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Benefits
of RFMD® Power Flattening Circuit
By Bobby L. Johnson, Applications Engineer, RF Micro
Devices
Introduction
Large variations of output power and current into a mismatched
load can affect efficiency and possibly compromise the PA’s
ability to maintain the minimum output power necessary to
prevent dropped calls. It is increasingly more important
to correct the power variation at the PA level in the handset.
This becomes even more important at type approval in order
to receive carrier compliance for TRP and SAR. The requirements
TRP, Total Radiated Power, and SAR, Specific Absorption
Rate, are tests that the carriers and the governmental agencies
have placed on mobile phone manufacturers to better improve
the quality of service and protect the user.
Increased current causes the handset to transmit
more power. This excess power needs to be dissipated into
the antennae. Power that does not get absorbed by the antennae
is radiated and dissipated into the phone materials and/or
the user; possibly exceeding the SAR absorption rate of
1.6 watts per kilogram. Likewise, power variation in the
negative direction could result in the handset failing minimum
TRP and dropping calls. This tradeoff can be a difficult
balance to achieve. RFMD achieves this through the introduction
of the Power Flattening Circuit. The RF3196 has an integrated
power flattening circuit that prevents the PA from high
current conditions when a mismatch VSWR such as 3:1 is presented
to the output of the PA.
Advantages of RFMD Power Flattening
When a mismatch is presented to the output of the PA, its
impedance is varied and could bring the load into high output
power regions on the Smith Chart. As the output power increases,
so does current consumption. The current consumption can
become very high if not monitored and limited. When considering
the architecture of the transmit chain and the limited isolation
through the switch, any mismatching at the antennae can
load the output of the power amplifier.
A mismatch can be created by a broken antenna, setting the
phone on or near a metal object, or just by the position
of the phone in relation to the user’s head. The design
of the antennae and the power amplifier’s ability
to deliver constant output power are key to how well the
phone is affected under adverse conditions.
The power versus current ellipse is plotted as a function
of phase where output power is on the y-axis and current
on the x-axis. The thinner and narrower the ellipse is,
the better the PA’s performance into mismatch. In
Figure 1, the RF3166 was used in this test
as the control part without power flattening to compare
the results of the RF3196. The ellipse is wider and taller
for the RF3166; this is because the output power is varying
approximately 3.2dBm in output power and 1.74A in current.
With the addition of the power flattening circuit, it is
apparent in Figure 2 that the RF3196 performance
into mismatch is greatly improved. The output power variation
is less than 1.5dBm and the current varies approximately
1Amp. Another advantage is that the max current drawn into
mismatch is less that 2.1A, so there is the added advantage
of improved efficiency.
The power flattening circuit monitors current through an
internal sense resistor. As the current changes, the loop
is adjusted in order to maintain output power. The result
is flatter power and reduced current into mismatch, such
as when a 3:1 load is presented to the output.
This is possible because of the linear relationship between
output power and current. In Figure 3,
is output power and current swept over phase into a 3:1
VSWR. It is apparent that the current and output power are
increasing and decreasing together.
Power Flattening Implementation
The original RFMD Power Star® power control circuit
uses a single feedback loop at the collector to keep the
PA in constant saturation. The power flattening circuit
adds a second loop to feedback a Vsense voltage. The Vsense
voltage is sensed across an internal sense resistor on the
module. This Vsense voltage is compared to a reference voltage.
This reference voltage is set by design into 50ohms, where
the current mirror ratio is set to control the amount of
current in the feedback loop that adjusts the gain of the
PA to correct for the swings in impedance. Figure
4 is a simplified diagram of the feedback loop
with the sense resistor.
If the current through Rsense has increased the collector
voltage, Vcc will be decreased. Likewise, if the current
through Rsense decreases, Vcc will be increased. The Vcc
voltage is controlled by internally adjusting the Vramp
control voltage to keep the power flat. The constant sensing
of the Rsense voltage and the adjustment of the collector
voltage, depending on the current through Rsense, is what
keeps the power flatter and improves current variation.
With the power flattening circuit implemented, the circuit’s
operation is evident when the power and current are now
plotted. The previous condition of a linear relationship
between power and current is now reversed. In Figure
5, is the Power vs Current over phase.
Summary
Output power and current variation into a mismatched load
can compromise the PA’s ability to maintain the minimum
output power, control maximum radiated power, and meet the
requirements of governmental agencies and cellular service
providers. RFMD’s RF3196 PA with integrated Power
Flattening Circuit senses a voltage across an internal sense
resistor. This voltage is fed back to compare to a reference
voltage that is set into 50 ohms. Then it adjusts Vramp
to reduce current and keep the power flat during mismatch
conditions. As TRP and SAR compliance is increasingly more
important in the market, this feature makes the RF3196 the
premiere power amplifier under adverse conditions.
RF Micro Devices
www.rfmd.com
TXTLINX.COM 70
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