Rainbow Electronics MAX2653 - Manual (Page 15)

MAX2651/MAX2652/MAX2653
GSM900 and DCS1800/PCS1900
Dual-Band, Low-Noise Amplifiers
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Detailed Description
Gain Select
The MAX2651/MAX2652/MAX2653 offer a step gain in
the LNA to optimize the receiver’s dynamic range. A
logic-level low at GAIN reduces the active LNA’s gain
by about 20dB and reduces supply current by 3.5mA.
Shutdown Functionality
The MAX2652/MAX2653 offer a low-current shutdown
feature. Drive SHDN low to power down the LNA and
reduce supply current to less than 0.25µA.
Applications Information
External Components
The MAX2651/MAX2652/MAX2653 require matching
circuits at their inputs and outputs for operation in a
50
Ω system. The application circuits in Figures 1, 2,
and 3 describe the matching circuits for each device’s
LNA; suggested component values, suppliers, and part
numbers are listed in the MAX2651/MAX2652/
MAX2653 EV kits manual. These values are optimized
for best simultaneous noise figure, gain, and return loss
performance.
Input and output impedance matching networks are
very sensitive to layout-related parasitics. It is important
to keep all matching components as close to the
device as possible to minimize the effects of stray
inductance and stray capacitance of PC board traces,
particularly for the 1800MHz and 1900MHz bands.
Using the Collector Load Resistor
to Set Gain
The MAX2651/MAX2652/MAX2653 provide open-col-
lector output stages to allow an external resistor to set
the gain. The collector pull-up resistors set the gain for
each LNA to about 18dB. Lower gains are achieved by
reducing this resistance, and higher gains are
achieved by increasing it. The maximum achievable
gain is defined by the maximum collector current
swing. Note that the value of the collector gain-setting
resistor principally defines the LNA’s output imped-
ance, and that the matching networks are tuned to
match this impedance to 50
Ω. Redefining the LNA gain
by changing the collector gain-setting resistor requires
retuning the output matching networks.
Layout and Power-Supply Bypassing
A properly designed PC board is essential to any
RF/microwave circuit. Be sure to use controlled imped-
ance lines on all high-frequency inputs and outputs.
Proper grounding of the GND pins is fundamental; if
the PC board uses a topside RF ground, connect all
GND pins directly to it. For boards where the ground
plane is not on the component side, it’s best to connect
all GND pins to the ground plane with plated through-
holes close to the package.
To minimize coupling between different sections of the
system, the ideal power-supply layout is a star configu-
ration with a large decoupling capacitor at a central
V
CC node. The V CC traces branch out from this central
node, each leading to a separate V
CC
node on the PC
board. A second bypass capacitor that has low ESR at
the RF frequency of operation is placed at the end of
each trace. This arrangement provides local decou-
pling at the V
CC pin. At high frequencies, any signal
leaking out of one supply pin sees a relatively high
impedance (formed by the V
CC trace inductance) to
the central V CC node and an even higher impedance to
any other supply pin, as well as a low impedance to
ground through its bypass capacitor.
Chip Information
MAX2651 TRANSISTOR COUNT: 272
MAX2652 TRANSISTOR COUNT: 272
MAX2653 TRANSISTOR COUNT: 253