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problems with a class B amplifier (high voltage)

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OM222O:
Hi all, I made a thread previously regarding high voltage safety and design considerations here:
https://www.eevblog.com/forum/projects/high-voltage-safety-and-design-considerations/

all in all, I relieved a lot of good feedback regarding the design and since there were no major issues, I decided to stick with the class B output amplifier. However finding high voltage BJTs was very difficult and only 1 or 2 parts came up which were any good for the job! I tried using fets which at first seemed like a great idea since I could find tons of them for high voltage applications with suitable SOA, etc. but after running the simulations, I noticed the resistor I used to eliminate crossover distortion (R5) , was actually making the FETs useless since it was too low of a value  :-DD I bumped it to 1k since the 25mA target goal is right on the limit of what the op amp can do, which is why I need the boosting stage to begin with. now the fets do the job but the gate-source voltage is outside the rating of the fets (picture attached) :palm: In hindsight that's obvious since 10v (max Vgs) at 25mA is about 400\$\Omega\$



now to the main questions:
1)the gates and sources are shared, so I can't simply use a zener since it clamps to 0.3V on one half of the signal which is no good! maybe 2 zeners back to back can do the trick? I'm not sure if they are fast enough though (<1mS reaction time)

2)Is there another way to reduce the crossover distortion without having R5? (I'm not sure how I can bias a FET to make a class AB amplifier  :palm:)

3)Maybe other output element types would be more suitable? perhaps I should look into high voltage IGBTs?

4)Any other suggestions about a better / simpler approach (not about using lower voltages or a H bridge with single supply approach for driving the inductor , just about solutions to the amplifier problem please! I'd like to keep this as close to the attached schematic as possible since I have designed it and it's important for my final submission since I have to explain how and why things work :P)

strawberry:
BJT MOSFET sziklay compound and biasing will be as regular BJT Vbe=0.65V

OM222O:
can you please provide a schematic or more details? as I mentioned before, it's almost impossible to find a high voltage BJT, otherwise, I would have used them for the output stage to begin with (although I was looking for ones with enough DC SOA and TO-220package for heat sinking, so maybe they're not that rare!). this would also help with the crossover distortion and allow me to remove R5, but this doesn't fix the Vgs issue (in fact it makes it worse since op amp will shoot to the positive or negative rail when source is at almost 0 and Vgs becomes 100 volts).

Zero999:
Why create another thread? It's less confusing if you keep it all in the same thread.

The ADHV4702-1 can almost do it. Add a couple of pass BJTs, which only need to pass 5mA or so.

MagicSmoker:

--- Quote from: OM222O on July 09, 2019, 06:44:31 pm ---...
However finding high voltage BJTs was very difficult and only 1 or 2 parts came up which were any good for the job...

--- End quote ---

From what I recall of your other thread - which was a real dog's breakfast, to use the old British colloquialism - you need to drive a highly inductive actuator with up to +/-100V at 25mA, right? And you couldn't find appropriate complementary BJTs with the necessary >200V or so Vceo rating to fit the bill?

This defies belief because there should be hundreds of pairs suitable for the job: matched NPN/PNP pairs with moderate voltage and current rating are commonly used in audio amplifier driver and output stages, after all.

Here's the start of a Farnell UK search for single BJTs that are in stock; you can further whittle down the list by Vceo rating, package, Ic rating, etc.:

https://uk.farnell.com/w/c/semiconductors-discretes/transistors/bipolar-transistors/single-bipolar-junction-transistors-bjt?range=inc-in-stock

I suggest searching for PNP first then looking at the datasheet to see if there is a recommended complementary NPN part. Or you can simply infer the likelihood of a pair being complementary from their specs, as happens to be the case for the first two results in the above search (2SA1943 and 2SC5200; NB - 2SA parts are always PNP while 2SC parts are always NPN).

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