Dear Dick, LF group,
Not too sure about linear amplifiers, but FETs are certainly better
for the switching - type LF PAs that are currently popular. The
reason for this is similar to those given for the use of MOSFETs in
switching power supplies, which have much in common. These are:
-Switching speed an order of magnitude greater, giving lower
switching losses
-The base current drive waveform must be carefully controlled for
optimum performance from bipolars, while MOSFETs only require
an "on-off" square wave from a low impedance source.
-MOSFETs do not suffer from "secondary breakdown"
phenomena, which means that the transient voltages and currents
in a bipolar PA need to be more carefully controlled to prevent
failure due to the formation of hot spots within the transistor. This
makes MOSFETs more robust against spikey switching waveforms.
The situation seems more complicated with "linear" amplifier
designs. Significant problems arise because a linear amplifier
inherently dissipates power, wheras most modern power
MOSFETs are designed to operate as high efficiency switches
with low power dissipation. This results in devices with high current
and voltage ratings being produced in small packages, which are
unable to dissipate the heat produced in a linear application, and
so must be operated at much below their ratings. This in turn
produces problems with biasing - a much-vaunted advantage of
MOSFETs is the negative temperature coefficient of the drain
current vs. gate bias voltage characteristic; the current falls as the
device gets hotter, so supposedly ensuring stable biasing, easy
parallel operation etc, etc. In fact, this only applies when the
device is operating relatively close to it's maximum current; when
used at lower currents, as you are forced to do by the power
dissipation issue, the tempco is in fact positive, and thermal
runaway can occur as with bipolars. The Mosfets designed for
audio PAs are much better in this respect, but cost more, and are
only available in relatively low ratings.
Also, there seems to be a dearth of reasonably high frequency,
high power, low cost bipolar devices, whilst MOSFETs get cheaper
all the time, and are inherently fast enough without needing to go to
expensive, specialised RF devices
I think the main problem with MOSFET PAs at the moment is that
not all design issues have been addressed properly yet. For
example, the transformer push-pull linear and class D designs are
prone to produce big voltage spikes, which mean death for any PA
device. The Decca circuit and derrivatives demonstrate that robust
and reliable designs can be achieved, however.
Cheers, Jim Moritz
73 de M0BMU
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