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Applications Information:
Designing with the
MGA-72543 RFIC
Amplifier/Bypass Switch
Description
The MGA-72543 is a single-stage,
GaAs RFIC amplifier with an
integrated bypass switch. A
functional diagram of the
MGA-72543 is shown in Figure 1.
The MGA-72543 is designed for
receivers and transmitters operat-
ing from 100 MHz to 6 GHz with
an emphasis on 1.9 GHz CDMA
applications. The MGA-72543
combines low noise performance
with high linearity to make it
especially advantageous for use in
receiver front-ends.
BYPASS MODE
power consumption. The adjust-
able current feature of the
MGA-72543 allows it to deliver
output power levels in excess of
+15 dBm (P
1dB
), thus extending
its use to other system applica-
tions such as transmitter driver
stages.
The MGA-72543 is designed to
operate from a +3-volt power
supply and is contained in a
miniature 4-lead, SOT-343 (SC-70)
package to minimize printed
circuit board space.
The output of the MGA-72543 is
internally matched to provide an
output SWR of approximately 2:1
at 1900 MHz. Input and output
matches both improve at higher
frequencies.
Driver Amplifier
Applications
The flexibility of the adjustable
current feature makes the
MGA-72543 suitable for use in
transmitter driver stages. Biasing
the amplifier at 40 – 50 mA
enables it to deliver an output
power at 1-dB gain compression
of up to +16 dBm. Power effi-
ciency in the unsaturated driver
mode is on the order of 30%. If
operated as a saturated amplifier,
both output power and efficiency
will increase.
Since the MGA-72543 is internally
matched for low noise figure, it
may be desirable to add external
impedance matching at the input
to improve the power match for
driver applications. Since the
reactive part of the input of the
device impedance is capacitive, a
series inductor at the input is
often all that is needed to provide
a suitable match for many appli-
cations. For 1900 MHz circuits, a
series inductance of 3.9 nH will
match the input to a return loss of
approximately 13 dB.
As in the case of low noise bias
levels, the output of the MGA-
72543 is already well matched to
50
Ω
and no additional matching
is needed for most applications.
When used for driver stage
applications, the bypass switch
feature of the MGA-72543 can be
used to shut down the amplifier
to conserve supply current during
non-transmit periods. Supply
LNA Applications
For low noise amplifier applica-
tions, the MGA-72543 is typically
biased in the 10 – 20 mA range.
Minimum NF occurs at 20 mA as
noted in the performance curve
of NF
min
vs. I
d
. Biasing at cur-
rents significantly less than
10 mA is not recommended since
the characteristics of the device
began to change very rapidly at
lower currents.
The MGA-72543 is matched
internally for low NF. Over a
current range of 10 – 30 mA, the
magnitude of
Γ
opt
at 1900 MHz is
typically less than 0.25 and
additional impedance matching
would only net about 0.1 dB
improvement in noise figure.
Without external matching, the
input return loss for the
MGA-72543 is approximately 5 dB
at 1900 MHz. If desired, a small
amount of NF can be traded off
for a significant improvement in
input match. For example, the
addition of a series inductance of
2.7 to 3.9 nH at the input of the
MGA-72543 will improve the input
return loss to greater than 10 dB
with a sacrifice in NF of only
0.1 dB.
RF
INPUT
RF
OUTPUT
AMPLIFIER
Figure 1. MGA-72543 Functional
Diagram.
The purpose of the switch feature
is to prevent distortion of high
signal levels in receiver applica-
tions by bypassing the amplifier
altogether. The bypass switch can
be thought of as a 1-bit digital
AGC circuit that not only pre-
vents distortion by bypassing the
MGA-72543 amplifier, but also
reduces front-end system gain by
approximately 16 dB to avoid
overdriving subsequent stages in
the receiver such as the mixer.
An additional feature of the
MGA-72543 is the ability to
externally set device current to
balance output power capability
and high linearity with low DC
AGILENT [ AGILENT TECHNOLOGIES, LTD. ]
