One of the few non-CMOS circuits left in wireless devices like cell phones these days is the power amplifier (PA). Most wireless radios use gallium-arsenide (GaAs) bipolar PAs, which up to now have been the only PAs really up to the task.
Making 26 to 28 dBm of linear power (roughly 500 to 600 mW) as usually required at frequencies up to 2.7 GHz isn’t easy, but the fussy high-mobility GaAs bipolar junction transistors (BJTs) do an excellent job. However, the BlackSand Technologies BST34 and BST35 series linear CMOS PAs can directly replace the existing GaAs PAs in most 3G handsets, tablets, and dongles.
Designing A CMOS PA
RF PAs are a special case of CMOS design. There are no standard large signal models or CAD tools, so engineers are on their own in the design process. While getting to the desired frequency range is made simpler by going to smaller geometries of CMOS (130 nm in this case), this technique conflicts with the voltage needs (up to the 4.2-V lithium-ion battery output) to get the desired power.
None of this deterred the members of the BlackSand design team, who learned their craft designing a 2G CMOS PA while at mixed-signal leader Silicon Labs. Instead, they came up with a unique stacked architecture that beats the higher-voltage problem and power requirements. The result is a genuine 3G CMOS linear PA that you can make in any standard CMOS fab. With more than 70 patents in its CMOS PA portfolio, BlackSand may be in line to become the CMOS RF PA leader.
What’s The Big Deal?
As smart phones and 3G methods have developed and been quickly adopted, the front-end circuitry of a cell phone has gotten much more complex. The need for multiband coverage to handle older 2G technologies along with the newer 3G technologies has meant the incorporation of multiple PAs.
For example, Apple’s iPhone has six PAs (two for GSM and four for 3G) just to cover the bands in use. Four of those PAs must be linear to handle the CDMA or HSPA being used. The iPhone uses the expensive GaAs types of PAs, and they’re one of the most power-hungry parts in a cell phone as well as one of the most expensive. The need for multiple PAs is only going to increase with the addition of the 4G technologies and bands, not to mention the growing demand in tablets and other mobile devices.
The big question is if the GaAs fabs can keep up. So far they have. With only a handful of GaAs fabs to serve multiple vendors, shortages could occur with any significant increase in demand or supply-chain glitches. The problem could also be compounded by a shortage of gallium, which is one of those rare earth materials that are hard to come by. With China controlling the rare earth supplies these days, it may be time to get serious about an alternative.
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Introducing The BST34 And BST35
The BST34 series (Fig. 1) is a pin-compatible drop-in replacement for existing GaAs PAs. With its 3- by 3-mm, 10-pin package, it can be adopted with no redesign of existing printed-circuit boards. It has a built in directional coupler on the output that’s daisy-chain compatible. The series complies with all 3GPP WCDMA and HSPA standards. And, it features a full 28-dBm high power output mode and a 19-dBm medium power mode.
The output impedance is a standard 50 Ω. The efficiency is 40%, which isn’t bad for a linear PA these days. Other features include digital bias control, digital calibration of internal offsets, and internal sensors to optimize the performance. The BST34 is available in three versions depending on the frequency of operation:
- BST3401: Band 1 UMTS 2100 (1920 to 1980 MHz)
- BST3402: Band 2 PCS (1850 to 1910 MHz)
- BST3404: Band 4 AWS (1710 to 1755 MHz), Band 9 Japan (1749.9 to 1784.9 MHz), Band 10 Latin America (1710 to 1779 MHz)
With its direct replacement benefit and the backing of the huge CMOS supply chain, the BST34 series also offers the benefit of greater reliability simply because CMOS isn’t subject to thermal runaway, unlike GaAs devices.
Comprising the BST3501, BST3502, and BST3504, the BST35 series (Fig. 2) offers all of the features of the BST34 and supports all of the same frequency bands while adding a built-in precision forward power detector. This device, accurate to less than ±0.5 dB, allows a greater total radiated power (TRP) while meeting the Food and Drug Administration’s specific absorption rate (SAR) concerns.
All mobile devices experience real-world antenna interference, meaning that the antenna can be detuned when the handset is held to the face or when it’s near other devices. The result is a higher voltage standing wave ratio (VSWR) that goes on the reduced power output. While most PAs today use a power detector, they don’t consider any mismatch error that isn’t accounted for.
The BST35’s power detector covers any mismatch loss or directivity error resulting in a higher possible handset TRP. The feedback to the power control circuits lets the BST35 PAs deliver up to a 2-dB improvement in output power compared to GaAs amps. This boosts signal range, reduces the number of dropped calls, and results in a higher 3G data rate.
Samples of the BST34 and BST35 will be available in the second quarter of 2011 with full production coming in the second and third quarters.