AoE x-Chapters, using power MOSFETs as linear transistors

Why does it need a current source?

--

John Larkin         Highland Technology, Inc 
picosecond timing   precision measurement  

jlarkin att highlandtechnology dott com 
http://www.highlandtechnology.com

They don't wanna pay for fast, low voltage op amps or logic and such. The circuit is gonna be in a semi-disposable product so cost is not no-object :(

The idea i'm messing with is bring the frequency down to something more sensible from the self-resonant of a couple hundred MHz with some low-loss inductance in series. Then use some negative resistance to compensate for the bad Q down there. Can all be done with FETs and BJTs, at low voltages.

I'm not under any kind of NDA so I'll post my result if I don't fail and am sent to work camp in forest.

Hey, if you guys don't cover it, who will? If it were easy, nobody would look to you for hints. :)

Bob Cordell brings up a few points in his book on audio power amps that you could repeat, or perhaps reference in the bibliography. E.g., source resistors alone may not be enough to make up for poorly- matched FETs, but FETs from the same manufacturing lot are usually easy to match. Lateral MOS behaves (very) differently than VMOS due to its tempco, but lateral parts are expensive and hard to come by. Gate resistors are more important than usual when paralleling FETs due to their tendency to oscillate, but there goes your phase response.

He suggests Zobel networks between the gate and drain terminals as an alternative. He also references a note on stability of paralleled FETs by Ed Oxner that I can't find.

-- john, KE5FX

** However, Larkin failed to do the *bleeding obvious thing* and use MOSFETs characterised for liner operation - ie Lateral types as made by Hitachi and others. Another example of his massive and insane prejudice against any and all things that even smack of "audio".

Wot a cretin.

.... Phil

Imagine anybody wanting to go faster than 20 kHz. The very _idea_!

Cheers

Phil Hobbs

You might want to experiment with piglet (Eric Wagner)'s idea of adapting a foldback current-limit circuit into a negative- resistance current source. We wrote it up for section 2x.12, in the AoE x-Chapters book, see draft copy on DropBox.

I played with SPICE and added a 100uA 1.5-volt low-voltage version. It might be fast enough, if you scale the resistors to run it at 2mA or even 10mA. You might do a pnp version.

--
 Thanks, 
    - Win

This cretin designs and sells a lot of electronics, and has fun doing it. I guess I'm dumb and happy.

Can you suggest some part numbers for high power n and p channel lateral fets? Something in the 200 volt, 400 watt ballpark?

I'm not fundamentally prejudiced against audio, but most audio design is really bad and mostly subjective. And boring.

We're using a very nice TI class D "audio" amp in a couple of designs now.

--

John Larkin         Highland Technology, Inc 
picosecond timing   precision measurement  

jlarkin att highlandtechnology dott com 
http://www.highlandtechnology.com

On 7/26/19 11:29 AM, Winfield Hill wrote:

Hi Win, thank you, very nice!

If I'm allowed a more luxurious ~2.5 volts I came up with this modified Lambda diode circuit. It uses a JFET and pnp for the negative resistance, plus current mirror for biasing up the diode (it won't start into a lossy inductor on its own) and some Colpitts-like feedback.

The 10u has 35 ohms of series resistance to represent a somewhat crappy L.

I am concerned about Vgs spread on real JFETs, though, and whether a circuit of this topology will actually oscillate in the real world consistently. It can be fiddly to start in LTSpice. An AC 1 current source into the node shows a current gain of about 20dB into the inductor

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Instead of "With a little prodding" you might substitute, "For a few haycorns". :^)

GH

What oscillation frequency do you envision? From 200 MHz down to low MHz? Will your circuit work up that high?, I see 10k resistors, etc.?

BTW, your added inductor esr doesn't show on the schematic (it's better to add parts to show these things explicitly).

--
 Thanks, 
    - Win

Exicon has the ECW20N20/ECW20P20, which are 200-volt 250-watt parts in TO-264. ECF20N20/ECF20P20 seem to be the same dice, in TO-3. These types look as if they're two 125-watt dice in a single package.

Their ProFusion arm sells 'em directly.

As far as I can tell, Exicon is the only company making the lateral-MOSFET dice these days; there may be multiple companies packaging them.

1-2MHz is fine. The actual lossy inductor will be about 0.5uH to 4 uH, I intend to "swamp" it with a somewhat larger low-loss inductor so it starts reliably despite the variation. And then the idea is that the oscillation frequency will deviate a bit in response to the smaller one being stretched/compressed. a few 10s of khz deviation is fine

Got it

So... I would say though, that I am not particular enamoured of the Sandler technique of adding subthreshold modelling. In principle, its a little bit precarious. It has potential for convergence issues. Its correcting for a zero current discontinuity by adding a feedback loop.

It would be preferable to simply add a controlled current using a B source in parallel to the mosfet. One would then use an exp() function directly rather than using a diode. One could then wrap that with a tanh() to terminate its effect at larger currents.

A case in point, is... at my day job... we had an updated kit from X-Fab for the process that were using just this last couple of weeks. All my existing spice tests failed due to convergence issues. I traced it down to some modelling problems in the natural mosfets. They had newly introduced B Sources into the subckts, for reasons unknown. They were flagging up convergence issues all over the place. A bare mosfet worked ok, but once one put them into a real circuit, it fell apart.

So, if a mainstream fab can release a model kit that fails...

It might be worth mentioning in that section that LTSpice does model the non-linear gd capacitance and has a subthreshold extension as well. Being that millions of people use LTSpice, its getting to be a tad moot that Spice3 don't support certain features. Additionally, there are other spices that also do, with even other enhancements possible worth a mention... :-)

-- Kevin Aylward

- SuperSpice

On 7/26/19 2:32 PM, Winfield Hill wrote:

Here is a time domain of the fleshed-out idea oscillating OK in the sim. A few hundred cycles of the resonant frequency injected into the tank gets it moving, in the simulation.

There's a gyrator in series with the lossy-L to bring the "self-resonant frequency" down.

The desired result is that when the inductance of the lossy-L is changed (by compressing or stretching the physical coil) the frequency deviates a bit.

That's about it.

Anyway it looks good here but I'm concerned if it will be annoying to get operating reliably on a breadboard in particular the part-to-part tolerance on the JFET, any suggestions would be appreciated on that front

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SYMATTR SpiceLine Rser=0 Cpar=0 SYMBOL res 624 544 R0 SYMATTR InstName R8 SYMATTR Value 10k SYMBOL voltage -288 512 R0 WINDOW 123 0 0 Left 0 WINDOW 39 0 0 Left 0 SYMATTR InstName V2 SYMATTR Value SINE(0 1 1Meg 0 0 0 1000) SYMBOL res -80 400 R90 WINDOW 0 0 56 VBottom 2 WINDOW 3 32 56 VTop 2 SYMATTR InstName R9 SYMATTR Value 10k SYMBOL res 1184 1520 R0 SYMATTR InstName R10 SYMATTR Value 50 SYMBOL res 1328 1440 R0 SYMATTR InstName R11 SYMATTR Value 100k SYMBOL cap 1472 1456 R0 SYMATTR InstName C5 SYMATTR Value 10p TEXT -352 48 Left 2 !.tran 100m TEXT 1592 1488 Left 5 ;Lossy L TEXT -632 296 Left 5 ;Kickstarter (sim only)

On 7/26/19 3:30 PM, bitrex wrote:

There was an error with that .asc here is the corrected version:

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SYMATTR SpiceLine Rser=0 Cpar=0 SYMBOL res 624 544 R0 SYMATTR InstName R8 SYMATTR Value 10k SYMBOL voltage -288 512 R0 WINDOW 123 0 0 Left 0 WINDOW 39 0 0 Left 0 SYMATTR InstName V2 SYMATTR Value SINE(0 1 1Meg 0 0 0 1000) SYMBOL res -80 400 R90 WINDOW 0 0 56 VBottom 2 WINDOW 3 32 56 VTop 2 SYMATTR InstName R9 SYMATTR Value 10k SYMBOL res 1184 1520 R0 SYMATTR InstName R10 SYMATTR Value 50 SYMBOL res 1328 1440 R0 SYMATTR InstName R11 SYMATTR Value 100k SYMBOL cap 1472 1456 R0 SYMATTR InstName C5 SYMATTR Value 10p TEXT -352 48 Left 2 !.tran 100m TEXT 1592 1488 Left 5 ;Lossy L TEXT -632 296 Left 5 ;Kickstarter (sim only)

No, it's not going to work right. That bias network is wrong.

On 7/26/19 11:29 AM, Winfield Hill wrote:

Apologies for the message/.asc spam earlier, but here is a working revision:

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SYMATTR SpiceLine Rser=0 Cpar=0 SYMBOL res 624 512 R0 SYMATTR InstName R8 SYMATTR Value 10k SYMBOL voltage -400 512 R0 WINDOW 123 0 0 Left 0 WINDOW 39 0 0 Left 0 SYMATTR InstName V2 SYMATTR Value SINE(0 1 1Meg 0 0 0 1000) SYMBOL res -192 400 R90 WINDOW 0 0 56 VBottom 2 WINDOW 3 32 56 VTop 2 SYMATTR InstName R9 SYMATTR Value 10k SYMBOL res 1184 1520 R0 SYMATTR InstName R10 SYMATTR Value 50 SYMBOL res 1328 1440 R0 SYMATTR InstName R11 SYMATTR Value 100k SYMBOL cap 1472 1456 R0 SYMATTR InstName C5 SYMATTR Value 10p SYMBOL npn 432 672 M0 SYMATTR InstName Q5 SYMATTR Value BC546B TEXT -464 48 Left 2 !.tran 10m TEXT 1592 1488 Left 5 ;Lossy L TEXT -744 296 Left 5 ;Kickstarter (sim only)

:) I get a mention in the great book - Yay!

piglet