RF power amp that can withstand infinite VSWR 2

[Renovator1]

Hey all. I got roped into building an RF power amplifier for a physics professor friend and I wanted some second opinions on which way to go with the design. The amplifier needs to deliver up to 300Vpp and 100W across the frequency range of 0.1 to 1 MHz into a purely capacitive load (~200-1000pF), preferably without tuning...

Much as I'd like to, I don't believe I can get away with whipping up a quick-and-dirty push-pull Class B out of some spare SMPS parts. It seems like I've only got two choices here, but I'd love to hear otherwise:

1) Single-ended Class A (with either a choke or a cascode current source feeding the drain to at least get 25% eff.)

2) high-speed current feedback op-amp driving a FET voltage gain stage then a FET current gain stage (ala EDN's Design Ideas from April 26, 2001).

The distortion requirement isn't too critical, so that's a plus, at least. Any ideas or comments would be most welcome.

-Jeff

 

[Renovator1]

Well, my strong suit in electronics is high power smps design so, yes, much of RF is a bit foreign, but not nearly as strange and incomprehensible as, say, the digital world...

I am familiar with transmission line transformers (let's call them TLTs for short), though if anything qualifies as black magic in electronics they are it.

From what I understand, the "windings" in TLTs must have a characteristic impedance that is the geometric mean between the primary and secondary. So, all I need to do here is figure out the output impedance of the op-amp and the input impedance of the MOSFET then either select the twisted pair or coaxial cable with the right impedance... oh boy, that doesn't sound like a productive way to spend one's time. Probably best to just go with a conventional transformer.

Since I design a lot of switchers, I tend to have lots of ferrite cores laying about the place, and it was one of these that I intended to co-opt for this little project until I got all worried from reading how Class B push-pull power amplifiers hate infinite VSWR loads (hence, this thread). That said, I still haven't found a single definite explanation as to what exactly occurs at the transistor's terminals when an amplifier it is part of attempts to drive a purely reactive load*... And yes, I've googled quite a bit as well as read through all the books I have even remotely relevant to the subject.

My intuitive analysis is that at worst either the voltage or the current will double through the C-E or D-S, but I can't confirm this and so my biggest worry is that the figure is actually much higher.

 

[Warpspeed]

I am a switchmode man myself, working on an interesting multi kilowatt project right now in fact.

With a switching supply, the switch is either open or closed most of the time, and the very violent drama of actual switching is a fleeting effect, perhaps lasting only hundreds of nanoseconds each switch cycle. While total switching losses can still rise to some fearsome numbers, the massive dissipation regime of simultaneous high voltage and high current combined can hopefully be kept to fairly short duration each switch cycle.

The problem with class B is that while it is in conduction, it is never entirely on, or entirely off, but stuck permanently in the act of "switching". There will be many simultaneous volts and amps together across the switch to burn things up.

Any effect in the switch such as current crowding, or second breakdown will be pretty lethal to the output devices, so mosfets are probably about the only feasible choice for something like this.

What you are building is more like a constantly varying dissipative series regulator than a switching supply.