RF3158PCBA-41X RFMD [RF Micro Devices], RF3158PCBA-41X Datasheet - Page 19

• Manufacturer Part Number: RF3158PCBA-41X
• Description: QUAD-BAND GSM/EDGE/GSM850/EGSM900 /DCS/PCS POWER AMPLIFIER MODULE
• Manufacturer: RFMD [RF Micro Devices]

Design Considerations
There are several key factors to consider in the implementation of a mobile phone transmitter solution using the RF3158:
• System Efficiency
• Power Variation due to Supply Voltage
• Power variation due to Temperature
• Noise Power
• Loop Stability and Loop Bandwidth Variation across Power Levels
• Transient Spectrum
Rev A2 DS070615
The RF output match can be designed to improve system efficiency by presenting a non 50Ω load. Output matching cir-
cuits for the RF3158 should be a compromise between system efficiency and power as well as EDGE linearity. Optimal
matching for GSM mode alone may degrade the linear performance beyond system specifications.
Output power does not vary due to supply voltage under normal operating conditions. By regulating the collector voltage
to the PA the voltage sensitivity is essentially eliminated. This covers most cases where the PA will be operated. However,
as the battery discharges and V
This cannot be avoided as a certain supply voltage is required to produce full output power. System specifications must
allow for this power decrease.
Switching Transients due to low battery conditions are reduced by the saturation detection circuit in RF3158. The satura-
tion detection circuit consists of a feedback loop which detects FET saturation. As the FET approaches saturation, the cir-
cuit adjusts the V
integrated into the CMOS controller and requires no additional input from the user.
RF3158 output power variation due to temperature is largest at low power levels and decreases at the upper power lev-
els. This follows the ETSI specification limits which allow a larger tolerance over extreme conditions at low power levels.
Since output power is controlled by an analog input, factors other than the power amplifier will have an effect on total sys-
tem power variation. The entire system containing the RF3158 should be tested to determine whether compensation is
necessary. At high temperatures and low battery voltages, the PA cannot support as high of an output power. In this con-
dition, increasing V
The bias point of the RF3158 is kept constant and the gain in the first stage is always high. This has the effect of main-
taining a consistent noise power which does not increase at reduced output power levels. For that reason, noise power is
at its highest when V
tem receive band noise power failures, but it may amplify noise from other sources. Care must be taken to prevent noise
from entering the power amplifier.
The design of a proper power control loop involves trade-offs affecting stability, transient spectrum and burst timing. In
non-PowerStar® architectures, backing off power causes gain variation which can affect loop bandwidth. In RF3158 the
loop bandwidth is determined by V
tained since amplifier bias voltage is constant.
Switching transients occur when the up and down power ramps are not smooth enough, or suddenly change shape. If the
control slope of a PA has an inflection point within the output power control range, or if the slope is too steep, switching
transients will result. In RF3158 all stages are kept constantly biased and the output power is controlled by changing the
collector voltage according to Equation 1. Inflection points are eliminated by this design. In addition, the steepness of the
power control slope is reduced because V
power amplifiers.
CC
RAMP
voltage in order to ensure minimum switching transients. The saturation detection circuit is
RAMP
will not provide more output power, so compensation may not provide the intended result.
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is at its maximum. The RF3158 does not create enough noise in the receive band to cause sys-
BATT
CC
approaches its lower operating limit, the output power from the PA will start to drop.
regulator bandwidth and does not change over output power. Loop stability is main-
RAMP
actively controls output power over a larger voltage range than many other
RF3158
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