11-376
RF7115
Rev A0 060808
Power Control Theory of Operation
Most power control systems in GSM sense either forward power or collector/drain current. The RF7115 uses RFMD’s
Power StarR collector voltage control instead of a power or current detector. A high-speed control loop is incorporated to
regulate the collector voltage of the amplifier while the stages are held at a constant bias. 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 -15dBm, 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 RF7115 regu-
lating, there are several key factors to consider in the implementation of a transmitter solution for a mobile phone. Some
of them are:
(Eq. 1)
•
Effective efficiency (
η
EFF)
•
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
•Post PA loss
•
Insertion loss in receive ports
•
TX power leakage into the RX ports
•
Performance during VSWR
•
Time needed to implement the solution
•
Needed board area for the solution
RF IN
RF OUT
H(s)
VRAMP
TX ENABLE
VBATT
P
dBm
10
2 V
CC
V
SAT
–
⋅
()
2
8 R
LOAD
10
3
–
⋅⋅
-------------------------------------------
log
⋅
=
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