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Old 11th February 2008, 09:07 AM   #1
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Question Cascode Output Stages

Only recently I came across a still very interesting article about Cascode amp design written by Nelson Pass (see link). I saw that the article dates from 1978, so I am sure many off you know this article for some time (or years).

Itís plain from a technical point of view that the Cascode topology is favorable in many cases. But after searching this forum and reading some of the threads, the general opinion seems to be that Cascodes are liked in VAS stages but are disliked in Output Stages.

Technically, I donít understand the reason for this. This means apparently that in Output Stages the disadvantages are greater than the advantages (besides the extra components). But what are these disadvantages? Why is the result of a Cascode Output Stage not what one would expect regarding the technical benefits.

I also wonder; did anyone perform a FAIR a-b comparison between a normal Output Stage and a Cascode equivalent? And which one sounded best? With fair I mean that in an amplifier only the Output Stage was modified between non Cascode and Cascode and the rest of the circuit and poer supply was left the same.

Peter

http://www.passlabs.com/pdf/articles/cascode.pdf
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Old 11th February 2008, 09:44 AM   #2
AndrewT is offline AndrewT  Scotland
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Hi,
the Leach double barreled is cascoded throughout including the output stage. Download both Low Tim and Double to read about the design.
There are a few threads discussing the cascoded version of the Leach and one running now that is organising a group buy.
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Old 11th February 2008, 11:52 AM   #3
Bonsai is offline Bonsai  Taiwan
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The main reason output cascodes were (are) used is to overcome SOA limitations of the output devices. This was a real concern 30 years ago when th e Leach was developed, or before that the Ampzilla (I think the Ampzilla was one of the earlier commercial implementations).

I really doubt the neccessity for a cascode output now days. There are great output devices with good SOA - much better than 30 years ago. The more common practice now is to parallel output devices to improve power handling.

Cascodes are used in VAS stages to reduce the effect of Miller capacitance- so the rationale in the VAS application is different to the output stage.

Hope this helps.
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Old 11th February 2008, 02:14 PM   #4
jcx is online now jcx  United States
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I would call the Leach output "Totem pole", not "Cascode"

in the Leach the series output devices simply split the V drop between them and both see (1/2 of) the varying V

Cascode operation reduces the V swing across one device by orders of magnitude

Cascode operation also "costs" more output V drop than the saturation V of the 2 devices - you need substantially more operating V than the saturation V for the cascoded transistor to show improved characteristics from cascode operation

in power amps paralleling the same transistors for higher output current/less β droop is often a bigger improvement and doesn't hurt the power output

The β22 is a active project amp that has a Cascode output:
http://www.amb.org/audio/beta22/
http://headwize.com/ubb/showpage.php?fnum=3&tid=6923
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Old 11th February 2008, 09:09 PM   #5
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Thanks for the interesting replies.

An important question remains: which sounds best? Cascode or non cascode output stage.
I never trust a design or a design idea until Iíve heard the results. Iíve build things where people raved about but were disappointed with the results afterwards. When a design (or an idea of myself) is interesting enough, I always start to build a test setup to see if I like it or not. And also to be able to tweak it until itís performing at its max. Only after that a definite version is made.

Peter
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Old 12th February 2008, 05:22 PM   #6
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Why not ask Mr. Pass?
He's on this forum site.
PM him directly.
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Old 12th February 2008, 09:47 PM   #7
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Cascode outputs not only help with heat dissipation in the output devices, but they reduce the effective capacitance that the front end has to drive. That's no small benefit, particularly if you're using MOSFETs in the output. Yes, if you're using a follower, the Cgs is pretty much out of the picture, but that's not the only capacitance you have to worry about.
As to pitfalls...well, there are cascodes and there are cascodes. If you look at the schematic for one of the old Threshold amps, you'll see that the cascode devices were run from a resistive divider. So what? Doesn't everyone? Well...
If I recall correctly (don't have that schematic in front of me at the moment, so I could be wrong), the non-rail end of that resistor divider was attached to the output, not ground. This makes perfect sense if your goal is to swing as much voltage as possible at the output because the cascode devices "get out of the way" when the output followers come barreling through. But as usual, there's a price to be paid. When the cascodes start swinging along with the followers (think of the bellows on an accordion) the operating characteristics as no longer as fixed as you might wish.
So is it the end for cascoded outputs?
Of course not.
If you're willing to give up some voltage swing, you can fix the resistors to ground instead of the output. If you want to take it a step further you can either bootstrap the Gate/base voltage reference or give that reference a small, dedicated supply at a higher voltage. Giving the circuit plenty of voltage at the rails is a good idea, as you'll have to set the cascodes comparatively far from the outputs in order to leave sufficient voltage swing. Yes, that could get intimidating, but this is DIY and the masthead says something about fanatics. Well, do you want to be a fanatic or don't you?
There are problems. Then there are solutions. Then there are more problems. Then there are more solutions. It's all a question of how badly you want to investigate the topology. It can be done--has been done, in fact--and some variant on the basic concept could very well be the Next Big Thing.

Grey
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Old 12th February 2008, 11:19 PM   #8
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If my memory serves me correctly the data presented by Nelson Pass in his paper referred to many SUBSTANTIAL benefits from cascoded outputs in the quality of sound and dynamics. There was very little discussion about using cascode to increases the rail voltage and hence amplifier output.
I am hoping to experiment with this idea when and if we get the Leach Superamp project worked out. As Grey suggested Dr. Leach did not use the same output design as Nelson Pass but maybe some of the benefits will be apparent on experimentation. Learning this firsthand is one of the unique and fun parts of DIY and having this forums wealth of knowledge to draw upon. Tad
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Old 12th February 2008, 11:20 PM   #9
Blues is offline Blues  United States
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Quote:
Originally posted by myhrrhleine
Why not ask Mr. Pass?
He's on this forum site.
PM him directly.
I'm betting Mr. Pass will soon come out with a PL amp having an output stage of cascoded power jfets...he has given us Zv9 and F3 already...high power will follow.
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Old 13th February 2008, 07:12 AM   #10
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Grey,

The circuit in the link is an example of what you mention in your post.
http://www.audio-circuit.dk/Schemati...d_Stasis_1.pdf

I was thinking: wouldnít it be more accurate to drive the upper part of the Cascode from the INPUT instead of the OUTPUT. One of the main goals of this construction to keep the Vce of the output device constant. Because the voltage at the output has added distortion, time lag and a small voltage attenuation, wouldnít it be better to drive both upper and lower parts of the Cascode from the SAME input voltage? Itís also very important that both parts of the Cascode (upper and lower) are identical. So if you use a darlington or triple darlington for the lower part, use the same darlington or triple darlington for the upper part.

A few years ago I was working on a tube Cascode stage and measured the how constant the voltage of the lower part of the Cascode was held. It was clear then that the best result were obtained when the upper part got itís signal from the input.

Peter
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