SiGe Challenges GaAs Handset Dominance

There was a time when cellular-handset vendors wouldn't have thought twice about using GaAs power amplifiers (PAs) in their designs. Now that silicon-germanium (SiGe) technology has achieved the performance levels of GaAs-based amplifiers, however, those days may be gone. One of the developers of such SiGe-based devices is the aptly named SiGe Semiconductor. This fabless company recently announced the availability of several silicon-based CDMA PAs: the SE5103L, SE5106L, and SE5107L. These power amplifiers claim to deliver comparable performance to GaAs at a lower cost and with tighter integration (SEE FIGURE).

The company specializes in the use of SiGe BiCMOS process technology. With this technology, it claims to bring distinct advantages to the front-end design portion of mobile devices, such as cellular phones. Bipolar technology enables a high level of device performance. The CMOS process, on the other hand, allows for very tight levels of functional integration. With regard to PA design, this process results in high performance. Most importantly, that high performance level can be tightly controlled over the device's entire power range rather than just at peak power.

Silicon-based processes have certain advantages over their GaAs-based counterparts. Chief among these strengths is the integration of different device types on the silicon substrate. For example, SiGe Semiconductor's new line of CDMA PAs incorporates many of the "large-signal" components that are typically found on the baseband side of a handset.

Those large signals result from the voltage swings, which are considered "big" by the digital-baseband side of the radio system. Such voltage swings are needed to drive the output signal into the handset PA prior to reaching the antenna. By integrating these signal-amplifying components on the front-end side of the radio, the digital-baseband portion can continue to shrink in size. This shrinkage results in power savings, a smaller device footprint, and decreased bill of materials.

All three of the company's recently announced CDMA amplifiers are aimed at 900-MHz handset applications. This segment is considered the most predominant application in the CDMA world. More specifically, these products are two-stage CDMA amplifiers for the 824-to-849-MHz band. Each has an output power of 28 dBm at a 50-dB adjacent-channel power ratio (ACPR) and 41% power-added efficiency.

In other words, all three power amplifiers provide a peak power of 28 dBm. At that level, they achieve an impressive efficiency of 41%. After all, efficiency relates to the amplifier's consumption of battery current. Equally important is the linearity, which is a measure of the amount of noise that is present in the output signal. The linearity also indicates the level of interference that's presented to an adjacent spectrum channel.

At 800 kHz away from the transmitting signal, FCC regulations state that the linearity must be 43 dBc below the level of transmission. SiGe Semicon-ductor's line of CDMA PAs provides a linearity of 50 dBc at this point. It more than meets the FCC requirements. Impressively, this product line also boasts the same performance level as today's GaAs amplifiers.

Another standout feature of these CDMA PAs is their inclusion of bias control (i.e., the amount of current that's consumed by the amplifier). The SE5103 includes digital bias control. In contrast, the SE5106 and SE5107 provide digital and analog bias control, respectively. Bias control is a vital power-amplifier feature. Typically, PAs consume 50% of a handset battery's total available power. This consumption is directly related to the talk time that's permitted on the handset.

The digital interface is a simpler interface. At 16 dBm, the bias control switches from a high-current setting to a low-current mode. The analog control approach is more complex to implement. Here, one must continuously adjust the amplifier's quiescent-current consumption. Less power is needed, for example, as a person gets closer to a base station.

Power detection is another function that's integrated in the company's CDMA amplifiers. This function is needed to ensure that each handset is in compliance with FCC power regulations. Too much power or noise from the amplifier can cause interference with adjacent spectrum channels. Incorporating a power-detection function on the chip die also helps to lower the overall bill of materials (BOM).

All three PAs come in a QFN package. The SE5103 sells for $0.80 apiece in unit quantities of 100,000. The SE5106 and SE5107 cost just $0.85 each per 100,000 units. All of these power amplifiers are available now.