Quad-Band GSM Integrated Power Amplifier
By Mike Zybura and Bobby L. Johnson, RF Micro Devices
This article presents the RF3133, a 7 × 10 × 1.4 mm AlGaAs/GaAs Heterojunction Bipolar Transistor (HBT)-based 3 V GSM quad band PowerStar power amplifier (PA) module utilizing integrated passive components for all purposes but low frequency bypassing. The techniques employed facilitate the integration of high quality passive components onto existing chips with superior performance, lower cost, smaller size than conventional surface mount design passive components and extremely tight tolerances. The architecture is a direct extension of previous work with integrated collector power control, and performance levels match or exceed state-of-the-industry standards with 55% typical Power Added Efficiency (PAE) for the GSM band at 35 dBm of output power, and 50% PAE at 33 dBm of output power for the D/PCS bands. The small footprint is a direct result of integrating both the power control and passive elements.
Figure 1. The RF3133 is a 7 × 10 × 1.4 mm laminate-based, internally matched, quad-band PowerStar PA module with integrated power control.
Traditionally, much of the matching and bypass functions in cellular PAs are implemented using surface-mounted passive components. Here, with the exception of very inexpensive low frequency bypassing capacitors, all bypassing and matching is integrated onto the GaAs chip. Similar quality factors have been achieved with the integrated passives providing excellent PAE across all bands. The integration of these components permits a priori modification of reactance values based on in situ measurement during part fabrication. Furthermore, rather than a random distribution of part values and Qs which is observed in Surface Mount Design (SMD) parts, capacitor values vary in unison, which helps stabilize the impedance presented to the PA. Figure 2 illustrates simulated load impedances as seen at the collector of the output stage of the PA from a standard two section low pass output match (inset).
Figure 2. Simulated real part (Ohms) of the load impedance as seen at the collector of the PA for a standard two section low pass output match at 900MHz versus capacitor variation.
The cases presented show the impact of a 10% variation in capacitor value in which both shunt capacitors move up/down in unison, and then in opposing directions. Integrating capacitors also reduces the expense of materials handling and procurement for the numerous surface mount components employed across PA module offerings. The inductive portions are constructed from both micro-strip portions integrated in the laminate and bond wires, each of which are extremely reproducible.
An additional feature included in the RF3133 is integrated power control. Incorporating this function on the PA eliminates the need for several components used with conventional power control including the directional coupler, buffer amplifier, attenuator, power control ASICs and associated SMD components. This reduces the ultimate phone area and cost (by roughly $0.90), while increasing yields due to fewer piece parts. A significant savings also is achieved in calibration time. The power control architecture utilized is accurate and repeatable over process variation. The measurement uncertainty in the test equipment is greater than the variation in the PA output power versus Vramp (the power control pin) with considerable margin. Figure 3 illustrates the repeatability of RF Micro Devices' power control chip.
Figure 3. Process repeatability of the RFMD(r) integrated power control function.
The red lines indicate the ETSI limits and the blue lines show the variation in output power. One standard deviation is less than 0.2 dB, and six standard deviations is less than 0.6 dB. This accuracy and repeatability facilitates faster phone calibration. The phone now can be calibrated at a single point, or the process can be removed all together.
Figure 4. Typical GSM band maximum power and efficiency.
Figure 5. Typical DCS/PCS band maximum power and efficiency.
The cost per second to calibrate is approximately $0.04. Thus a single point calibration on the phone with an RFMD(r) PA can save $0.30 to $1.10 per phone. The repeatability analysis was conducted using a single ramp waveform for all power levels and over all frequencies. Customers easily can generate the waveform using the RFMD Engineer's Advantage ramp synthesis tool. This tool will generate the ramp function for all of our collector controlled power amplifier modules, and is available for customers on the data CD generated with each of our PowerStar products. It includes characterization data, datasheets, application notes, the ramp synthesis tool, power sweeps, and other useful information that help engineers implement RFMD's power controlled PAs.
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