RF3145PCBA-41X RFMD [RF Micro Devices], RF3145PCBA-41X Datasheet - Page 16

• Manufacturer Part Number: RF3145PCBA-41X
• Description: QUAD-BAND GSM/EDGE/GSM850/DCS/PCS POWER AMPLIFIER MODULE
• Manufacturer: RFMD [RF Micro Devices]

The components following the power amplifier often have insertion loss variation with respect to frequency. Usually, there
is some length of microstrip following 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
RF3145 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 V
frequency, most synthesizer designers use very wide loop bandwidths to quickly compensate for frequency variations
due to the load variations presented to the V
The RF3145 presents a very constant load to the V
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 RF3145 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 RF3145 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 (for instance, when the input power from the V
voltage). The burst timing then appears to shift to the right, especially at low power levels. The RF3145 is insensitive to a
change in input power and the burst timing is constant and requires no software compensation.
Switching transients occur when the up and down ramp of the burst is not smooth enough, or suddenly changes shape.
If the control slope of a PA has an inflection point within the output power range, or if the slope is simply to steep, it is dif-
ficult to prevent switching transients. Controlling the output power by changing the collector voltage is (as described ear-
lier) based on the physical relationship between voltage swing and output power. Furthermore all stages are kept
constantly biased so inflection points are nonexistent.
2-534
F
CB
TOT
(C
RF3145
=
GS
F1
and C
+
F2 1
--------------- -
SG
G1
APC
–
for a FET) vary over the bias voltage. The same principle used to make varactors is present in the
voltage is swept. Although this could present a problem with frequency pulling the transmit V
+
-------------------
G1 G2
F3 1
⋅
–
CO
.
CO
CO
. This is because all stages of the RF3145 are run at constant bias.
decreases (or increases) with respect to temperature or supply
Rev A4 050919
(Eq. 5)
BE
CO
and
off