Bob Cordell Interview: BJT vs. MOSFET - Page 22 - diyAudio
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Old 1st December 2006, 08:15 PM   #211
mlloyd1 is offline mlloyd1  United States
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i suspect bob might have been generalizing a bit again.

we all know nelson biases his mosfets hotter than (most) conventional designs.

mlloyd1
PS - man o man, these are some GREAT discussions. reminds me of the old days of netnews before the web and AOL. i just wish charles hansen was up to joining in ...

Quote:
Originally posted by Nelson Pass

Well Bob, that would be me.
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Old 1st December 2006, 08:21 PM   #212
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Quote:
Originally posted by Nelson Pass


Well Bob, that would be me.


Boy, did I goof!

Well, I know you are not a fool, that's for sure. So I guess I'm wrong, or at least I was way too quick to make a generalization.

My apologies.

OK, so what is your reasoning? Is your avoidance of the VAS buffer just confined to a particular topology? I'm trying to figure out how far apart we are on this, and under what conditions we agree/disagree.

Cheers,
Bob
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Old 1st December 2006, 08:32 PM   #213
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Me too!
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Old 1st December 2006, 08:38 PM   #214
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Bob, why should we use mosfets, IF we have to drive them with buffers as well? It's not an improvement over bipolars.
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Old 1st December 2006, 09:02 PM   #215
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Quote:
Originally posted by john curl
Bob, why should we use mosfets, IF we have to drive them with buffers as well? It's not an improvement over bipolars.
I am not Bob, but when I add mosfet source follower in parallel to degraded emitter follower I can pump much more current per case without degradation of sonic qualities. I.e. I use 2 cases up to 30A peak total instead of 10 of them.
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Old 1st December 2006, 09:03 PM   #216
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My apologies for embarrassing you, it's just the in me.

Of course we agree that if we want a fast circuit whose
distortion remains low at high frequencies we need to toss
current into the Gates. I don't place as much emphasis on this
as you do, but I address it by running more current through
the Vas. In the X600.5 the symmetric voltage gain stage will
peak out at about 100 mA. We also take advantage of balanced
output stages (halving the slew requirement), and last but not
least, we bias the amplifiers high.

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Old 1st December 2006, 09:04 PM   #217
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Quote:
Originally posted by john curl
Bob, why should we use mosfets, IF we have to drive them with buffers as well? It's not an improvement over bipolars.

John, that's a fair question. I've never considered that an advantage of using MOSFETs was any absence of a need to buffer the VAS for them.

So, clearly, whatever MOSFETs bring to the table as an improvement over bipolars, at least in my paradigm, must come even in the situation where there are predrivers and even drivers. Their benefit, if any, is not associated with a reduction in circuit complexity, at least in my designs.

One thing I like about them is their speed, both small-signal and the ability to turn them on and off.

Another thing I like about them is that the driver that they do need doesn't have to be as beefy, since there is no beta issue and no beta droop. For example, I don't think I could use folded emitter followers to drive bipolars; the standing current would have to be way too high in order to supply the worst-case base current.

I agree that in the absence of error correction, they are at a disadvantage in smaller amplifiers in that they need higher idle bias to achieve a given THD, at least in Class AB. In bigger amplifiers, where there tends to be more idle bias to go around anyway, I think this relative disadvantage pretty much goes away.

Although I have not had a chance to see real test-based SOA data on the ring emitter transistors, I think you have made a lot of progress in convincing me that the absence of second breakdown in the MOSFETs may not be as big an advantage over bipolars (at least the ring emitter devices) as I originally thought. At least on paper. I'm still very interested in seeing how well the ring emitter transistors hold up under a destructive test.

Bob
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Old 1st December 2006, 09:12 PM   #218
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Quote:
Originally posted by Nelson Pass
My apologies for embarrassing you, it's just the in me.

Of course we agree that if we want a fast circuit whose
distortion remains low at high frequencies we need to toss
current into the Gates. I don't place as much emphasis on this
as you do, but I address it by running more current through
the Vas. In the X600.5 the symmetric voltage gain stage will
peak out at about 100 mA. We also take advantage of balanced
output stages (halving the slew requirement), and last but not
least, we bias the amplifiers high.


Sounds good to me!

BTW, in your view is a balanced output stage different than a bridged output? A while back, bridging of amplifiers got a bad rap, it seems. Have we just changed the semantics to protect the innocent, or is there a substantive difference in architecture between an amplifier with a balanced output and one with bridged outputs? I'm assuming both have single-ended inputs, so that that part of the issue does not cloud the discussion.

Bob
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Old 1st December 2006, 09:20 PM   #219
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Bob & Nelson,

Sorry if I stirred the pot, but it's not often that we get to see two giants in this field agree to disagree.

Everyone have a great weekend.

Larry
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Old 1st December 2006, 09:30 PM   #220
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100 mA, Nelson ? How many gates in parallel are driven in this case ?
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