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Low cost 3 v RF power amplifiers for AMPS.

With all the talk about digital wireless communications it's hard to remember that the advanced mobile phone system (AMPS) is the largest selling cellular phone system in use today. AMPS is a basic 850 MHz narrowband FM, frequency division multiple access (FDMA) system that was first offered to the public in 1983. Currently, all of the advertised low cost cellular phones are part of the AMPS. These phones represent over 80 percent of the new phone market.

All dual-mode digital cellular phones must fall back to AMPS because digital cellular systems are not available throughout the US, whereas AMPS is available in all major and minor US population centers. When a caller attempts to make a digital call in most of the US using a dual-mode phone the phone switches from digital to AMPS operation. Even many of the new personal communication services (PCS) phones have a fall back to AMPS mode built in. Therefore, AMPS will be maintained as long as there is a cellular system in the US.

Although the benefits of low power 3 V systems have been promoted for the new cellular and portable phone systems, manufacturers must continue to build and sell the old standard AMPS equipment. For this reason, the focus of the new 0.5 [[micro]meter] GaAs technology, which had been concentrated on high performance digital systems, has recently been redirected toward developing a low cost, high efficiency GaAs powered FET for the high volume AMPS market.

As a result, two new low cost, high performance 3 V GaAs RF power amplifier products, the ceramic model KGF1607 and the plastic model KGF1637 GaAs power FETs have been developed. These new transistors are optimized for AMPS cellular phone applications, feature high power output of greater than 31.5 dBm and operate with high peak current capabilities of greater than 4.5 A at 850 MHz. The power efficiency for the ceramic model power FET is greater than 70 percent and in excess of 60 percent for the lower cost, plastic packaged model. This feature ensures longer battery life and, consequently, longer talk time and standby time for cellular phone users.

The ceramic GaAs power FET also features high gain at high current with the ultra-low impedance drive required for cellular and PCS applications. The unit is supplied in a high thermal- and RF-efficiency ceramic surface-mount package, as shown in Figure 1. The package is ideal for high efficiency, miniature packaging systems. The power FET is suitable for use in the final transmitter stage amplifier in portable voice and data communication devices using analog FM or FDMA.

The ceramic device has been designed for optimum external matching at a 3.4 V operating voltage. The FET operates at 850 GHz, [V.sub.DS] = 3.4 V and [I.sub.DSQ] = 300 mA, and typically produces 32 dBm power output with an input of 22 dBm. Figure 2 shows power output and power-added efficiency (PAE) vs. input power. Figure 3 shows the typical S-parameters at [V.sub.DS] = 3.4 V and [I.sub.DS] = 420 mA. The FET's maximum power dissipation is 5 W and the maximum channel temperature is 150 [degrees] C. The ceramic surface-mount package thermal impedance is 15 [degrees] C/W.

Both the plastic packaged and the ceramic models use the same transistor chip. The plastic packaged unit is supplied in an industry-standard SOT-89 plastic package, which allows for overall lower cost systems. The plastic packaged unit's maximum ratings and typical operating characteristics are the same as for the ceramic model with the exception the minimum drain efficiency (60 percent) and the package thermal resistance (20 [degrees] C/W).

Both devices are now in full production. Engineering and qualification samples are available immediately. Large quantity deliveries ([greater than]1000 pieces) can be scheduled within 14 weeks. Prices: $8.76 for 1000 pieces (ceramic); and $7.60 for 1000 pieces (plastic).

Oki Semiconductor, Sunnyvale, CA (800) 654-6388.
COPYRIGHT 1996 Horizon House Publications, Inc.
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Title Annotation:advanced mobile phone system
Publication:Microwave Journal
Date:Jan 1, 1996
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