Gate resistor for lateral power MOSFET?

[Frank Miles]

I'm designing a power amplifier using some Exicon lateral
MOSFETs as the final common-source stage. These will be
used in various experimental systems. Each only needs
to run around +/-50V out, up to 4A, and driving sinusoidally
between 50 and 400kHz. The load can be approximated as a
lossy series LC circuit, driven near resonance with R
generally 10-20 ohms. There's an additional >200pF of
capacitance on each output source to ground due to the
capacitance to the heat sink.

These MOSFETs have huge capacitances:
N ch: Ciss 900pF, Coss 500pF
P ch: Ciss 1.8nF, Coss 850pF

MOSFET "lore" (particularly from audio power land) says that
you need fairly substantial series gate resistors to protect
these FETs. Unfortunately the dominant pole in the amplifier
is in the preceding voltage-gain stage, so I'm having some
HF stability issues when I use resistors of the recommended
value (>300 ohms). I'd like these amplifiers to tolerate
a variety of loads, not require tweaking for particular
systems.

QUESTION : are these resistors necessary? How can a proper
value be determined?

Thanks for any insights!
-F

[John Larkin]

Mosfets sometimes like to RF oscillate, especially when used in linear
applications, especially with the source not grounded. A gate
resistor, 10s of ohms, will usually kill that. I know of no other
"protection" function of a resistor, but maybe there could be a gate
zener or something that needs protection.

I'd add a resistor to the layout and stuff it with some low value that
doesn't compromise your loop. If you see oscillation (which would be
10s or maybe 100s of MHz) you could increase the resistance or try a
ferrite bead or something.

Laterals have relatively low Gm, don't they? That might reduce the
tendency to oscillate.


--

John Larkin Highland Technology, Inc
picosecond timing precision measurement

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

[Frank Miles]

Thanks, John. I haven't observed any misbehavior that I would want
to fix with a gate resistor - just trying to prevent difficulties
in circumstances that I failed to foresee.

I like the ferrite idea (assuming that the mechanism is the unintended
oscillator). I'll look for a part that's good and lossy above the
band that the loop has to operate over, lower resistance below.

-F

[John Larkin]

On Wed, 28 Sep 2016 22:22:08 -0000 (UTC), Frank Miles

Wild guess, use a bead that's 30 ohms at 100 MHz.

[Winfield Hill]

Frank Miles wrote...

>
> MOSFET "lore" (particularly from audio power land)
> says that you need fairly substantial series gate
> resistors to protect these FETs.

If the Vgs drive voltages go substantially beyond
the listed gate-breakdown voltages, and cause a
serious leakage event, series resistors won't help.

The only reason for a series resistor, as others
have stated, is to dampen RF oscillation. I'd not
expect a problem, given the low gm of lateral parts,
but I've seen ferrite beads on the gates of these
parts in Hafler's MOSFET amplifiers. Nonetheless,
they also placed 470-ohm resistors in series with
the gates, according to manual schematics. But I
do not believe such large values are necessary.


--
Thanks,
- Win

[Phil Allison]

cassiope wrote:
>
> I'm designing a power amplifier using some Exicon lateral
> MOSFETs as the final common-source stage.
>

** You mean connected as source followers ?

> These will be
> used in various experimental systems. Each only needs
> to run around +/-50V out, up to 4A, and driving sinusoidally
> between 50 and 400kHz. The load can be approximated as a
> lossy series LC circuit, driven near resonance with R
> generally 10-20 ohms. There's an additional >200pF of
> capacitance on each output source to ground due to the
> capacitance to the heat sink.
>
> These MOSFETs have huge capacitances:
> N ch: Ciss 900pF, Coss 500pF
> P ch: Ciss 1.8nF, Coss 850pF
>
> MOSFET "lore" (particularly from audio power land) says that
> you need fairly substantial series gate resistors to protect
> these FETs. Unfortunately the dominant pole in the amplifier
> is in the preceding voltage-gain stage, so I'm having some
> HF stability issues when I use resistors of the recommended
> value (>300 ohms). I'd like these amplifiers to tolerate
> a variety of loads, not require tweaking for particular
> systems.
>
> QUESTION : are these resistors necessary? How can a proper
> value be determined?
>

** Gate resistors of about 200ohms are necessary, but not sufficient to ensure freedom from parasitic oscillations. As you have realised, lateral mosfets have lots of power gain up into the VHF range and a little stray L or C will set easily them off. Wiring layout should be done with this in mind, keeping all tracks short as possible. You can avoid heatsink capacitance by isolating it from ground and connecting all the mosfet sources directly so it floats at output voltage.

An output stabilising network is essential, consisting of a inductor of about 5uH with RC Zobels on both ends to supply ground.

The operating bias current can play a role in HF stability, but try to keep it close to 100mA per device since this corresponds with the inflexion point in the tempco curve - where it goes from positive to negative.


.... Phil

[Winfield Hill]

Phil Allison wrote...

>
> As you have realised, lateral mosfets have lots of power
> gain up into the VHF range and a little stray L or C
> will set easily them off.

Ordinary VMOS power MOSFETs have *much* higher gm in the
linear region than lateral MOSFETs, and techniques that
work for them should be conservative for the laterals.


--
Thanks,
- Win

[Phil Allison]

Winfield Hill wrote:

>
>
> The only reason for a series resistor, as others
> have stated, is to dampen RF oscillation. I'd not
> expect a problem, given the low gm of lateral parts,
> but I've seen ferrite beads on the gates of these
> parts in Hafler's MOSFET amplifiers. Nonetheless,
> they also placed 470-ohm resistors in series with
> the gates, according to manual schematics. But I
> do not believe such large values are necessary.
>
>

** What you "believe" is hardly relevant to anyone but you.

I hope you agree changing the gate resistor value downwards lessens the stability margin, rather than improves it - so is not very likely to help the OP.

As someone who has spend a LOT of time puzzling over stability problems in power amplifiers using Hitachi and Semelab lateral mosfets, I can assure you none of them have much stability to spare.

Any of them WILL develop parasitic oscillations in the range of 2 to 10MHz, with resistive or even no load connected, when any of the zobel components become damaged. Supersonic oscillation is a common event in the cruel world of live music sound reinforcement and quickly destroys the 100nF film capacitors and 4.7 ohm resistors typically employed - often invisibly.

If the circuit uses 1 watt film resistors here, upgrading them to 5 watt WW types is very likely to induce parasitics - due to the inductance of such components. Adjusting the idle bias to a lower value does the same in some models - which can then be cured by adding another RC zobel from the common source point to supply ground.

Sometimes the oscillations are continuous at a low level while and other times they only appear superimposed on one half wave when a 4ohm load is driven to near full output.

BTW: to evaluate how prone a certain Perreaux mosfet model was to supersonic oscillation, I EXTERNALLY tried various low value capacitors from output to input and carefully advanced the volume. Full power oscillation at 60kHz or higher was the result every time until the value used was below 22pF.




.... Phil

[Frank Miles]

On Wed, 28 Sep 2016 19:52:00 -0700, Phil Allison wrote:

> cassiope wrote:
>>
>> I'm designing a power amplifier using some Exicon lateral MOSFETs as
>> the final common-source stage.
>>
>>
> ** You mean connected as source followers ?

Yes, quite so, I realized after I'd posted it that I'd mis-written, but
since no one had seemed to notice I didn't correct the error.

>> These will be used in various experimental systems. Each only needs to
>> run around +/-50V out, up to 4A, and driving sinusoidally between 50
>> and 400kHz. The load can be approximated as a lossy series LC circuit,
>> driven near resonance with R generally 10-20 ohms. There's an
>> additional >200pF of capacitance on each output source to ground due to
>> the capacitance to the heat sink.
>>
>> These MOSFETs have huge capacitances:
>> N ch: Ciss 900pF, Coss 500pF P ch: Ciss 1.8nF, Coss 850pF
>>
>> MOSFET "lore" (particularly from audio power land) says that you need
>> fairly substantial series gate resistors to protect these FETs.
>> Unfortunately the dominant pole in the amplifier is in the preceding
>> voltage-gain stage, so I'm having some HF stability issues when I use
>> resistors of the recommended value (>300 ohms). I'd like these
>> amplifiers to tolerate a variety of loads, not require tweaking for
>> particular systems.
>>
>> QUESTION : are these resistors necessary? How can a proper value be
>> determined?
>>
>>
> ** Gate resistors of about 200ohms are necessary, but not sufficient to
> ensure freedom from parasitic oscillations. As you have realised,
> lateral mosfets have lots of power gain up into the VHF range and a
> little stray L or C will set easily them off. Wiring layout should be
> done with this in mind, keeping all tracks short as possible. You can
> avoid heatsink capacitance by isolating it from ground and connecting
> all the mosfet sources directly so it floats at output voltage.

Unfortunately I have three channels of amplifiers, and the physical
design (this is a "feature improved" replacement of a legacy device)
makes separate heat sinks kinda unlikely/difficult. But now that I see
this I should revisit this, though it would take a huge package redesign
to accommodate separate heat-sinks for N and P devices and retain
compatibility with the existing units. So far the heat sinks have
metal exposed to the outside world, which would have to change if not
grounded.

How did you determine the 200 ohms?

> An output stabilising network is essential, consisting of a inductor of
> about 5uH with RC Zobels on both ends to supply ground.

Ouch. 5uH has ~12ohms reactance at 400kHz. I may need some inductance
here, but hopefully less. The usual Zobel impedances are also too low,
consuming far too much of the output at my much-higher-than-audio
frequencies.

> The operating bias current can play a role in HF stability, but try to
> keep it close to 100mA per device since this corresponds with the
> inflexion point in the tempco curve - where it goes from positive to
> negative.
>
>
> .... Phil

I noticed that. So far bias stability hasn't seemed a problem, I've got
a thermal sense diode and running at 75mA quiescent.

Phil - thanks so much for your recommendations! They have given me much
to think about.

-F

[Winfield Hill]

Phil Allison wrote...

>
>Winfield Hill wrote:
>
>> The only reason for a series resistor, as others
>> have stated, is to dampen RF oscillation. I'd not
>> expect a problem, given the low gm of lateral parts,
>> but I've seen ferrite beads on the gates of these
>> parts in Hafler's MOSFET amplifiers. Nonetheless,
>> they also placed 470-ohm resistors in series with
>> the gates, according to manual schematics. But I
>> do not believe such large values are necessary.
>
> ** What you "believe" is hardly relevant to anyone but you.
>
>I hope you agree changing the gate resistor value downwards
> lessens the stability margin, rather than improves it - so
> is not very likely to help the OP.

Normally adding extra poles is worse for stability.
But when I read your horror stories below, I see
that my experience is certainly not relevant here!

--
Thanks,
- Win

[Winfield Hill]

Winfield Hill wrote...

I retract this statement, and bow to Phil's comments.


--
Thanks,
- Win

[Phil Allison]

cassiope wrote:
>
>
> >>
> >>
> > ** Gate resistors of about 200ohms are necessary, but not sufficient to
> > ensure freedom from parasitic oscillations. As you have realised,
> > lateral mosfets have lots of power gain up into the VHF range and a
> > little stray L or C will set easily them off. Wiring layout should be
> > done with this in mind, keeping all tracks short as possible. You can
> > avoid heatsink capacitance by isolating it from ground and connecting
> > all the mosfet sources directly so it floats at output voltage.
>
>
> Unfortunately I have three channels of amplifiers, and the physical
> design (this is a "feature improved" replacement of a legacy device)
> makes separate heat sinks kinda unlikely/difficult. But now that I see
> this I should revisit this, though it would take a huge package redesign
> to accommodate separate heat-sinks for N and P devices and retain
> compatibility with the existing units.

** Lateral mosfets have all have the case connected to the source, so one hestsink can be used for all devices in a channel without any insulators.

I assume you are not using source ballast resistors as most designs leave them out.

> So far the heat sinks have
> metal exposed to the outside world, which would have to change if not
> grounded.

** Another option is to ground the sources of all the mosfets, via a common heatsink that can serve more than one channel. This requires that the centre point of the PSU be available to serve as the output as in this Haffler schematic:

http://bmamps.com/Schematics/Hafler/Hafler_9300,_9500_Schematic.pdf

>
> How did you determine the 200 ohms?
>

** I didn't, but many other designers have settled on values of 220 ohms to 470 ohms per device for the job. Often N channel devices having a higher value than used for the P channel ones.

> > An output stabilising network is essential, consisting of a inductor of
> > about 5uH with RC Zobels on both ends to supply ground.
>
> Ouch. 5uH has ~12ohms reactance at 400kHz. I may need some inductance
> here, but hopefully less. The usual Zobel impedances are also too low,
> consuming far too much of the output at my much-higher-than-audio
> frequencies.
>

** Yeah, that is a problem.

Any chance you could operate the mosfets outside the NFB loop with the gates linked via 220ohm resistors? Lateral mosfets start conducting at 0.4V so there would be a little crossover distortion and the output impedance would be around 1 ohm.

.... Phil

[boB]

I would imagine that the gate resistor could be very high in value, at
least until it gets in the way of the desired slew rate or high end
frequency response of the amplifier ?

boB

[Frank Miles]

On Thu, 29 Sep 2016 18:52:53 -0700, Phil Allison wrote:

> cassiope wrote:
>>
>>
>> >>
>> >>
>> > ** Gate resistors of about 200ohms are necessary, but not sufficient to
>> > ensure freedom from parasitic oscillations. As you have realised,
>> > lateral mosfets have lots of power gain up into the VHF range and a
>> > little stray L or C will set easily them off. Wiring layout should be
>> > done with this in mind, keeping all tracks short as possible. You can
>> > avoid heatsink capacitance by isolating it from ground and connecting
>> > all the mosfet sources directly so it floats at output voltage.
>>
>>
>> Unfortunately I have three channels of amplifiers, and the physical
>> design (this is a "feature improved" replacement of a legacy device)
>> makes separate heat sinks kinda unlikely/difficult. But now that I see
>> this I should revisit this, though it would take a huge package redesign
>> to accommodate separate heat-sinks for N and P devices and retain
>> compatibility with the existing units.
>
>
> ** Lateral mosfets have all have the case connected to the source, so one hestsink can be used for all devices in a channel without any insulators.
>
> I assume you are not using source ballast resistors as most designs leave them out.

Actually I'd put some in :( figuring that they'd be convenient to measure
the quiescent current. Before I fixed my bias generator (using the
thermal sense diode attached to one of the MOSFETS), the current wasn't
as stable as I wanted so I bumped these to 0.22ohms.

>> So far the heat sinks have
>> metal exposed to the outside world, which would have to change if not
>> grounded.
>
>
> ** Another option is to ground the sources of all the mosfets, via a common heatsink that can serve more than one channel. This requires that the centre point of the PSU be available to serve as the output as in this Haffler schematic:
>
> http://bmamps.com/Schematics/Hafler/Hafler_9300,_9500_Schematic.pdf

I'd seen this design earlier - cute! IIRC it requires (one? two?) separate
floating power supplies for each channel, unfortunately, which makes the cure
probably worse than the disease in my situation.

>> How did you determine the 200 ohms?
>>
>
> ** I didn't, but many other designers have settled on values of 220 ohms to 470 ohms per device for the job. Often N channel devices having a higher value than used for the P channel ones.

Certainly the N channel device has less capacitance so the disparity makes
sense. I'd sure like to understand the need for these resistors better than
the WAG about HF oscillations.

>> > An output stabilising network is essential, consisting of a inductor of
>> > about 5uH with RC Zobels on both ends to supply ground.
>>
>> Ouch. 5uH has ~12ohms reactance at 400kHz. I may need some inductance
>> here, but hopefully less. The usual Zobel impedances are also too low,
>> consuming far too much of the output at my much-higher-than-audio
>> frequencies.
>>
>
> ** Yeah, that is a problem.
>
> Any chance you could operate the mosfets outside the NFB loop with the gates linked via 220ohm resistors? Lateral mosfets start conducting at 0.4V so there would be a little crossover distortion and the output impedance would be around 1 ohm.
>
> .... Phil

That's something I'm hoping to explore today. The feedback network is
a standard pair of parallel RC (identical t.c.s); so I'm going to try
connecting the output-side resistor to the "real" output, and the
capacitor to the output of the voltage gain stage. The breakpoint is
around 1MHz, so distortion may be tolerable.

[Frank Miles]

On Fri, 30 Sep 2016 00:54:45 -0700, boB wrote:

[snip]

>
> I would imagine that the gate resistor could be very high in value, at
> least until it gets in the way of the desired slew rate or high end
> frequency response of the amplifier ?
>
> boB

The input capacitances are so high that I'm observing ringing step
responses with "higher" (300ohm) gate resistors with no load; and more
problems with reactive loads. Thus my effort to better understand
what's going on...

-F

[George Herold]

I know little about these issues, but would it make any sense to try a big
ferrite bead? Maybe with a little R too.

George H.

[Frank Miles]

On Fri, 30 Sep 2016 15:53:36 +0000, Frank Miles wrote:

[snip]

> The feedback network is
> a standard pair of parallel RC (identical t.c.s); so I'm going to try
> connecting the output-side resistor to the "real" output, and the
> capacitor to the output of the voltage gain stage. The breakpoint is
> around 1MHz, so distortion may be tolerable.

Early testing looks very good! Stability seems fine with a few different
loads, distortion (at least as viewed on the 'scope) is negligible, and
BW and slew rates are at least twice as fast as I need. This is with gate
resistors around 240 ohms (no beads, at least not yet).

It would still be nice to have a better understanding of why these
resistors are needed. For now I'll content myself with a fuller testing
with more reactive loads. Hopefully that will shake out any weaknesses.

Thanks Phil and everyone else that contributed to the discussion!
-F

[Winfield Hill]

Frank Miles wrote...
> Frank Miles wrote:
>> [ snip ]

>> The breakpoint is around 1MHz, so distortion may be tolerable.
>
>Early testing looks very good! Stability seems fine with a few different
>loads, distortion (at least as viewed on the 'scope) is negligible, and
>BW and slew rates are at least twice as fast as I need. This is with gate
>resistors around 240 ohms (no beads, at least not yet).
>
>It would still be nice to have a better understanding of why these
>resistors are needed. For now I'll content myself with a fuller testing
>with more reactive loads. Hopefully that will shake out any weaknesses.
>
>Thanks Phil and everyone else that contributed to the discussion!
> -F

I wasn't in the discussion, but would like to point
out / remind anyone interested, of my 200 W amplifier
project of last fall. This has a 1000 V/us slew rate
and a DC to 10 MHz response (-3dB rolloff frequency).
But ahem, it doesn't use lateral MOSFETs.


--
Thanks,
- Win

[Dave Platt]

>Early testing looks very good! Stability seems fine with a few different
>loads, distortion (at least as viewed on the 'scope) is negligible, and
>BW and slew rates are at least twice as fast as I need. This is with gate
>resistors around 240 ohms (no beads, at least not yet).
>
>It would still be nice to have a better understanding of why these
>resistors are needed. For now I'll content myself with a fuller testing
>with more reactive loads. Hopefully that will shake out any weaknesses.

My recollection/belief is that you're dealing with the equivalent of
the "grid stopper" resistors that were used with many vacuum-tube
amplifier circuits.

MOSFETs, like tubes, have a significant amount of capacitance at their
gates/grids (both gate-to-source, and the Miller capacitance). The
leads/traces connected to the gates have a non-zero inductance... so
you've got a series-resonant circuit, with the gate right in the
middle of it (maximum-voltage-excursion point). This can be a recipe
for instability, up to and including parasitic oscillation at the
resonant frequency. A term I recall from the vacuum-tube days was
"snivets" (these were thin vertical lines on a TV screen, caused by
parasitic oscillation of this general sort in the output tube).

Putting a stopper resistor in series with the gate or grid (ideally,
close to the device) both rolls off the bandwidth (resistor R
interacting with gate C) and kills the Q of the L/C resonant circuit.
It quiets the shrieking and screaming no end and can help keep the
Magic Blue Smoke where it belongs :-)

Anyhow, that's my (possibly-faulty) recollection and analysis.

I had to deal with a somewhat-related problem some years ago,
trouble-shooting a simple twin-audio-tone oscillator designed for
doing IM analysis of single-sideband ham transceivers. Very simple
circuit, which a guy built based on an article in QST - two twin-T
audio oscillators using 2N2222 transistors. It did oscillate, but the
frequencies and amplitudes were unstable and it'd misbehave if you
brought your fingers near the circuit. The owner couldn't figure
out the cause, and gave me the box as a "use this for parts if you
like" gift.

A bit of poking around with a spectrum analyzer showed that this
"audio" oscillator was breaking into RF parasitic oscillation at
upwards of 100 MHz!

Sticking a ferrite bead around the base and collector leads of each
2N2222 killed the Q of the resonances and fixed the problem. A
resistor of a few tens of ohms in the base would probably have done
just as well.

[Phil Allison]

cassiope wrote:

>
> > I assume you are not using source ballast resistors as most designs
> > leave them out.
>
> Actually I'd put some in :( figuring that they'd be convenient to measure
> the quiescent current. Before I fixed my bias generator (using the
> thermal sense diode attached to one of the MOSFETS), the current wasn't
> as stable as I wanted so I bumped these to 0.22ohms.
>

** Using a diode like that could result in over compensation.

Lose the 0.22ohms, especially if they are WW types.

> >
> > ** Another option is to ground the sources of all the mosfets,
> > via a common heatsink that can serve more than one channel.
> > This requires that the centre point of the PSU be available to
> > serve as the output as in this Haffler schematic:
> >
> > http://bmamps.com/Schematics/Hafler/Hafler_9300,_9500_Schematic.pdf
>
> I'd seen this design earlier - cute! IIRC it requires (one? two?) separate
> floating power supplies for each channel, unfortunately, which makes the cure
> probably worse than the disease in my situation.
>

** The main DC supply needs to be floating but drive can be from an op-amp running on +/-15V rails with centre grounded. Note how there are no load isolating components - not even an RC zobel.



.... Phil

[Frank Miles]

I'm sorry, Win - I didn't recall that. I have no vested interest in the
lateral MOSFETS. Is there a web link or at least a title that I might
search for?
Thanks!

[Winfield Hill]

Frank Miles wrote...

> Winfield Hill wrote:
>> Frank Miles wrote...
>>> Frank Miles wrote:
>>>> [ snip ]
>>>> The breakpoint is around 1MHz, so distortion may be tolerable.
>>>
>>>Early testing looks very good! Stability seems fine with a few
>>>different loads, distortion (at least as viewed on the 'scope) is
>>>negligible, and BW and slew rates are at least twice as fast as I need.
>>>This is with gate resistors around 240 ohms (no beads, at least not

>>>yet). [snip]

>>
>> I wasn't in the discussion, but would like to point
>> out / remind anyone interested, of my 200 W amplifier
>> project of last fall. This has a 1000 V/us slew rate
>> and a DC to 10 MHz response (-3dB rolloff frequency).
>> But ahem, it doesn't use lateral MOSFETs.
>
> I'm sorry, Win - I didn't recall that. I have no
> vested interest in the lateral MOSFETS. Is there
> a web link or at least a title that I might search
> for?

I don't remember the s.e.d. subject, and just now
I couldn't find the folder I made on DropBox, for
you s.e.d. blokes and a DIY-audio discussion, so
I created a new one, see AMP-70A under s.e.d.
https://www.dropbox.com/sh/mkoqdo5y0b9fevm/AACBM7iDdTyRLFADqLgj7ljNa?dl=0


--
Thanks,
- Win

[Frank Miles]

That was so nice for you to post this, thanks! I remember seeing it
now, though I'd forgotten it before I began the project with the power
amp requirement.

One prominent note in your schematic warns about using the 1-ohm output
apparently due to lack of protection. You may not have needed the low-
resistance output (though that seems strange given the 5A - losing over
1kW is a lot of heat!) - but did you find that active shutdown schemes
were too slow to protect your amplifier? (I haven't yet done this with
my amplifier)

[Phil Allison]

cassiope wrote:
>
>
> - but did you find that active shutdown schemes
> were too slow to protect your amplifier? (I haven't yet done this with
> my amplifier)
>

** And you won't need to.

From the capacitance figures you quoted, I see you have gone for the high power "double die" devices rated at 200V,16A and 250W.

Full protection only requires gate zeners of about 3.3V to limit current to about 5amps.

In the event of high dissipation, when the chip temp reaches about 165C, max current falls preventing further heating and the device saves itself - amazing but true.



.... Phil

[Winfield Hill]

Frank Miles wrote...
> Winfield Hill wrote:
>> Frank Miles wrote...

>>> Winfield Hill wrote:
>>>>
>>>> I wasn't in the discussion, but would like to point out / remind
>>>> anyone interested, of my 200 W amplifier project of last fall.
>>>> This has a 1000 V/us slew rate and a DC to 10 MHz response
>>>> (-3dB rolloff frequency).
>>>> But ahem, it doesn't use lateral MOSFETs.
>>>
>>> I'm sorry, Win - I didn't recall that. I have no vested interest in
>>> the lateral MOSFETS. Is there a web link or at least a title that I
>>> might search for?
>>
>> I don't remember the s.e.d. subject, and just now I couldn't find the
>> folder I made on DropBox, for you s.e.d. blokes and a DIY-audio
>> discussion, so I created a new one, see AMP-70A under s.e.d.
>> https://www.dropbox.com/sh/mkoqdo5y0b9fevm/AACBM7iDdTyRLFADqLgj7ljNa?
>dl=0
>
>That was so nice for you to post this, thanks! I remember seeing it
>now, though I'd forgotten it before I began the project with the power
>amp requirement.
>
>One prominent note in your schematic warns about using the 1-ohm output
>apparently due to lack of protection. You may not have needed the low-
>resistance output (though that seems strange given the 5A - losing over
>1kW is a lot of heat!) - but did you find that active shutdown schemes
>were too slow to protect your amplifier? (I haven't yet done this with
>my amplifier)

Another CAD schematic version includes a fast shutoff scheme,
but I didn't implement that on this prototype. The amplifier
is (almost) fast enough to be a 50-ohm RF source, but the real
usefulness comes from its high-current output, despite what
the drawing says. We've been using two of them that way for
almost a year, and nothing failed. Actually, we fried the
electrode system, but the amplifier survived. Knock on wood.

Using BJTs at high currents and high Vce invites disaster; the
180V 2SC4883A 2SA1859A have a much smaller SOA than I'd like.
That's because they're small die parts with fT = 60 and 120MHz.
We've been lucky so far. I also designed a fast SOA-sensing
protection circuit, but haven't implemented that either. As
it stands, the AMP-70A design is just a high-speed test bed.
I have modifications extending its slew rate to 2 or 3kV/us.
Keep up this kind of business long enough and eventually
something will blow out!


--
Thanks,
- Win

[Frank Miles]

Wow. Too good to believe? Gonna have to check it. 3.3V zeners are so
soft, but the NFB may hide some of that. Great observation, Phil, will
have to check further.

[Frank Miles]

That kind of speed from a power amplifier? Wow! The first vertical
deflection amplifier I designed for a Tek 'scope only had ~10kV/us,
obviously only for driving a well-defined load (deflection plates).

I don't need this kind of bandwidth or slew rate for my current
application. So far the lateral MOSFET design is simpler and
considerably more compact. So long as I can make it reliable!