Electronics > RF, Microwave, Ham Radio

Looking for advice to create/implement "grid driver" for vacuum tube RF PA

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--- Quote from: wb0gaz on April 13, 2024, 12:18:04 pm ---On use of a transformer (but not necessarily because of using a transformer) - I now recall a tube operating in class C is biased beyond cutoff, then grid driven hard enough to overcome the prevailing bias. Is there any benefit in operating a tube in class E mode (like a FET would be in RF generator), or does that even make sense with a tube?

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Well, you'll need a damper diode at least.  Note that class E produces a flattened waveform because the amplifier device is switching, and in particular, it switches on for negative current too -- think FET body diode.  You also need screen current limiting (preferably instantaneous, but that's nowhere near feasible at this frequency so average it must be), or use triodes only (at expense to saturation voltage, and, much more drive power required).  Otherwise, you need to operate in such regime where plate current is largely positive, which greatly limits load impedance range.

Sweep circuits themselves were class E, so it's not impossible.  That's just a very constrained application: no load variance to speak of, so it can be tuned just right.

Class E offers somewhat better efficiency, but you'll find heater power is always a significant fraction of the total and a more traditional class C amplifier (maybe driven into saturation/compression to some degree) will probably be more flexible, while being tolerant of load variation, and giving acceptable efficiency.

Obviously, these all need harmonic filtering at the output, and a tuned load is best.  A tuned load also provides impedance matching, so you can use a pi network for example, no transformer required.

--- Quote from: wb0gaz on April 13, 2024, 03:04:20 pm ---Current limiting on plate ... yes good catch ... high-side current sensor in plate supply would be needed but never seen one (as an IC) with ~1kv withstand voltage. If I use resistive voltage dividers (2) to step down to a common IC sensor inputs (2), resistor precision and thermal stability probably renders impractical as any resistance difference in the two paths would create large difference at the current sensor inputs.

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Easy, use one of the transconductance type ICs and cascode it:

Since there's a bias divider too, just chain them down to handle arbitrary voltage ratings.  P-ch aren't available in as diverse ratings as N, so it could take a few stages, but it's always doable.  Make sure Vgs is constrained on each one; probably the divider is backed with capacitors so AC (startup and short circuit conditions, ripple) divides evenly as well, and use G-S zeners to ensure drain capacitance doesn't mess up the next one down, etc.

Also, since your power supply is probably custom, might as well use a single-winding type and ground-return sense it.  This is trivial whether iron-core or SMPS.  (Not trivial if you're using a boost or something like that, but there's not much you'd want to boost from, let alone in a single stage, to that kind of voltage. And isolation is almost certainly required, so might as well make it a flyback or resonant supply.)


Thanks, Tim --- now you've provided ample homework assignments so I'm going to cease asking questions for a little while until I catch up on the info you've kindly provided!!!



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