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RF7115
Rev A0 DS060808
7628 Thorndike Road, Greensboro, NC 27409-9421 · For sales or technical
support, contact RFMD at (+1) 336-678-5570 or sales-support@rfmd.com.
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 dia-
gram.
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 out-
put power, supply fluctuations are the dominate mode of power variations. With the RF7115 regulating, 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 (
ηEFF)
• 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
Talk time and power management are key concerns in transmitter design since the power amplifier is the leading current con-
sumer in a mobile terminal. Considering only the power amplifier's efficiency does not provide a true picture for the total sys-
tem efficiency. It is important to consider effective efficiency which is represented by
ηEFF. (ηEFF considers the loss between
the PA and antenna and is a more accurate measurement to determine how much current will be drawn in the application).
ηEFF is defined by the following relationship (Equation 2):
(Eq. 2)
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
⋅
=
η
EFF
10
P
PA
P
LOSS
+
10
------------------------------
10
P
IN
10
--------
–
V
BAT
I
BAT
10
⋅⋅
-------------------------------------------------
=
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