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Folded/shunt cascode differential question
Folded/shunt cascode differential question
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Old 1st February 2018, 02:23 PM   #11
merlin el mago is offline merlin el mago
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Very interesting Rod.
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Old 1st February 2018, 03:22 PM   #12
SpreadSpectrum is offline SpreadSpectrum  United States
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It looks like pretty impressive performance for gain that high.
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Old 1st February 2018, 08:12 PM   #13
Sodacose is offline Sodacose  United States
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Click the image to open in full size.

Some quick rough guesses as to values. The 150V base reference would probably be a 0D3. Around 100x gain.

This puts 5mA through the PNP (MJE350 just because I have some). Less current would allow for a larger anode resistor, less dissipation across the PNP, and higher collector quiescent voltage. OTOH, more transistor current increases gm. But is that advantage swamped by the relatively low gm of the tube?
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Old 1st February 2018, 09:02 PM   #14
Wavebourn is offline Wavebourn  United States
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Folded/shunt cascode differential question
Collector quiescent voltage would depend on base voltage. Higher value of anode resistor would limit it's upper value. Also, you need to protect b-e junction by a diode from reverse breakdown.
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Old 2nd February 2018, 07:36 AM   #15
Merlinb is offline Merlinb  United Kingdom
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I have often wondered if you could find a vacuum diode with complementary characteristics to the lower tube in a shunt cascode, to get the distortion down...
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Old 2nd February 2018, 07:43 PM   #16
PRR is offline PRR  United States
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Quote:
Originally Posted by Sodacose View Post
...more transistor current increases gm. But is that advantage swamped by the relatively low gm of the tube?
The transistor is working common-base. Its Gm hardly matters as long as it is high. The stage gain is Gm(tube) * RL(bjt) for practical purpose. (Never forgetting "RL" includes load and stray-capacitance suckage.)

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Originally Posted by Sodacose View Post
...It's the potential for a single gain stage (if you can call cascode a single stage) that has really drawn me...
Don't get over-drawn yet.

The "folding" doesn't, by itself, change the action. It is still a cascode. With some simplifying assumptions they both work the same.

The folding does un-stack the supply voltage (at the cost of those plate loads). If you need large swing from small supply, this may be compelling.

The "simplifying" is assuming all devices flow the same current. This is "only natural" in cascode, though a diversion is possible. Un-equal current is "easy" in the folded form. Why would we do that? The Gm which matters is the tube(s), and good Gm means high current. However it *may* be that the BJT does not need such a high current. While current "usually" increases as you go from input to output, sometimes we do otherwise. The folded form can do this by choice of top and bottom current sources. You "can" do this in the cascode but the diversion adds unnecessary parts, may not save supply power, or may want an added supply.
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Old 2nd February 2018, 08:37 PM   #17
PRR is offline PRR  United States
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Quote:
Originally Posted by Merlinb View Post
I have often wondered if you could find a vacuum diode with complementary characteristics to the lower tube in a shunt cascode, to get the distortion down...
This wonderment leads down the rabbet hole. Diode compensation has been used in the past, notably to improve DC drift without an extra triode, but there's more to explore.

Somebody (you?) recently posted a hollow-diode distortion for musicians. (Fender did it 20+ years back: diode-strapped 12AX7 worked just like a pair of Ge or Si diodes.) Your site has a clever curve tracer which would find possible "complements".

I'm sure you've seen the pre-distortion diode plan in the LM13700 (below). For the same THD you can run 5X the level; or for the same level you can reduce THD an astonishing 30:1. (Yet musicians seem to prefer the soft-onset to the sudden clipping.)

I'm simming a thingie which uses complementarly driven tube grid-cathode impedances to good effect and some advantages over the way it has been done since the 1930s; but I'm not sure of my thoughts yet, and it has no application to what Sodacose is doing.
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Last edited by PRR; 2nd February 2018 at 08:45 PM.
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Old 3rd February 2018, 01:12 AM   #18
Sodacose is offline Sodacose  United States
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Quote:
Originally Posted by PRR View Post
The Gm which matters is the tube(s), and good Gm means high current. However it *may* be that the BJT does not need such a high current. While current "usually" increases as you go from input to output, sometimes we do otherwise. The folded form can do this by choice of top and bottom current sources.
That's the meat of what I was wondering. Thanks!

I misspoke about collector voltage in the earlier post though. Obviously decreasing collector current lowers collector voltage (everything else constant).

Seems like I'm looking to strike a balance between transistor dissipation and a high enough collector voltage to avoid clipping the output. Not that I need extreme swing with the current plan (shooting for around 100x), but still an important aspect of this circuit.
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Old 3rd February 2018, 04:51 AM   #19
Hearinspace is offline Hearinspace  Canada
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Folded/shunt cascode differential question
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Originally Posted by SpreadSpectrum View Post
It looks like pretty impressive performance for gain that high.
It makes a really nice phono stage.
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Old 3rd February 2018, 09:23 AM   #20
Rod Coleman is offline Rod Coleman  United Kingdom
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Folded/shunt cascode differential question
Hello Ian, I wondered if you had completed your version. Which triode worked best for you?
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