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GSM/GPRS power amplifiers use CMOS technology
The IC Insider: Reverse engineering the Axiom AX502 CMOS power amplifier.
By Randy Torrance, Chipworks -- EDN, 2/26/2009
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CMOS technology is vastly preferred over GaAs. CMOS processing is of course much cheaper than GaAs. GaAs manufacturing capacity is very limited, so second sources of supply are difficult to arrange. Also, most GaAs PAs use external passive components and multi-chip modules, adding even more to the cost and supply issues.
There are three main reasons for the difficulty in migrating PAs to CMOS. First, CMOS breakdown voltages are typically under 10V, but a 2W PA needs to deliver up to 20V. Second, the PA output must be matched to 50 ohms, requiring high-Q inductors. These inductors are very difficult to implement in CMOS. Finally, CMOS does not easily operate at high power levels at 1.8 GHz.
Axiom has overcome these challenges and has designed the first GSM/GPRS power amplifiers using CMOS technology. This has required some very sophisticated innovations and new design techniques.
AX502 process technology
The Axiom AX502 device is fabricated using a 7-metal single polysilicon 0.13-µm CMOS process. The top two metal layers, M7 and M6, are unusually thick, about 3.4 microns each. These are the thickest metal layers we have seen on a consumer chip. The thick metal layers are required to increase the inductor Q-factor to a level that allows this PA to meet its specifications. Unfortunately, these two thick metal layers also make the delayering process for reverse engineering very challenging. By using some innovative R&D (and a lot of sweat and tears), we developed a new delayering technique allowing us to get all the way to the bottom layers with no damage to the edges (Figure 1).
With much thanks to our delayering lab, Chipworks has now completed a full analysis of the Axiom AX502.
AX502 architecture
Axiom's AX502 is a fully integrated quad-band GSM/GPRS power amplifier. The device supports GSM 850, E-GSM, DCS, PCS bands, and GPRS class 12. It has an integrated 50-ohm input/output matching circuit. High-accuracy power control and ramping is achieved via a closed loop power-control system. Applications include any dual, triple, or quad-band GSM/GPRS cellular handset or data module. The device implements all of the functions between the transmitter output and the transmit/receive switch, including the power gain stages, small-signal control circuitry, and 50-ohm matching.
The AX502 uses a technology know as Distributed Active Transformer (DAT). This technology was first developed at Caltech, and many papers are available on the theory behind it (References 1, 2, 3). This technology allows high-power and high-speed circuitry in silicon CMOS technology. Multiple differential amplifiers feed multiple primaries in this architecture. Each primary is implemented with a slab of metal, and each secondary winding is implemented as a single loop. The transformation ratio is then given by the number of primaries.
The chip is implemented in two sections, with the left side being the L-band PA (used for 1800 MHz and 1900 MHz), and the right side being the H-band PA (used for 850 MHz and 900 MHz). The two sections are shown overlaid on a top level metal (M7) die photo in Figure 2. Each of these sections is a stand-alone DAT that uses active devices to control the current direction and magnitude on the winding.
Both H-BAND and L-BAND DATs have primary winding sections, second primary winding sections, and secondary winding sections, as shown in Figure 3. Each of these windings is actually implemented in both metal 7 and metal 6. Each primary winding and second primary winding has an associated push-pull amplifier as shown in the figure.
The layout, or topology, of these push-pull drivers is arranged in a circular shape to increase overall output power capacity and efficiency. It should be noted that the output power is proportional to the number of push-pull drivers. Hence, this topology creates one distributed amplifier having individual radiating RF power outputs. During operation, both H-BAND and L-BAND DATs can operate with both primary winding and second primary winding active, or having only one of them active, depending on the power and current requirements of the device. A reverse-engineered schematic representation of one of these DATs is shown in Figure 4.
Each push-pull amplifier pair at each primary winding section can be controlled so that the current flowing on the primary winding section and second primary winding section alternates in direction and magnitude that creates a magnetic field that induces an electromotive force (EMF) on the secondary winding. The electromotive force causes current to flow in the secondary winding based on the impedance of that winding and circuit. Hence, the current through the push-pull amplifier pairs can be controlled to adjust both the current and voltage induced on the secondary winding. Figure 5 shows the 4 push-pull amplifier pairs of both the primary and second primary winding of the L-BAND core, and the 3 push-pull amplifier pairs of both the primary and second primary winding of the H-BAND core.
AX502 devices
There are many interesting devices on the AX502. Of course, the implementation of the primary winding transformers is one of these. But some other devices also caught our eye.
The capacitors seen in the schematic of Figure 4 are compensating capacitors that assist in controlling the impedance of the transistors at the fundamental frequency. They also decrease the levels of the overtones at the output and assist in providing the transistors with suitable impedances for use as a switching amplifier. They are laid out using vertical plates rather than horizontal as is more customary. The capacitors are shown in Figure 6. The main screen on the right shows the Metal 4 layer. The smaller screens on the left show all layers directly above and below this Metal 4 area. On the left screens the views in order from the top are substrate, poly, M1, M2, M3, M4, M5, M6, M7. All layers from poly up to M4 are minimum-pitch vertical wires. On top of all wires from poly to M3 are minimum-pitch vias. Effectively, Axiom has build capacitors using vertical plates running from poly to M4. The reason behind this is that in 0.13-µm technology and below, the line to line spacing is actually smaller than the layer to layer spacing. Hence, the dielectric is thinner in this direction, and the capacitance per mm2 is larger.
A second interesting component is the RF coupler. The RF detector detects the HBOUT and LBOUT frequencies via the RF coupler and feeds the RF signal to the power-control block, forming a closed-loop power controller, thus eliminating the need for external couplers, detectors, and error amplifiers. The RF detector contains an internal differential amplifier for each HBOUT and LBOUT frequency that detects the GSM, DCS, and PCS bands. Figure 7 shows the location of the RF couplers. A higher magnification view of the LBOUT RF coupler on both the Metal 7 and metal 6 layers is shown in Figure 8.
An all-CMOS power amplifier is certainly deserving of accolades and with over 100 schematics required to organize the device, there was no shortage of interesting things to talk about for this article. Axiom has done an impressive bit of circuit engineering and process development to make the GaAs guys pay attention.
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| Author Information |
 Randy Torrance leads the Circuit Analysis team for the Technical Intelligence group at Chipworks. During 22 years in the technology industry he has held senior technical and management positions in the IC design and electronic systems areas. He holds bachelor's and master's degrees in Electrical Engineering from the University of Waterloo. |
