Briliant! Thanks Manfred
Le 02.10.2012 17:52, Manfred Mornhinweg a écrit :
Gene,
I am puzzled as to why the solid-state devices seem to be more linear
than tubes ("fire-FET's"?). As I understand it, FETs are square-law
devices that generate mostly low-order and second-order distortion
products, while tubes are 3/2 power-law devices that generate more
and higher order distortion products. Does anyone out there with a
better comprehension than I have of the physics in this matter have
an explanation?
I will try to elaborate on this, but instead of thinking in terms of
square law and the order of distortion products, I prefer in this case
thinking simply about the instant voltages and currents along the RF
waveform. Not that the waveform is important, as a low pass filter
will sinusoidalize it anyway, but for analysis purposes it's good to
think about the RF waveform, because any distorsions there translate
to distorsions of the envelope waveform, which is of course not
corrected by any filter you can install between a power device and the
antenna.
The base curves of output current (plate or drain) divided by control
voltage (grid or gate), which you could measure at low frequency, are
basically:
- For a triode: Severely nonlinear and very dependent on plate voltage
- For a tetrode: Quite linear except for a certain area of the curve,
and not very dependent on plate voltage, but highly dependent on
screen voltage
- For a pentode: Quite linear and quite independent on plate voltage,
but highly dependent on screen voltage
- For a MOSFET: Severely nonlinear but quite independent on drain voltage
From these characteristics it's clear how each device needs to be
used. With a pentode, often no further linearizing is required. You
just apply proper grid bias, a VERY STABLE screen voltage, some RF
drive, and you get a reasonably clean output that can be put on the
air directly. With a tetrode it's much the same, as long as the bias
and drive are selected such that the nonlinear part of the curve is
avoided.
With triodes and MOSFETs it's not that easy. These are simply too
nonlinear to be used without "ironing flat" their curves. This is done
with negative feedback, which lowers the gain but improves linearity.
With triodes, the time-proven method to add a lot of negative feedback
in a simple way is running them in grounded grid configuration. A very
low gain results, typically just 10-15dB, often even less, along with
an acceptable degree of linearity, but usually not a very great one.
RF power MOSFETs cannot be easily run in grounded gate, because of the
extremely low input impedance and poor power gain that results. So the
negative feedback has to be added in other ways: Source degeneration,
which is already built into MOSFETs intended for linear service;
Overall feedback from the drains to the gates, sometimes with
frequency compensation; and sometimes more complex methods are used,
such as inductive drain-source feedback (bootstrapping). These
techniques result in good gain and linearity.
Modern MOSFETs have extremely high power gain, and this allows adding
a lot of negative feedback and still having enough gain left over for
practical use. It is this large amount of negative feedback that ends
up giving well designed MOSFET amplifiers their high level of linearity.
It would be possible to make a very low distortion tube amplifier, by
using a pentode in grounded cathode configuration, which has good
linearity and very high power gain by itself, and then adding strong
negative feedback to cut down the gain to 12dB or so. But it seems
that nobody is doing this. Probably there would be stability problems,
because tubes have such a poor ratio of output capacitance to plate
load resistance, that there is no option but to use them with narrow
band, moderately high Q, tuned impedance matching circuits, to absorb
that capacitance. These circuits introduce enough phase shift to bring
an amplifier with strong negative feedback to the point that at some
frequencies the feedback might rotate into positive, and whops, you
have an oscillator!
MOSFETs instead have a much better ratio of output capacitance to
drain load resistance, allowing them to be used with broadband
matching transformers on HF, or very low Q tuned circuits, on VHF and
UHF. This makes them far more stable in feedback amps.
A particular linearity problem with MOSFETs that has to be addressed
is that their internal capacitances vary very much with voltage. They
are basically varicaps. Specially the output and reverse transfer
capacitances vary strongly over the RF cycle, causing distortion. The
higher the frequency, the worse this effect becomes. The designer of a
solid state amplifier must be aware of this issue, and handle it
properly. If he "thinks tubes" and assumes these capacitances are
constant, an amplifier with poor linearity will result. Negative
feedback helps a lot in reducing this form of distortion. Swamping
with external capacitance also helps some. But in many cases it can be
advisable to avoid driving the drains very close to the sources,
because that's when the problem becomes worst. So this requires
leaving some headroom between the power supply voltage and the RF
voltage at the drains, and this costs efficiency. That's why a low
distortion linear amp using MOSFETs often can't match the plate
efficiency of a pentode amp with similiar distortion performance. It
makes up for that by not needing any heater power.
Manfred
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