On Sun, 02 Oct 2016 09:43:41 -0700, Winfield Hill wrote:
> 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!
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!
Reply by Frank Miles●October 2, 20162016-10-02
On Sat, 01 Oct 2016 21:10:31 -0700, Phil Allison wrote:
> 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
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.
Reply by Winfield Hill●October 2, 20162016-10-02
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
Reply by Phil Allison●October 2, 20162016-10-02
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
Reply by Frank Miles●October 1, 20162016-10-01
On Sat, 01 Oct 2016 12:19:35 -0700, Winfield Hill wrote:
> 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
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)
Reply by Winfield Hill●October 1, 20162016-10-01
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?
On Fri, 30 Sep 2016 14:07:16 -0700, 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).
>>
>>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.
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!
Reply by Phil Allison●October 1, 20162016-10-01
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
Reply by Dave Platt●September 30, 20162016-09-30
>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.
Reply by Winfield Hill●September 30, 20162016-09-30
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