RF7115_1 Datasheet, PDF (12/22 Pages) RF Micro Devices – QUAD-BAND GSM850/GSM900/DCS/PCS TRANSMIT MODULE

RF7115
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
Star^R 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.
TX ENABLE
VRAMP
VBATT
H(s)
RF IN
RF OUT
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:
P_dBm
=
10 ⋅ log
-⁽--2----^â‹---^V----^C----^C----^–-----^V----^S---^A---^T---⁾--²-
8 ⋅ R_LOAD ⋅ 10^–3
(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
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 η_EFF. (η_EFF 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).
η_EFF is defined by the following relationship (Equation 2):
P_{----P---A----}+_-----P_{----L---O---S---S}
-^P---^I--^N-
η_EFF = ^1----0--_V---_B----_A---_T¹---⁰-_â‹---_I--_B---_A----_T^–----_â‹-1--1-0--0--¹--⁰--
(Eq. 2)
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Rev A0 DS060808