[AMPS] Excess hfe and ft

John Lyles jtml@lanl.gov
Fri, 22 May 1998 17:35:48 -0600


Hi,

Jon  (KE9NA) is correct:

>Today the popular device is Laterally Diffused MOS FETS (LDMOS).  I
>really know very little about them since when I left the design world
>they were just becomming popular.

These are really taking the place in new RF power designs for UHF, that
used to be bipolar. We designed an amplifier 2 years ago using the MRF 899,
a bipolar for 150 W PEP at 805 MHz. The parts are made for cellular base
stations. Now Motorola, Ericsson, Thomson CSF, MA/COM PHI, are all making
LDMOS parts. The advantage of laterally diffused MOSFETS is that the
insulator layer on the bottom to isolate conventional power MOS drain,
which presented capacitance to ground. The LDMOS don't have the drain
underneath, so they can have directly grounded source. THis enhances their
use at UHF. TV transmitters and cellular systems for high linearity
multi-tone operation are working with LDMOS. Also, now moving into the 1.9
GHZ PCS bands I hear. There are LDMOS devices good for 80 watts from
Motorola now.

John
K5PRO



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