RF3140PCBA-41X RFMD [RF Micro Devices], RF3140PCBA-41X Datasheet - Page 11

• Manufacturer Part Number: RF3140PCBA-41X
• Description: QUAD-BAND GSM850/GSM900/DCS/PCS POWER AMP MODULE
• Manufacturer: RFMD [RF Micro Devices]

The switching transients due to low battery conditions are regulated by incorporating the following relationship limiting
the maximum V
tery compensation required for extreme conditions is covered by the relationship in Equation 4. This should be added to
the terminal software.
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 cou-
plers due to variation in the coupling factor over frequency, as well as the sensitivity of the detector diode. Since the
RF3140 does not use a directional coupler with a diode detector, these variations do not occur.
Input impedance variation is found in most GSM power amplifiers. This is due to a device phenomena where C
C
power amplifiers. The junction capacitance is a function of the bias across the junction. 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 RF3140 presents a very constant load to the VCO. This is because all stages of the RF3140 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.
Noise power in PA's where output power is controlled by changing the bias voltage is often a problem when backing off of
output power. The reason is that the gain is changed in all stages and according to the noise formula (Equation 5),
the noise figure depends on noise factor and gain in all stages. Because the bias point of the RF3140 is kept constant
the gain in the first stage is always high and the overall noise power is not increased when decreasing output power.
Power control loop stability often presents many challenges to transmitter design. Designing a proper power control loop
involves trade-offs affecting stability, transient spectrum and burst timing.
In conventional architectures the PA gain (dB/ V) varies across different power levels, and as a result the loop bandwidth
also varies. With some power amplifiers it is possible for the PA gain (control slope) to change from 100dB/V to as high
as 1000dB/V. The challenge in this scenario is keeping the loop bandwidth wide enough to meet the burst mask at low
slope regions which often causes instability at high slope regions.
The RF3140 loop bandwidth is determined by internal bandwidth and the RF output load and does not change with
respect to power levels. This makes it easier to maintain loop stability with a high bandwidth loop since the bias voltage
and collector voltage do not vary.
An often overlooked problem in PA control loops is that a delay not only decreases loop stability it also affects the burst
timing when, for instance the input power from the VCO decreases (or increases) with respect to temperature or supply
voltage. The burst timing then appears to shift to the right especially at low power levels. The RF3140 is insensitive to a
change in input power and the burst timing is constant and requires no software compensation.
Rev A9 060217
V
F
CB
RAMP
TOT
(C
=
GS
≤
F1
3
-- - V
8
and C
⋅
+
CC
F2 1
--------------- -
RAMP
SG
G1
+
–
for a FET) vary over the bias voltage. The same principle used to make varactors is present in the
0.18
voltage (Equation 4). Although no compensation is required for typical battery conditions, the bat-
+
-------------------
G1 G2
F3 1
⋅
–
RF3140
BE
(Eq. 4)
(Eq. 5)
2-501
and