MAX2251
+2.8V, Single-Supply,
Cellular-Band Linear Power Amplifier
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Detailed Description
The MAX2251 is a linear PA intended for TDMA/AMPS
dual-mode applications. The PA is fully characterized in
the 824MHz to 849MHz U.S. cellular band. The PA con-
sists of a driver stage and an output stage; both are
independently biased using external resistors. The
MAX2251 also features an integrated power detector.
Bias Control
External resistors connected to C1 and A1 indepen-
dently set the bias currents of the driver and output
stages, respectively. An internal bandgap reference
fixes the voltages at C1 and A1. RBIAS1 is typically
47.5kΩand RBIAS2 is typically 11kΩ. The bias current
can be dynamically adjusted by summing a current into
the bias pin of interest with an external source such as
a DAC. See the Typical Operating Circuit. The Typical
Operating Characteristics graph, Supply Current vs.
Output Power, demonstrates the current saving with
throttleback at low-output power levels.
Power Detector
The on-chip power detector monitors the output power.
The power detector outputs a voltage proportional to
the output power. Connect a filter capacitor from
PD_OUT to GND to set the power detector time con-
stant. The integrated power detector eliminates the
need for an external detector circuit.
Applications Information
External Matching
The MAX2251 requires input, interstage, and output
matching circuits for proper operation. See the Typical
Operating Circuit for suggested component values.
Use high-quality components for L2 and C12 in the out-
put-matching circuit for highest efficiency. The
MAX2251 EV kit uses a trace as a pullup inductor
(approximately 2nH) for the interstage matching.
Mode Selection
MAX2251 features two modes of operation: high-linear
mode and high-gain mode. For TDMA operation, drive
MODE high or connect to VCC. For AMPS operation,
drive MODE high for best PAE, or drive MODE low for
best gain.
Layout and Thermal Management Issues
The MAX2251 EV kit serves as a layout guide. Use con-
trolled impedance lines on all high-frequency inputs
and outputs. Connect GND to the PC board ground
plane with as low inductance path as possible. The
GND pins also serve as heat sinks. Connect all GND
stage of the PA.
capacitor to set time constant. The settling time is typically 2µs with a 4700pF capacitor.
B4 together.
the driver stage.
board trace.
operation.
operation.