Preliminary
2-269
RF3110
Rev A0 010921
2
Where PPA is the output power from the PA, PLOSS the
insertion loss, PIN the input power to the PA and PDC
the delivered DC power.
The RF3110 improves the effective efficiency by mini-
mizing the PLOSS term in the equation. A directional
coupler may introduce 0.4dB to 0.5dB loss to the tran-
sit path. To demonstrate the improvement in effective
efficiency consider the following example:
Conventional PA Solution:
RF3110 Solution:
The RF3110solutionimproves effectiveefficiency 5%.
Output power does not vary due to supply voltage
under normal operating conditions if VRAMP is suffi-
ciently lower than VBATT. 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
approaches its lower power range the maximum output
power from the PA will also drop slightly. In this case it
is important to also decrease VRAMP to prevent the
power control from inducing switching transients.
These transients occur as a result of the control loop
slowing down and not regulating power in accordance
with VRAMP.
The switching transients due to low battery conditions
are regulated by incorporating the following relation-
ship limiting the maximum VRAMP voltage (Equation 2).
Although no compensation is required for typical bat-
tery conditions, the battery compensation required for
extreme conditions is covered by the relationship in
Equation 4. This should be added to the terminal soft-
ware.
(Eq. 4)
Note: Output power is limited by battery voltage. The
relationship in Equation 4 does not limit output power.
Equation 4 limits the VRAMP voltage to correspond with
the battery voltage.
Due to reactive output matches, there are output power
variations across frequency. There are a number of
components that can make the effects greater or less.
The components following the power amplifier often
have insertion loss variation with respect to frequency.
Usually, there is some length of microstrip that follows
the power amplifier. There is also a frequency
response found in directional couplers due to variation
in the coupling factor over frequency, as well as the
sensitivity of the detector diode. Since the RF3110
does not use a directional coupler with a diode detec-
tor, these variations do not occur.
Input impedance variation is found in most GSM power
amplifiers. This is due to a device phenomena where
CBE and CCB (CGS and CSG for a FET) vary over the
bias voltage. The same principle used to make varac-
tors is present in the power amplifiers. The junction
capacitance is a function of the bias across the junc-
tion. This produces input impedance variations as the
Vapc voltage is swept. Although this could present a
problem with frequency pulling the transmit VCO off
frequency, most synthesizer designers use very wide
loop bandwidths to quickly compensate for frequency
variations due to the load variations presented to the
VCO.
The RF3110 presents a very constant load to the VCO.
This is because all stages of the RF3110 are run at
constant bias. As a result, there is constant reactance
at the base emitter and base collector junction of the
input stage to the power amplifier.
P
PA = +33 dBm
P
IN =+6 dBm
P
LOSS =-0.4 dB
V
BAT =3.5 V
I
BAT =1.1 A
η
EFF = 47.2%
P
PA = +33 dBm
P
IN =+6 dBm
P
LOSS =0 dB
V
BAT =3.5 V
I
BAT =1.1 A
η
EFF = 51.72%
V
RAM P
3
8
--- V
BAT T
0.18
+
⋅
≤
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