Got any suggestions as to which devices meet your criteria ?
73, Dick, W1KSZ
On Tue, Oct 21, 2014 at 12:26 PM, Manfred Mornhinweg <manfred@ludens.cl>
wrote:
> John, and all,
>
> There is another group online on AM phone that does a lot of class E
>> transmitter development, and they too are using plastic switchmode
>> devices in
>> new ways.
>>
>
> Using class E (or D, or F) on AM is easier than on SSB, because AM isn't
> sensitive to phase changes, as long as the whole AM signal shifts phase
> together. SSB instead is very sensitive to this. So AM can tolerate some of
> the AM-to-PM conversion that typically happens in MOSFETs, while with SSB
> it has to be carefully handled, to reduce it to a level low enough that
> doesn't cause trouble.
>
> I have been in contact with some experimenters who don't realize that
> getting very good envelope linearity is NOT good enough for SSB - and that
> in addition to it, good phase linearity is needed.
>
> So, for a truly linear, multimode amplifier we can only partially apply
> the technologies used for AM.
>
> Regarding plastic switchmode FETs, I'm placing all my bets on them,
> because of the huge price advantage. But it's good to know how to do it!
> Many experimenters start the wrong way, by choosing the biggest FETs they
> can find. And there are some switchmode FETs that can handle 600V, 50A, and
> 600W, while costing just 3 bucks! The problem with these is that they are
> too slow. They might work well on 160m, poorly on 80m, and be unacceptable
> on any higher band.
>
> So, my experimenter friends, here comes a little bit of knowledge, which
> _is_ important for using cheap plastic switchmode FETs at RF:
>
> - You must look for _small_, low power FETs, and use enough of them in
> parallel, to get the power you want. That's the only way to use them
> throughout the HF range.
>
> - These should be high voltage, low current FETs.
>
> - Select those that have low enough capacitances, specially the Miller
> capacitance needs to be low.
>
> - Select those that have low enough gate resistance (not always specified,
> sometimes needs to be tested).
>
> - Select only those that have a fast enough dV/dt rating.
>
> - RdsON is uncritical. Generally the ones that have worse RdsON are the
> best ones for RF.
>
> - Transconductance should be low, as far as possible, for linear modes.
>
> The things that limit high frequency response of such a FET are:
>
> - Source lead inductance. To minimize it, use FETs in the smaller plastic
> encapsulations, and connect them extremely close to the body. Using many
> FETs in parallel means having many source leads in parallel, reducing total
> source inductance. A practical rule is that to operate up to 30MHz, you
> shouldn't have much more than 1 ampere of RF current in each TO-220 source
> lead. And that requires a high voltage supply, to get lots of power from a
> reasonable number of FETs. Higher than 50V, in any case.
>
> - Capacitances and gate resistance. The gate-source capacitance must be
> pulled up and down at RF rate, and worse than that, the Miller capacitance
> has to be pulled up and down by the whole drain swing, through the gate.
> This requires a lot of gate current. FET gates are _not_ high impedance, at
> RF! And the gates have an internal resistance, that forms a low pass filter
> together with the capacitances. Don't try to operate a broadband amplifier
> beyond this cutoff frequency! A resonant, narrow band amplifer can work a
> bit above that cutoff frequency, but gate drive becomes hard and lossy.
>
> - FET slew rate (dV/dT). Each FET has a limit as how fast it can slew. It
> will limit the slew rate, by turning partially on even when the gate is at
> low voltage, through several mechanisms. At some high enough frequency,
> instead of a sine wave on the drain you start getting a triangle wave, and
> the efficiency drops. This is a big limitation, and most switchmode FETs
> have trouble getting to 30MHz, due to this. Sometimes it's a good idea to
> run a 600V FET at 100V, to get one that is fast enough to slew through the
> whole swing withing the allowed time at 30MHz.
>
> I don't yet fully understand myself these FET slew rate limitations, so I
> would be grateful for any insight that exceeds what can be found in data
> sheets and application notes on the web.
>
> - RF circuits must be designed to never apply negative drain voltage,
> because that biases the FET's internal diode on, and those diodes are
> always too slow to turn back off in time, at RF rates.
>
> -
>
> There you have it. Now start experimenting! ;-)
>
> Manfred
>
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> http://ludens.cl
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