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Old 28th September 2004, 04:35 PM   #21
jam is offline jam  United States
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Mlloyd,

A folded cascode models as a single stage and is pretty fast. The disadvantage is the it has no current gain to speak of. It is a great topology if used correctly.

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Jam
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Old 28th September 2004, 04:45 PM   #22
mlloyd1 is offline mlloyd1  United States
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Default Re: I know of a few more...........

Jocko:

You're right, the Rowlands I was thinking about were the old beefy butt products from years gone back, not the girly man amps, I mean chip amps.

One of the things I miss most about the old rag "Audio" magazine was the schematic discussions to see who's thinking about or doing what and spending time trying to figure out why they would do it that way.

Then, talk to the designers during CES (back when they used to have them in Chicago) to see if I got it right.

Ahh, the good old days ...

PS - it's in the mail.
really ...

[QUOTE]Originally posted by Jocko Homo
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Old 28th September 2004, 04:52 PM   #23
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Default Yep, the old ones.......

But he has gone one step further away..............but don't tell CH........sssshhhhhh.......!

(Inside joke.)

I just had to fix one of "the old ones" a few weeks ago. Too damn heavy to lift onto my bench. Or from the customer's house. Had to fix it there, on the floor. Lightning strike took out the control circuitry.

Jocko
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Old 28th September 2004, 05:15 PM   #24
jcarr is offline jcarr  United States
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mlloyd:

If you include what the Japanese commercial audio designers have been doing, a folded cascode is not a particularly uncommon topology. Quite a few Luxman designs, likewise from Marantz, same with Denon, some from JVC, Stax... I don't know if I am to be considered a member of the Japanese audio design community, but FWIW I've also used folded cascodes commercially.

Not many folded-cascode designs have been published in the Japanese DIY audio magazines, however. The better-known designers like Kaneda, Kubota, Yasui et. al haven't published any folded-cascode designs, AFAIK.

Regarding the pros and cons of folded cascodes, I've written quite a few related posts on this forum. Using the search engine with the terms "folded cascode" and user name jcarr, I get 21 hits. Poor utilization efficiency of the voltage rails is an obvious weak point of folded cascodes, unless you sandwich something like a current-mirror stage between the folded cascodes and the final outputs.

regards, jonathan carr
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Old 28th September 2004, 05:16 PM   #25
MikeB is offline MikeB  Germany
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Quote:
Originally posted by Ouroboros
seems to give better performance. At least SPICE says so!
Which particular performance do you mean ?
The folded cascode is not great for low thd, but for speed...

Mike
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Old 28th September 2004, 05:55 PM   #26
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Quote:
Poor utilization efficiency of the voltage rails is an obvious weak point of folded cascodes
jcarr: could you please elaborate on that one. I think quite the opposite unless i have misunderstood your point.

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Old 28th September 2004, 06:02 PM   #27
Mr Evil is offline Mr Evil  United Kingdom
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Quote:
Originally posted by Lars Clausen


jcarr: could you please elaborate on that one. I think quite the opposite unless i have misunderstood your point.

The output voltage swing is limited to the reference voltage of the common base transistors. That's why the output stage of the circuit I posted needs some voltage gain.
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Old 28th September 2004, 07:19 PM   #28
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Mr. Evil: aaaa ok, but that doesn't have to be a weaker point in SFC stages, than any other kind of amplifier stage. You can always use a lower Vref
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Old 28th September 2004, 07:53 PM   #29
jcarr is offline jcarr  United States
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Lars: The output signal of the folded cascode stage cannot swing any more than the biasing voltage that is applied to the bases of the folded cascode devices. As regards voltage swing and power supply voltage utilization efficiency, the key voltage levels are those that bias the folded cascode devices, not those on the power supply rails. Put another way, the voltages that bias the folded cascodes should be subtracted from the power supply rails. You could reduce the biasing voltages, but this should be done while considering the behaviour of the current sources and sinks that feed and drain the folded cascode stage.

A folded cascode is not the appropriate topology if you want to design a rail-to-rail amplifier design. Of course, it is possible to circumvent this limitation - at the cost of extra circuitry. But rather than do this, it is probably better to use a different topology if maximum signal voltage swing from a given power supply voltage is an important design goal.

I touched on similar things here, along with other comments:

http://www.diyaudio.com/forums/showt...260#post117260

Although at that time my discussion was on the basis of a non-complementary folded cascode.

More here:

http://www.diyaudio.com/forums/showt...092#post159092

As I said before, try the search engine.

regards, jonathan carr
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Old 29th September 2004, 12:25 AM   #30
Mr Evil is offline Mr Evil  United Kingdom
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Here's an updated schematic. Q7/8 added as per Konrad's suggestion. With C2/3 they stop cross-conduction, although they do add about 10dB to each harmonic distortion component. I also swapped the zeners/current sources so they are *ahem* the right way around now.
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