Class AB common source mosfet power amplifier

That's the topology I use in my NMR gradient amps, which range from

+-12 volts at 3 amps up to +-180 at 100 amps. I've posted pics of the big fet/heatsink assembly a couple of times.

I use an opamp per fet (up to 32 of them!) to close a loop on the voltage drop across the source resistors. That gives very tight control of each fet's current and lets me use small sense resistors.

The driver has to intelligently split positive and negative drives and maintain idle currents, and shift the drives up to the power rails. That's not trivial to do right.

Note that this config is like an ldo regulator: it has a high open-loop output impedance, so loop dynamics is a lot more sensitive to loads than a more conventional source-output amp.

John

This topology can be problematic because of the wild variation of the gain depending on the load impedance and current. I assume you are running in the class AB, right? Nevertheless it can be worked out however you may have to add the heavy correction capacitors between drains and gates (and maybe some nonlinear elements too it the goal is the high performance).

But why this topology? It is unlikely that you need R-R operation at

+/-80V. Why not a trivial pnp-npn emitter follower?

BTW, if you can drop the rail voltage to somewhat +/-50V, you can probably use an audio IC like TDA7294.

Vladimir Vassilevsky DSP and Mixed Signal Design Consultant

Right

OK

I don't *need* rail to rail. As I said, it is mainly so I can bolt the devices to the same heatsink without insulation which would reduce the heat transfer. Power mosfets have the drain connected to the tab, so this implies the topology above.

Not for this - the final thing will have 8 mosfets paralleled for each shown above, and will also use a copy driven in antiphase to get full bridge drive. 32 mosfets in all,~10kW pulsed, > 1kW rms!

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John Devereux

That was my first idea, then I got diverted by all the audio power amplifier designs out there! You even have the same number of mosfets (my final version will be a full bridge configuration).

That's where I started looking for other ideas...

Makes sense, thanks.

I do have a common drain version - that simulates OK - but common source would simplify construction.

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John Devereux

Wow! Who's playing lead guitar during that concert?

--
Regards, Joerg

http://www.analogconsultants.com/

[...]

Yes - should drive a decent size sub-woofer!

Its actually for a vibrator. (Not that sort! A voice-coil actuator for vibration testing).

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John Devereux

So the output stage operates as the current source and the dynamic response is completely determined by the parameters of the load. Assuming the unconditional stability, closing the global feedback loop by the output voltage would be problematic because there is only ~45 degrees of phase available for everything.

Yes, this is not very trivial as this schematic is prone to the different kinds of ill behavior.

Vladimir Vassilevsky DSP and Mixed Signal Design Consultant

One other note: mosfets spec'd for switching use have a bad habit of exploding when used in linear mode. We tested a lot of "300 watt" power fets, bolted to a solid copper block, and most of them couldn't dissipate 300 watts for 100 milliseconds when running at higher drain voltages. Imagine loud noises. It has to do with nonuniform heating or something. IXYS has some fets specially designed to not explode in this mode.

So test your candidate fets to destruction to make sure you have margin! Or go switchmode.

John

We're driving an NMR gradient coil with a current source, and feeding back on sensed current. So having a high-z output stage makes us nearly insensitive to load impedance. So use a low-z stage if you're feeding back on voltage, and a high-z stahe if you feed back on current.

Well, we did do it right.

John

[...]

The datasheets do show SOA for various duration pulses, which I will respect.

Will do.

That seemed even more complicated (and I do not need to save power).

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John Devereux

See ftp://ftp.rowland.org/pub/hill/ris-496-1.pdf

Actually, properly done, this can be a very stable amplifier configuration. Here's an example of such an amplifier I designed, with a little help from Tony Williams, R.I.P. It has been discussed here on s.e.d. several times, including when I was doing the design. It uses BJTs instead of MOSFETs (which I came to regret, SOA, ouch!), but you may find it useful to study anyway.

"John Devereux"

** For heaven's sake, forget stupid switching fets and use complementary LATERAL mosfets for linear apps.

The case is the source, they operate in parallel like a dream, are not prone to thermal runaway and inherently protect themselves if the chip goes overtemp.

Egs:

These are 16A, 250W dual chip devices.

Same numbers without the "D" are single chip versions.

...... Phil

They seem quite rare, likely expensive, and I was not too worried about distortion.

...but you make a good case!

Thanks Phil - I had not realised you could get anything with case connected to source. I will investigate these tomorrow.

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John Devereux

Wow, thanks, I will study it. Am I right in saying this is a variant of the circuit warned against here (fig 16)?

.

(Hey, it's like one of your "bad circuits" sections!)

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John Devereux

"John Devereux" "Phil Allison"

** Farnell have them.

...... Phil

Doggone, U6, with its power pins cascoded into the complementary HV transistors, is hauntingly familiar. Not to mention D3 and D4, except that I like to add LED's there, too.

John

LOL. So, I'm reading this thread, and the guy is asking if he can solve this problem where the transistors all have to be mounted on the same heatsink (or two). And then another guy chimes in with a solution that completely removes the problem of heat, and the guy shoots it down...

;-)

C'mon...switching ain't all that hard...hell, I did it with tubes!

Rak(on) ~= 100 ohms on that 38HE7. :)

Tim

-- Deep Fryer: A very philosophical monk. Website @

I found an old schematic I created in 1997. I quickly drew it in Ltspice..

351Kbyte

Don't ask about the output network.. I've completely forgotten why I did that and it's looks weird to me now..

D from BC

Don't be silly. First, in reality the configurations are totally different. Second, do the calculations and see the damaging effect in OUT1 of a too-small emitter resistor. My output stages are mirrors - do you see any mirrors in either OUT1 or OUT2 --- no I didn't think so. That's OK, I knew you were pulling my leg.

Go read the circuit discussions in s.e.d.