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Old 30th June 2013, 08:49 AM   #11
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I'm sure that would work - it's a standard push-pull VAS output. Myself I intend to use the Exicon laterals though I will not drive them directly from the VAS. When I was testing I had the high impedance feedback connected to the VAS output directly. When I inserted a buffer, performance instantly got much better.

The reason why I'd buffer MOSFETs is because at HF, their gate capacitance becomes a notable lower resistance and thus a load on the VAS if not buffered.

I can get away with 15 ohm gate stoppers on the output devices when I locally compensate the fets with a 47pF to stop them from oscillating. It makes for a very fast output
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Old 30th June 2013, 09:13 AM   #12
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I would do the same with laterals...you could play with a value for N/P gate stoppers and compensation caps, they should not be the same as m-fets are different, but you allready know that I guess ;-)
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Old 30th June 2013, 02:25 PM   #13
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Yep 15 ohm for the P channel and 18 ohm for the N channel.

In the sim I've changed the cascodes a little; I now turned them into a Hawksford cascode and THD performance noticably increased. Now to put it in practice again and see if I get to keep at least the current performance. I'm definitely going to keep the Hawksford cascode configuration
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Old 30th June 2013, 02:56 PM   #14
PMI is offline PMI  United States
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Quote:
Originally Posted by MagicBox View Post
Yep 15 ohm for the P channel and 18 ohm for the N channel.
At first glance, wouldn't the gate resistor values be further apart, if you are trying to equalize the diff gate capacitance of the mosfets, or am I getting confused here? I mean more like 1:1.5 instead of 1:1.2 ?
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Old 30th June 2013, 08:14 PM   #15
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Yeah, to equalize the lowpass filters that are created by the stopper and gate capacitance. Those values I proposed are what's in MF80, another project of mine and they work out well in practice.
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Old 30th June 2013, 08:35 PM   #16
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Originally Posted by MagicBox View Post
Yeah, to equalize the lowpass filters that are created by the stopper and gate capacitance. Those values I proposed are what's in MF80, another project of mine and they work out well in practice.
My instinct is to just calculate back from the published gate capacitance which would give me a ratio of 1:1.5 (I do not have the datasheet in front of me). At least for the Exicons and the Alfets.

However, why I asked: I have been told that the gate capacitance is published in the "Typical" column in part because it varies a lot from one production run to the next. Can't verify this, but the person who told me has reason to know this and to have measured it, something I can't do. Plus, given the nature of a typical circuit board, even one with short traces, the suggestion as made to me was to use something in the 1:1.3~1.35 range.

Sadly, this is all just based on hearsay on my part, which is why the values you posted jumped out at me, since the ratio is also different. So, do you base this on past experience, measurement, educated guess? Seems odd to me that you, and another engineer in a field unrelated to audio, would come up with the same thing, unless there is a reason

I also note that the complementary Alfet was on production hold for a while b/c the dies for one of the two components did not meet spec, while production of the N- and P-channel in separate packages, continued.
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Old 1st July 2013, 06:31 PM   #17
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Hi PMI,

I didn't reach the values by using the exact datasheet capacitance values and calculate ratios. I merely approximated knowing the P part is the slower part. I actually measured the amp output to pick values that resulted the best symmetry which could be measured by how quick artifically induced oscillation faded away. No special procedures, just error and trial on a real output stage. The design uses 2 Exicon N parts and 2 P parts for a relaxed 80 watts into 4 ohms

I'm still going ahead with posting the schematic - it's not "secretly special". Instead it's cool to see that indeed topology is the main factor in a circuit's performance, followed by choice of components. The circuit has been consistently performing in the sim and in real as well now. It'll be fun sharing it and perhaps people will have ideas for improvement.

Eventually I want to scale it up to an output power of 100W into 4Ohm as a prototype.

I wonder if mr. Cordell would have anything to say about my little circuit and perhaps tell a bit about squarewave testing
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Last edited by MagicBox; 1st July 2013 at 06:41 PM.
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Old 1st July 2013, 09:03 PM   #18
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Hello

If you still want to publish your schematic, I would be currious to see it.

Thank

Bye

Gaetan
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Old 1st July 2013, 09:07 PM   #19
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Interesting that you arrived at something similar empirically. That gives me a bit more confidence that I was given the right information.

Quote:
Originally Posted by MagicBox View Post
...a bit about squarewave testing
I was told once that relying on square waves leads to pushing the envelope too far in favor of a wide bandwidth v. a noise free circuit, but everyone is still doing it...

For comparison, here is a link to some test results I posted a while ago in Shaan's PeeCeeBee thread:

Square Wave Test Results
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Old 1st July 2013, 09:13 PM   #20
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I was told once that relying on square waves leads to pushing the envelope too far in favor of a wide bandwidth v. a noise free circuit, but everyone is still doing it...
[/URL]
Yes, there's something addictive about them especially if you can get them up to 100KHz in simulation and even better if it can then be repeated in a real circuit.

Wonder how many are watching this thread suffering the suspense.
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