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Folded/shunt cascode differential question
Folded/shunt cascode differential question
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Old 3rd February 2018, 09:36 AM   #21
Rod Coleman is offline Rod Coleman  United Kingdom
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Folded/shunt cascode differential question
About the gm value of the cascode device (the PNP transistor in a shunt cascode).

The bipolar transistor measures and sounds better than any other device, because of the higher gm. Choosing one with a low Cob is helpful for bandwidth in high-gain designs, and a high and stable beta helps with low distortion. The ZTX558 and the Fairchild KSA1381E are worth adding to your basket, if your are at Farnell or Mouser for parts.

You can compare measurement and sound of a series cascode with different devices in the upstairs position, like FETs and triodes, against a bipolar (NPN in the case of series cascode). I found that that even low-Hfe non-optimal NPNs were better than triodes/FETs.

With a practical high-gain stage, there is little need to worry about the gm. Running 1 to 5mA in the PNP is fine. The spectrum of the driver stage in my post above was taken with about 4mA of idle current in the PNP - a ZTX558.
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Old 21st February 2018, 02:50 AM   #22
Hearinspace is offline Hearinspace  Canada
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Folded/shunt cascode differential question
Rod !
Yes, I've built two so far , one as shown here with step-up transformer on the input and the other with jFet front end. Haven't played much with other triodes yet and not sure I'll bother as they sound so good as is.
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Old 8th March 2018, 02:02 PM   #23
Sodacose is offline Sodacose  United States
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I'd like to make a couple of boards for a basic folded cascode to use in future projects. Does anyone have any comments on the below two options? The MOSFET CCS is certainly simpler, but the BJT ring of two might be more widely available.

Also interested to hear whether this use of LR8 makes sense. I haven't played with these before but it looks like it could make a pretty simple voltage reference here.

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This will also be my first time creating boards so any helpful tips would be very appreciated.

edit: not sure if I'd keep the trim pots, btw. probably add a way to bypass them or replace after adjustment in circuit.
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Old 8th March 2018, 05:17 PM   #24
Hearinspace is offline Hearinspace  Canada
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Folded/shunt cascode differential question
Quote:
Originally Posted by Rod Coleman View Post
but I need to add a low-frequency servo at some point, to avoid the need to occasionally adjust the current-source.
Hi there Rod,
I just noticed your above comment. Why the need to readjust the current source?
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Old 9th March 2018, 09:42 AM   #25
Rod Coleman is offline Rod Coleman  United Kingdom
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Folded/shunt cascode differential question
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Originally Posted by Hearinspace View Post
Hi there Rod,
I just noticed your above comment. Why the need to readjust the current source?
Hello Ian!

Yes, with very high gain stages, that also requires large output swings: the output must stay near to half the anode voltage, or the available swing is diminished. The stability is very good if you use high quality parts and a stable base drive, but when you need swings amounting to most of the base/anode voltage, adjustment of the idle position might be needed from time to time. Mine need doing at the start and end of summer (that brief interval of the UK calendar). With lower gain or required swing, and especially your RIAA stages, it won't be needed.
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Old 9th March 2018, 09:43 AM   #26
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'd like to make a couple of boards for a basic folded cascode to use in future projects. Does anyone have any comments on the below two options? The MOSFET CCS is certainly simpler, but the BJT ring of two might be more widely available.

Also interested to hear whether this use of LR8 makes sense. I haven't played with these before but it looks like it could make a pretty simple voltage reference here.

This will also be my first time creating boards so any helpful tips would be very appreciated.

edit: not sure if I'd keep the trim pots, btw. probably add a way to bypass them or replace after adjustment in circuit.
My thoughts - some changes are needed:

1. The Q1 PNP base-protection diode is mandatory;
2. Anode stopper, noted above, highly recommended, to avoid need to chase instabilities in this high-gain stage;
3. Bipolar current source degrades the power supply isolation, and will oscillate on its own, as drawn. The MJE350 C-E is drawn reversed. The DN2540 cascode is easy to use, and is well-worth the inclusion.
4. LR8 uses a band-gap reference, and is not intended for low-noise applications. This kind of regulator is relatively noisy, and the noise increases with output voltage, so beware excessive noise, especially in RIAA applications, where I suspect it will not be quiet enough.
5. Layout: short trace lengths, especially around Q1. Leave reasonable gaps from B and E of Q1 to low-voltage nodes, to avoid leakage risk.

My current version includes a discrete low-noise shunt regulator for the base, for use in driver or RIAA stages. DN2540 cascode is included. The rest of the boards in the lot are available - Rod Coleman Shunt Cascode - Power Tube Driver
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Old 9th March 2018, 02:27 PM   #27
Sodacose is offline Sodacose  United States
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Rod,

Thank you so much for the feedback!

1. I will add this; thank you.

2. I was thinking this would be off the board, but maybe I'm misunderstanding. This is a stopper b/w tube anode and the BJT/CCS junction?

3. Doh on the MJE350 c-e. I'm leaning now towards a depletion MOSFET anyways to keep things simple. I've used 10M45S more in the past but have nothing against DN2540 other than I don't think I have any atm.

4. I'll research LR8 noise a bit more and/or other options. Seems a shunt arrangement (like you mentioned) is a good option. Initially I was thinking a 0D3 would make a very 'pretty' voltage reference, but it doesn't jive well with the board and keeping connections short.

5. Noted, thank you again. I'm planning to cut my teeth on a CCS board or two and then trying a folded cascode board. I have not done boards in the past but I'm starting to enjoy the layout process.

When you mentioned your board earlier, I Googled it to see if you were making it available for purchase. Not finding anything, I started down this road. I hope you don't take my project as any kind of comment on what you've already done! Apparently I'm just not good enough at Google. Now that I've built up some thought inertia in this direction, I'll try to see the board project through.
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Old 9th March 2018, 06:11 PM   #28
Sodacose is offline Sodacose  United States
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Maybe a better option for a voltage reference

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Old 9th March 2018, 07:48 PM   #29
Hearinspace is offline Hearinspace  Canada
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Folded/shunt cascode differential question
Quote:
Originally Posted by Rod Coleman View Post
Yes, with very high gain stages, that also requires large output swings: the output must stay near to half the anode voltage, or the available swing is diminished. The stability is very good if you use high quality parts and a stable base drive, but when you need swings amounting to most of the base/anode voltage, adjustment of the idle position might be needed from time to time. Mine need doing at the start and end of summer (that brief interval of the UK calendar). With lower gain or required swing, and especially your RIAA stages, it won't be needed.
Thanks Rod. I understand that side of the equation but really was asking about the root of change in circuit performance that necessitated the adjustment.
Framing it between summer start and end is interesting. What's the change, humidity ? I assume you have heating at home, or perhaps that's what the tube amp is for?
Seriously, at first read of your original mention, I'd wondered if it was due to changes in the 6Э5П's emission.
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Old 11th March 2018, 04:56 PM   #30
Sodacose is offline Sodacose  United States
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Been doing some reading up on voltage reference noise. Here's a great thread:

Some noise measurements for LEDs and zener diodes

Jones also has a nice section of zener and gas reg measurements.

I'm thinking one of the following options:

1 - TL431 like shown above, see TubeCAD (cons: noisy band pass reference, complicated, current requirements)(pros: easily adjustable bias and accuracy based on the resistor tolerance)

2 - Resistor with CCS to pull up voltage, see Bartola (cons: needs adjustment to reach correct bias voltage, lots of BJTs)(pros: low noise, low current)

3 - Zener with CCS load (cons: must find a quiet zener @ ~150V, increase current ot lower noise)(pros: simple, two terminal reference can swap in gas reg when noise is not critical)


references

1 T-Rex 300B Amplifier

2 http://www.bartola.co.uk/valves/2016...ascode-driver/

3 Jones

Anyone care to comment on the above approaches?
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