[Amps] Sad News for Eimac Users

[Angel Vilaseca]

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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