RF3158PCBA-41X RFMD [RF Micro Devices], RF3158PCBA-41X Datasheet - Page 17

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

A large portion of the total current in a linear amplifier is necessary to bias the transistors so that the output remains linear. In
an EDGE system where there are a range of output power levels used (PCLs), an amplifier biased to operate at a high power
will be very inefficient at low power levels. Conversely, an amplifier biased to operate at a low power will not be linear at high
power levels. The maximum linear power of an amplifier is determined during design, but can be adjusted to some extent by
the quiescent current through the amplifier transistors.
The RF3158 incorporates an analog bias control in EDGE mode. This allows the system designer to reduce quiescent current in
the power amplifier when operating at lower output power levels, resulting in improved efficiency. Low bias mode for the
RF3158 is basically a specific setting of the analog bias control for which characterization and test data has been collected. In
low bias mode the PA can only be operated at or below a specified output power level while maintaining linearity. It is conceiv-
able that a system designer could select a separate bias setting (V
operating levels. However, as bias changes, so does the gain, linearity, and temperature response. Balancing all these param-
eters across extreme operating conditions and process variation would likely be impossible in a volume production environ-
ment. At minimum, careful system characterization and planning would be required.
Power Ramping and Timing
The RF3158 should be powered on according to the Power-On Sequence provided in the datasheet. The power on sequence is
designed to prevent operation of the amplifier under conditions that could cause damage to the device, or erratic operation.
In the Power-On Sequence, there are some set-up times associated with the control signals of the RF3158. The most important
of these is the settling time between TXEN going high and when V
the "pedestal" and is required so that the internal power control loop and bias circuitry can settle after being turned on. The
RF3158 requires at least 1.5µs or two quarter bit times for proper settling of the power control loop.
The diagram below is the ETSI time mask for a single GSM timeslot.
The V
The ability to ramp the RF output power to meet ETSI switching transient and time mask requirements partially depends upon
the predictability of output power versus V
capable of meeting switching transient requirements with the proper raised Cosine waveform applied to the V
ramping waveform on V
transients at high power levels.
Rev A2 DS070615
RAMP
waveform used with the RF3158 must be created such that the output power falls into this power versus time mask.
RAMP
must not start until after TX_EN is asserted. A ramp of about 12us is required to control switching
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RAMP
response of the power amplifier. The PowerStar® control in the RF3158 is very
RAMP
RAMP
can begin to increase. This time is often referred to as
voltage) for each PCL to optimize efficiency across all
RF3158
RAMP
input. The
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