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Folded/shunt cascode differential question
Folded/shunt cascode differential question
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Old 31st January 2018, 04:28 PM   #1
Sodacose is offline Sodacose  United States
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Default Folded/shunt cascode differential question

I recently started reading a bit about the folded/shunt cascode (thanks, Rod Coleman for your interesting posts). It's a cool circuit and I have a potential project for it as the differential front end on a push-pull amp (Allen Wright style).

Click the image to open in full size.

I understand the basic workings and am comfortable enough to be able to calculate bias/dissipation/etc as a single-ended cap-coupled amplifier. However if simply mirroring the layout in a LTP arrangement, one runs into CCSs on top and bottom of the circuit.

Notwithstanding Morgan Jones, this is usually frowned on. But in the case of the folded cascode, I wonder if the multiple CCSs are an issue. My thinking is that surplus current is driven through the shunt anyways, and so CCS current differences at anode or cathode (with respect to the tube) are ok. Thus the two CCSs would not simultaneously be trying to set a different current. The cathode CCS sets current through the triodes and the anode CCS sets current through the triodes and the shunt.

Click the image to open in full size.

Has anyone tried this kind of circuit as differential? Maybe a current mirror instead of a CCS would make a good anode load, too?
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Old 31st January 2018, 09:28 PM   #2
Sodacose is offline Sodacose  United States
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I'll throw one more schematic in here too

Click the image to open in full size.

Ultimately I'd like to do something like this as differential for an AB2 PP amp (eg George's 6L6GC thread). The cascode should crank out decent gain and the followers will help drive the output tubes' Miller cap.
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Old 31st January 2018, 09:34 PM   #3
20to20 is offline 20to20  United States
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...Sir, would you like some LEDs with that...?
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Old 31st January 2018, 09:41 PM   #4
Sodacose is offline Sodacose  United States
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There would be plenty in the CCS, I'm sure.

Back to my original post though, I guess I could substitute the anode loads in a cascode LTP for resistors too. The tail CCS would still set the balance/splitting and the resistors would create some feedback for the transistor (I think).
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Old 1st February 2018, 03:18 AM   #5
PRR is online now PRR  United States
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Quote:
Originally Posted by Sodacose View Post
....My thinking is that surplus current is driven through the shunt anyways, and so CCS current differences at anode or cathode (with respect to the tube) are ok.....
Your thinking is correct. You need to be more confident.

But why stop there? Why have CCSes in the top? All you really need is much-much more than transistor emitter impedance hOE. Assume 1mA. hOE is 27 Ohms. Much-much-more is 2,700r for 1% accuracy. Pencil 5K for easy math. This leg has 2mA so 10V of drop, which is surely acceptable.

You have fixed bias on bases. With the resistors this wants to be referenced to the resistor supply. This actually does come out to be a "CCS", so we arrive at the same place.

The un-implemented tail CCS needs detailing. We can wrap NFB around the whole shebang by combining the transistor legs.
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Old 1st February 2018, 12:56 PM   #6
Rod Coleman is offline Rod Coleman  United Kingdom
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Folded/shunt cascode differential question
Quote:
Originally Posted by Sodacose View Post
I recently started reading a bit about the folded/shunt cascode (thanks, Rod Coleman for your interesting posts). It's a cool circuit and I have a potential project for it as the differential front end on a push-pull amp (Allen Wright style).

I understand the basic workings and am comfortable enough to be able to calculate bias/dissipation/etc as a single-ended cap-coupled amplifier. However if simply mirroring the layout in a LTP arrangement, one runs into CCSs on top and bottom of the circuit.

Notwithstanding Morgan Jones, this is usually frowned on. But in the case of the folded cascode, I wonder if the multiple CCSs are an issue. My thinking is that surplus current is driven through the shunt anyways, and so CCS current differences at anode or cathode (with respect to the tube) are ok. Thus the two CCSs would not simultaneously be trying to set a different current. The cathode CCS sets current through the triodes and the anode CCS sets current through the triodes and the shunt.


Has anyone tried this kind of circuit as differential? Maybe a current mirror instead of a CCS would make a good anode load, too?

Glad you're enjoying the shunt cascode!

I haven't implemented a differential, but it should work very well.

The shunt cascode will offer high-to-very-high gain and low distortion, if designed carefully.

My thoughts:

- The input and output are both referred the ground, so if possible, keep the cathode at a low-impedance to ground, and the anodes at a high impedance. I understand the motive for PRR's anode resistors, but depletion FETs (DN2540s are perfect here) improve the measured performance enough to justify the small extra effort - they are very easy to assemble. PNP or PFET CCS can be made to work, but the performance is degraded.

High impedance in the anode circuit, plus the fact that the instantaneous (and average) supply current is perfectly static, even at large output swings, mean that the gain-stage is quiet, and needs no supply regulation.

With a differential stage, some consideration must be given to the circuit balance, to account for the high gain. I would suggest that individual resistors are used in the cathodes of each triode, before joining at the long tail. At least 50Ω, as a starting estimate, depending on the gm of the triode. The gain is reduced, but the feedback also has a useful linearising effect. A dual triode would seem like a good idea for closer matching, and many pleasant TV bottles have high gm, low-voltage operation and come in duals.

I might prefer to use a resistor as the long tail, because common-mode rejection should not be the most important design choice, but of course, it depends on what feeds the inputs.

As usual with high-gain stages, care must be taken to avoid running out of phase margin. Watch out for loop-area of the grid, anode, CCS, and load-resistor circuits; stopper the anode as well as the grid.
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Old 1st February 2018, 01:12 PM   #7
Rod Coleman is offline Rod Coleman  United Kingdom
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Folded/shunt cascode differential question
In case anyone wonders about how good the shunt cascode is: here's some spectrum.

This is a 6Э5П in triode running at 202V and 32mA through my shunt-cascode explorer board. The load resistor returns to negative voltage in this case. The performance and sound is very pleasing, but I need to add a low-frequency servo at some point, to avoid the need to occasionally adjust the current-source.

The stage is driven directly by an ES9018 DAC, and the measurement is taken at an output of ~50V rms - almost 150V peak to peak.
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File Type: png 49V8rms-145Vpp-109V-idle.png (49.1 KB, 491 views)
File Type: jpg 20170120_085131.jpg (320.3 KB, 239 views)
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Old 1st February 2018, 01:49 PM   #8
Sodacose is offline Sodacose  United States
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Quote:
Originally Posted by PRR View Post
But why stop there? Why have CCSes in the top?

We can wrap NFB around the whole shebang by combining the transistor legs.
Thanks, PRR! I was part of the way there after thinking about it a bit more yesterday. The emitter sets the anode voltage and drop across the load, so using a resistor makes calculating the PNP current easy (given the cathode CCS). The PNP is creating a 'vertical' loadline for the tube (just like a regular cascode), so the load in parallel to it isn't so critical from the tube's perspective. I like the feedback idea too (kind of like Baby Huey).

Rod, I really appreciate your thoughts. The point about PSRR and CSS over resistor in the anode makes sense. This choice (for me) would probably be determined by tube choices and how easy a quiet supply would be to create.

It's the potential for a single gain stage (if you can call cascode a single stage) that has really drawn me to this circuit. I'd like to do a "simple" two-stage PP design with trioded EL34/KT88/etc outputs. If it works out, the next step would be an AB2 amp along the same lines (grid drivers add that extra layer of complexity).
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Old 1st February 2018, 02:05 PM   #9
SpreadSpectrum is offline SpreadSpectrum  United States
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Rod,

What kind of gain do you see for the stage in practical circuits?
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Old 1st February 2018, 02:21 PM   #10
Rod Coleman is offline Rod Coleman  United Kingdom
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Folded/shunt cascode differential question
For my 300B driver, the load resistor is 27K, and the gm of the 6Э5П starts at 30mS, degenerated to about 7mS by the action of a 110Ω cathode resistor, unbypassed.

So the gain in this case is 7e-3 * 27e3 = about 190 (45.5dB)

Meanwhile, a D3A with 24KΩ load, and 53.3Ω unbypassed will yield about 12.9mS and x309 gain (50dB), but I have not needed to use that yet.

In other practical terms, I have gone from DL103 Moving Coil to 300B grid in 3 stages of shunt cascode: 2SK369 JFET shunt cascode MC stage, 6H23 shunt cascode EQ stage, EF184 shunt cascode driver stage. All of these stages run open-loop, and degenerated by cathode resistors.
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