In my post earlier about the LDMOS transistors, it came garbled. Sorry,
here is a better explanation:
These are really taking the place in new RF power designs for UHF, that
used to be all bipolar. Yes, MOSFET's didn't make it to UHF and L band very
well. 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 is their lack of the standard beryllia or alumina layer on
the bottom to isolate the drain, needed in MOSFET process like the MRF150,
174 etc. It created shunt capacitance to ground. The LDMOS don't have the
drain underneath, so they can have directly grounded source in the
substrate, and higher frequency power gain. Plus, by eliminating the
ceramic, they have low thermal impedance to the flange, and they are
cheaper. The third order IMD appears to be similar to BJT technology of
similar levels. But the higher order IMD products generated are lower for
LDMOS. This explains interest in their use at UHF in new TV transmitters
(HDTV, Harris Corp) and cellular systems for high linearity multi-tone
operation. Also, they are now moving into the 1900 MHz wireless bands.
There are LDMOS devices good for 80 watts from Motorola now. They have
devices covering frequencies from 400-900 Mhz with a range of new parts.
Have hams written any articles about using LDMOS at 30 cm? QEX?
Interestingly, Hitachi made lateral MOS power transistors as far back as
the seventies, which were excellent for audio power. Hafler (kit) DH-series
power amplifiers on my stereo use multiple 2SJ49 P-channel and 2SK134
N-channel devices, from that vintage, to produce up to 250 watts per
channel. It hasn't died yet. But they were slow devices, only recent high
frequency processes have produced the LDMOS that is now taking place of BJT
at UHF power levels. While we don't expect to see devices as monstrous as
the MRF154 series of HOGFETs, LDMOS has some room to move towards the GaAs
range of frequencies.
Westinghouse (now Northrup-Grumman) in Baltimore, MD, has made silicon
carbide junction transistors that can operate around 200 deg C junction T
(i think that's correct). Now that makes transistors compete with
thermionics (tubes) at least with respect to outer temperature.
John
K5PRO
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