RF5146 Datasheet, PDF (14/18 Pages) RF Micro Devices – QUAD-BAND GSM850/GSM900/DCS/PCS POWER AMP MODULE

RF5146
Output power does not vary due to supply voltage under normal operating conditions if V_RAMP is sufficiently lower than
V_BATT. 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 V_RAMP 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 V_RAMP.
The switching transients due to low battery conditions are regulated by the V_RAMP limiter circuit. The V_RAMP limiter, a
new feature for the RF5146, consists of a feedback loop that detects FET saturation. As the FET approaches saturation,
the limiter adjusts the V_RAMP voltage in order to ensure minimum switching transients. The V_RAMP limiter is integrated
into the CMOS controller and requires no additional input from the user.
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. Power variation straight out of the RF5146 is shown in the tables below.
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
RF5146 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_BE and
C_CB (C_GS and C_SG for a FET) vary over the bias voltage. The same principle used to make varactors is present in the
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 RF5146 presents a very constant load to the VCO. This is because all stages of the RF5146 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 2),
F_TOT
=
F1 + ^F-----2----–-----1- + --^F----3-----–----1----
G1 G1 ⋅ G2
(Eq. 2)
the noise figure depends on noise factor and gain in all stages. Because the bias point of the RF5146 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 RF5146 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.
2-504
Rev A1 050928