperatures and may force early failures. The
RF2123 includes reference diodes in the bias circuit to
temperature compensate the RF transistors, thus limit-
ing the current through the bias network and protecting
the devices from damage. The same mechanism
works to compensate the currents due to ambient tem-
perature variations, and the part is remarkably consis-
tent over the full -30°C to +85°C commercial
temperature range.
While the part is safe under CW operation, maximum
power and reliability will be achieved under pulsed con-
ditions. The data shown in the following table were
taken with a 12.5% duty cycle and a 600ms pulse.
! "
The RF2123 performance is summarized in Table 1
below. A full data sheet is available from RF Micro
Devices.
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TECHNICAL NOTES
AND ARTICLES
Parameter
Frequency Range
Maximum CW Output Power
Total CW Efficiency
Gain at Max Power
Gain Control Range
Vpc Current
“OFF” Current
Voltage Range
Stability
Temperature Range
Typical Performance
890-915 MHz
4-W
55%
30 dB
45 dB, min
1 mA, max
10 uA, max
5.3 to 6V
Spurious <-60dBm
-30 to +85 C
Conditions
With specified load at 6V
at Max Output
Vpc < 0.2V
Output VSWR < 8:1
Operating
Table 1. RF2123 Performance Summary
The RF2123 HBT GSM Cellular Power Amplifier has
been introduced by RF Micro Devices. This amplifier
provides the best overall performance of any integrated
PA on the commercial market. Operating from a single
positive supply, efficiencies of 55% and power levels of
4-W are achievable from a single 16-lead SOIC surface
mount package at 6V. Power down and power control
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are integrated on-chip without additional components
required. The new power amplifier can be used to sim-
plify cellular phone design and improve operation, as
well as significantly reducing overall cost.
Copyright 1997-2000 RF Micro Devices, Inc.
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