[ARC5] Circuit Help Needed

[Tom Lee]

Thanks very much for reminding me of the Hunt and Hickman reference! Jim 
Williams (the late and great) ran a contest some years ago to uncover 
the original source of the circuit (and term). The authors themselves 
don't explain in the paper why they chose the name cascode, so there is 
an enduring minor mystery about the etymology. I was taught that the 
word derives from "cascading into the cathode" or something similar. 
Wikipedia offers a different speculation, in line with yours. Since I'm 
lazy, I prefer the one with fewer words, but de gustibus...

And, like you, I found the topology of Mark's regulator interesting and 
surprising. The general rule in negative feedback is "fewer gain stages 
is better" to reduce the likelihood of instability. The single-stage 
pentode (or cascode) is a better choice for that reason. But Mark's 
circuit demonstrates once again that there are many ways to skin a 
cat(hode).

--Cheers
Tom

-- 
Prof. Thomas H. Lee
Allen Bldg., CIS-205
420 Via Palou Mall
Stanford University
Stanford, CA 94305-4070
http://www-smirc.stanford.edu
650-725-3383 (public fax; no confidential information, please)

On 2/11/2017 10:01 AM, Kenneth G. Gordon wrote:
> On 11 Feb 2017 at 0:22, Tom Lee wrote:
>
>> If the unregulated supply has very poor filtering, then setting the average value high indeed
>> helps, as then the worst-case ripple never gets close to letting the supply drop out of regulation.
>> That said, 700V does seem like overkill. Beefing up the filter capacitor and reducing the
>> unregulated voltage to, say, around 500V, would cut dissipation in half, which can't be a bad
>> thing (except, maybe, in winter),
> Ha ha! :-)
>
>> without affecting regulation in any significant way.
>>
>> I no longer know what the cost of a suitable capacitor would have been at the time. Maybe it was
>> high enough to drive the design decision. Guess it's time to dig out some old radio mags to see
>> what the cost equation might have looked like then.
> I would find that info interesting too.
>
> I think that if I had Mark's power supply, I would 1) find a different, lower voltage, but
> current-capable transformer, and save the present one, then 2) Replace all the filter caps
> with more modern, higher capacity units where appropriate.
>
> Thank you, Tom, for your help with this. I usually must flounder around until I figure these
> sorts of things out. Sometimes I takes me a while.
>
> Once I properly diagnosed Mark's drawing, I found the circuit very interesting. I was not
> aware of a dual-triode cascade-connected control circuit in a regulated power supply before
> this, and this has been an excellent learning experience for me. The most common such
> circuits use a high-gain pentode. The circuit is simpler and works well. The Heathkit IP-17
> series is a good example.
>
> BTW, "CasCode" turns out to be a contraction of "cascaded triodes having similar
> characteristics to a pentode" by the original designers, Frederick Vinton Hunt and Roger
> Wayne Hickman in 1939. Ha ha!
>
> Ken W7EKB
>
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