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Cascode driver

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#21
TUBESMAN said:
Well if no one tried to patent it before then he deserves it.
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That might be how patents work. It is not how they are supposed to work. A patented idea is supposed to be new, not just previously unpatented.

As I read it, he keeps the main advantages of the cascode and reduces the disadvantages of it.
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No, he reduces both e.g input capacitance will rise. You can do the sums if you want some numbers.

In many cases a better circuit would be a simple grounded cathode triode. Less distortion, and adequate drive capability. If a lower output impedance is needed then a mu follower or bootstrapped CF could be used.
 
#22
Yeah that's why I like numbers. In theory, communism and Reaganomics work, in practice not so much. If I saw the gain, noise, bandwidth, output impedance and distortion of a cascode and "BJ's" (sorry about that) circuit. I'd be able the know the truth.
 
#23
You can find the cascode maths in most good old electronics textbooks. If your algebra is OK you can modify cascode maths to get 'BJ' maths. Bear in mind that it is difficult to calculate distortion for a valve circuit, but you can get a feel for how different circuits compare. Gain, impedance, bandwidth can be calculated.
 
#24
Ok some increase in input cap was indicated. But it's all a matter of degree. Compared to what? Design is a compromise. You describe a topography suitable for a SE😱 Phase shift,the number of gain stages and number of caps in series with the audio are considerations too. Yes it may be possible to DC this and that is a consideration. It's obvious you don't like the cascode for drivers, but I do wonder how bad is it really.
 
#25
Drivers need to handle large signals with low distortion, and have lowish output impedance. Cascodes do the opposite.

Cascodes have low input capacitance (not usually needed for a driver) and lower noise than a pentode (irrelevant for a driver).

About the only cascode characteristic which matches a driver is poor PSRR (doesn't really matter for a driver).
 
#26
 
#27
Like I said it is obvious you do not like cascodes. Low noise is important in a driver because it is an error signal that will mask detail and it is the job of an designer to make a circuit better than it needs to be. If input capacitance isin't a factor, why did you point to it as a fault of the BJ design? A differential cascode, my original post describes it as what I am interested in, I think it has good PSRR. Besides, that is not a factor to me because I will use low noise supplies. Yes a cascode without the CF has high output impedance. I am not considering a cascode without an output CF. A low output impedance with high standing current is needed to drive a high capacitance with low slew. Also, you know as well as I do that it is very difficult to calculate high order distortion accurately. I trust a FFT spectrum display more than a French Curve. I do think that MJ is as qualified as anyone on this forum to recommend a design and he specifically described, pg.504-505 of MJ4e, using a differential cascode with a CF as a high performance driver for a high capacitance load for high power tubes and it wasn't even the BJ circuit, possibly because of patent issues. He does say that a cascade of carefully designed differential amps COULD be better. Unless some one has test data showing poor performance of a cascode, even better, test of the BJ design, I will accept your opinion and take it for what it's worth.
Of course, no one should take my opinions as fact either, until they fire up the soldering iron and the FFT software on their computer and make measurements and compare apples to apples.
 
#29
Could it also be that commercial products are designed to be as cheap as possible and a cascode should have multiple power supplies to avoid HK voltage problems? As I said design is an art and science and you make the compromises you feel are right
 
#30
TUBESMAN said:
Could it also be that commercial products are designed to be as cheap as possible and a cascode should have multiple power supplies to avoid HK voltage problems? As I said design is an art and science and you make the compromises you feel are right
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yes, that is the idea, get your unit cost down so that selling price can be high, leading to more profits....😀

me too was fascinated with cascodes owing to the fine performance i got with Leach's doubled barreled amp....

wading thru the threads here and some pm's with more learned members made me had a rethink....cascode for bj't are fine, for tubes, that is another story...😀

good luck on your cascode venture...
 
#36
DF96 said:
If you read the patent carefully you will see that he admits the cascode has problems. The patent makes it less 'cascode-like' and so reduces both its advantages and disadvantages. My question is: why choose an inappropriate circuit and then reduce its problems? Why not choose a better circuit?

I would be surprised if nobody has thought of that idea before. The fact that a patent has been granted does not mean that the patent should have been granted.
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patents are mere ideas, the proponent's IP or so he thinks, but that it works in practice is another story....
 
#37
If someone has an idea, lets say he can make salt water into gasoline, and he doesn't patent it. Then someone does the work to document it and do it then they should get the credit. "Hard Work beats Talent, If Talent doesn't Work Hard". May not be fair but that's the way it is.
 
#40
i too dream of doing cascodes some day, but they are in the back-burner for now......lots of topologies to try out, so little time....

look up on the threads i linked, perhaps you can find something there that the likes of PRR, SY and others failed to see....lot's of luck....

the 1953 edition of Radiotron Designer's handbook, (can be downloaded free from Peter Millette's site,) page 533 discusses cascodes and points out to refferences.....there a lot of information available even before Johnson's 1987 patent...

this is from John Broskie....http://www.tubecad.com/march99/page2.html

this is from Nelson Pass....https://passlabs.com/articles/cascode-amp-design
 
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