A New RF 1 GHz L-DMOS product family
1 March 2000
Analogue, Mixed Signal, LSI
Agatronics
A new series of lateral diffused MOS field effect RF power transistors have been introduced by ST Microelectronics. They are assembled in standard RF flange, flangeless and surface mount ceramic packages. Specifically designed and processed for use in digital video broadcast and cellular base station power amplifiers for TDMA and CDMA, this family of L-DMOS products offers different power solutions from 6 W (SD57006) to 170 W (SD57170) and operating up to 1 GHz.
The latest generation of cellular base stations require RF power amplifiers with high peak envelope power (PEP). Until recently, the only way to achieve this kind of power was to use high power RF bipolar transistors which require the use of complex bias circuitry and class A operation in order to achieve the linearity (IMD) requirements. ST says the SD57xxx Series not only provides the required output power but also the right level of IMD under class AB operation. It also offers reliable operation under severe operating conditions, thanks to its 3 dB overdrive and 10:1 VSWR load mismatch capability. ST will soon introduce a new series of L-DMOS in 2 GHz technology to satisfy the emerging UMTS and existing PCN and PCS markets.
L-DMOS basic structure
The improved performance of RF L-DMOS are mostly related to the laterally diffused P+ sinker which connects to the source metal to the P+ substrate. As the substrate is directly soldered to the RF ground, the common lead inductance is minimised resulting in a higher common source RF gain. Furthermore, the elimination of the electrical isolator (typically the highly toxic BeO) improves the electrical and thermal performance of the package, besides being environmentally friendlier.
L-DMOS versus bipolar
L-DMOS transistors are today used successfully in digital cellular base station applications for high power amplification. L-DMOS have already proven their advantages versus bipolar transistors in terms of: higher power gain; input impedance more constant under varying drive levels; better IMD performances; easier to bias; gain control by varying the DC gate bias voltage; better thermal behaviour; and lower overall system cost.
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