Allison,
one more point: for the load always stay off the "knee" i.e. the
so called ohmic range. Above this point the MOSFET acts like a
voltage controlled (control voltage is the gate to source voltage)
current source.
Example: for VGS = 5 V stay awav from the range of VDS = 0 to
about 1 V. As easily be seen the "knee" gets worse for higher
drain currents.
A similar condition happens for the RD16HFF1, having a larger
ohmic region--? and in addition a limited maximum drain source
voltage. The IRF 510 can do better as it can be operated at higher
drain -source voltages.
The result: as the maximum drain -source voltage swing for the RF
signal is limited the maximum output power can be only achieved
through the output transformer and its transformation value.
There is a good explanation of this relationship in the
proceedings of the "UKW-Tagung Weinheim" a few years ago by the
author Prof. Dr Jochen Jirmann DB1NV.
? Henning Weddig
DK5LV?
Am 23.09.2018 um 17:21 schrieb
ajparent1/KB1GMX:
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Tim,
IMD can be caused by two things, saturation at high power and
cross-over distortion near idle conditions.
You missed one.? Anytime the output is not a uniform multiple of
the input.
Or what is know ans non linear behavior.
Good example the HFE of 3904 at 10ma is 100, at 100ma its 40.? So
is the?
waveform causes it to swing from 10 to 100ma the gain supplied is
varying.
The base current required to get 10ma and 100 ma is not ten times
higher
it is more like 25 times higher and the change is varying for
intermediate
currents.??
For power MOSfets this is pronounced at low gate voltages as first
we have
threshold where below that there is no conduction. as we go up
from that point
the curve is markedly curved then straightens and at some high
current bends
again.
For information look at the Ids VS Vds for Vgs curves.? From that
graph we can get a
load line that reflects input voltage.? Old tube hands likely the
curves looking like?
many power tubes.? ?It is on the page upper right corner.? Do not
let pulsed be a bother
as in some cases those curves represent power well above what the
device can do
sustained.? Needless to say its hard to run the drain to zero
volts and remain linear.?
It also indicates a Gm of about 1S? IE 1Vgs change causes a 1A
Drain current change.
Allison
