On the Subject of Cascoding

[GRollins]

On the subject of cascoding...
Okay, let's set aside bandwidth, capacitance, distortion, and whatever else you can say for cascodes. There's one point that I never see mentioned, which is that cascoding runs 180 degrees opposed to the "run 'em hot" school of thought. High rail voltages (tubes, anyone?) and high bias currents lead to better specs and better sound. Split that by cascoding and you'll end up with an anemic cousin of the original circuit. Yes, you gain this, that, and the other thing, but...
Just thought I'd toss that in the mix.

Grey

P.S.: Yes, you can jack the rails even higher, then run both devices hot, but that's another topic for another day.

[Peter Daniel]

Maybe reading this thread again would be in order:? The sound of cascodes

[Fred Dieckmann]

Seems to me that cascoding allows one to run devices at a higher current for a given device dissapation. Higher curent gives greater transconductance and one can use lower value drain, plate or collecter resistors for the same gain as the devices run at lower current with bigger load resistors. Inverted cascodes
even allow more DC current through the gain device than the load resistor lower rail voltage requirements for a given value load resistor. Distortion due to voltage varations across the gain device are reduced by cascoding to a much greater degree than increasing the DC voltage across gain device.

It seems to me that the "run 'em hot" school of thought makes a very good case for cascoding rather than against it.

Setting aside bandwidth, capacitance, distortion, and whatever else you can say for cascodes when disscusing them seems a bit strange since those are the very reasons that cascodes are used to begin with. I don't think you can have a useful disscussion on an approach by ignoring its merits.

Not running a bit hot,
Fred

P.S. I am glad to see you back as well and can't imagine you would stay gone for this long. I save my real conStErnation for a certain SomeonE who SEems to thrive on it....

[Peter Daniel]

Originally posted by Fred Dieckmann
\

P.S. I am glad to see you back as well and can't imagine you would stay gone for this long. I save my real conStErnation for a certain SomeonE who SEems to thrive on it....

Why do you ever have to argue with Someone, FrEd?

[Fred Dieckmann]

Maybe the same reason that moderators allow post like that below:

http://www.diyaudio.com/forums/showt...743#post231743

[Peter Daniel]

Originally posted by Fred Dieckmann
"Sorry, but without knowing just what your goals are, it's difficult to speak"

That never stopped you before.

Most importantly you will need to ignore vague advice from people who have never designed an amplifier and make post with no actual useful content. It would be advantageous to avoid overly simple advice.

Originally posted by Steve Eddy
Welcome back, Fred. How was rehab?

se

I already got in trouble once, for trying to implement more relaxed ways of running things around here. So my comments will be really short and not exactly what I want to say.

But, IMHO, moderators here are not for the reasons to watch what members say to each other, but only to protect them from incoming conflicts and emotional arguments.

It seems to me that you were the first to start the argument, with SE, with your subtle, yet apparent remark. Now, SE just replied to you the best he could. Again, personally, not as a moderator but as a member, I can't blame him for that.

[macka]

Peter,

Loved that link to the thread on cascodes above and it certainly was graced by the finest minds on the forum including Mr Pass and Hugh Dean who needs know introduction but is a true master of communication with technical literature on the Web.

I think Hugh sums up the difficulties of debating on forums perfectly well.

Ian

QUOTE

" You feel too intently, and maybe you think too much about these relatively unimportant things. Your post listing the dos and don'ts makes good common sense, but it is prescriptive, and best left unsaid."

" Rules can create resentment, and I wouldn't like to have all my posts put through those rules to check correctness. I have no wish to start a flame war, and never will (although I like to stir occasionally!) but believe me, if it happens, I'm outa here. I have survived many flame wars on joenet only by remaining silent; flames are fanned only by emotional responses. The brain damage is much worse if you respond; humans are often incensed by disagreement, yet they are drawn to it like moths to a flame. Leave the email unanswered if you don't agree with it; this works every time, as time dilutes everything."

"Now, may I return to my hobby horse, that pesky cascode? I am like a dog with a bone on this one, I love a friendly debate, and since I've built it and measured it, I know its performance, and the gain is huge. This is in sharp contrast to the diff pair, whose gain is trivial by comparison. "

I recall there is a ample coverage of cascodes also in the A75 article by Norman Thagard & Neslon Pass. This is an excellent read and clearly outlines the application of cascode and their variants

http://www.passdiy.com/pdf/A75p1.pdf

[grataku]

Can we go back to the thread subject, PLEASE?
Before certain users intervention people were slowly getting somewhere, now we are getting nowhere fast. (I have the distinct impression someone used this before)

Fred I have learned how cascodes really work from this site thank to Nelson, you, and a few others I can't remember right now.
Can you seriously tell me you don't understand what Grey is saying?

Peter why are you wasting time posting here about another thread that went to crap because you didn't do anything about it? How many more disposable threads do you need?

[Fred Dieckmann]

I understand what he is saying. I just don't agree with it. Hot means power. Power is voltage times current For given device power limit increasing the quiesent current will give higher transconductance and greater linearity than increasing the voltage at the expense of bias current. Cascoding does not prevent one from running devices "hot" The real reason for using cascodes is not to limit dissapation was the one of the points I was trying to make.

[GRollins]

In an uncertain world, it's good to see that some things remain constant: The sun rises in the east, gravity pulls things downwards, and Fred still hasn't learned manners & civilized public behavior. I was making bets with myself as to how long it would take him to show up. The only question was how long it would take him to start trying to pick a fight. To do so in a single post is a marvel of efficiency.
For those who believe that Fred's attitude towards me is based on anything as mundane as facts, I trust that his post above--wherein he carefully disregards the benefits of higher voltage simply to start an argument--will enlighten them.
Reminds me of the advice given to aspiring lawyers. If the facts are against you, argue the law. If the law is against you, argue the facts. In Fred's case, perhaps we could amend this to argue the current if the voltage is against you.
Case in point. I had the Aleph-X on the bench hooked to a distortion meter. Ran the rail voltage up--distortion dropped. Dropped it back--distortion rose. The current was held constant for all devices due to the current sources. For that matter, you can raise and lower current independent of voltage with the three pots I designed into the circuit (sneaky of me, eh?), and that has clear benefits, too.
But to argue the current in isolation? Sad. And the guy claims to be an EE.
The defense rests.
No doubt the prosecution will go on for quite some time...

Grey