Hitachi Semiconductor (America) is launching a new line of high-power amplifiers (HPAs), the first such commercial products built using the MOSFET process for CDMA cellphones.
Most CDMA designs today require an isolator between the high-power amplifier and the antenna. The isolator helps terminate antenna reflections, thus protecting the amplifier and decreasing intermodulation distortion. Next-generation designs built with MOSFET HPA modules need no isolator because the amplifier achieves excellent stability under high voltage-standing-wave-ratio (VSWR) conditions. Eliminating the isolator improves the signal budget by approximately 0.5 dB, produces a 5% savings in the current budget, and yields a 10% savings in power. It also saves board space and reduces the cost of the handset.
MOSFET devices have other important advantages, too. They are rugged, exhibiting no degradation when Vdd=5 V, and withstanding a 15:1 mismatch with no damage. They have very good quality and reliability characteristics. Significantly, MOSFETs have a negative temperature coefficient, so the power amplifiers don't experience thermal runaway, which can cause device destruction.
To obtain long talk times on batteries, the handset's efficiency at low output power levels is much more important than its efficiency at peak output power. A system design that uses a MOSFET amplifier can benefit from this fact. The HPA operates at maximum efficiency when used with a Vdd control circuit that leverages the silicon technology's superior low-voltage operating characteristics. Specifically, the efficiency obtained with a MOSFET amplifier and a Vdd control system is 2.2 to 3.4 times better than it is without a control system'"
Posted to the site on 4th June 2002
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