2-503
RF5146
Rev A1 050928
Theory of Operation
Overview
The RF5146 is a quad-band GSM850, EGSM900, DCS1800, and PCS1900 power amplifier module that incorporates an
indirect closed loop method of power control. This simplifies the phone design by eliminating the need for the compli-
cated control loop design. The indirect closed loop appears as an open loop to the user and can be driven directly from
the DAC output in the baseband circuit.
Theory of Operation
The indirect closed loop is essentially a closed loop method of power control that is invisible to the user. Most power con-
trol systems in GSM sense either forward power or collector/drain current. The RF5146 does not use a power detector. A
high-speed control loop is incorporated to regulate the collector voltage of the amplifier while the stage are held at a con-
stant bias. The VRAMP signal is multiplied by a factor of 2.75 and the collector voltage for the second and third stages are
regulated to the multiplied VRAMP voltage. The basic circuit is shown in the following diagram.
By regulating the power, the stages are held in saturation across all power levels. As the required output power is
decreased from full power down to 0dBm, the collector voltage is also decreased. This regulation of output power is
demonstrated in Equation 1 where the relationship between collector voltage and output power is shown. Although load
impedance affects output power, supply fluctuations are the dominate mode of power variations. With the RF5146 regu-
lating collector voltage, the dominant mode of power fluctuations is eliminated.
(Eq. 1)
There are several key factors to consider in the implementation of a transmitter solution for a mobile phone. Some of
them are:
•
Current draw and system efficiency
•
Power variation due to Supply Voltage
•
Power variation due to frequency
•
Power variation due to temperature
•
Input impedance variation
•
Noise power
•
Loop stability
•
Loop bandwidth variations across power levels
•
Burst timing and transient spectrum trade offs
•
Harmonics
V
RAMP
3 dB BW
300 kHz
-
+
-
+
H(s)
V
BATT
Saturation
Detector
RF IN
RF OUT
TX ENABLE
P
dBm
10
2 V
CC
V
SAT
–
⋅
()
2
8 R
LOAD
10
3
–
⋅⋅
-------------------------------------------
log
⋅
=
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