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Folded/shunt cascode differential question
Folded/shunt cascode differential question
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Old 11th March 2018, 06:23 PM   #31
mogliaa is offline mogliaa  United Kingdom
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Folded/shunt cascode differential question
I found that a Cascoded CCS with a top depletion FET and a lower J112 instead of the LND150 has much better temco. I run the J112 at about 1-1.4mA. You can implement a simple follower and reduce component number.
If you want to go better, then a shunt regulator as recommended by Rod is a good option
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Old 11th March 2018, 07:05 PM   #32
Sodacose is offline Sodacose  United States
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Great thanks for the update on your circuit!

By shunt regulator, are we talking about just a zener and transistor combo with a dropping resistor/ccs? Forgive my density. I'm still learning some of the common transistor arrangements (especially pnp coming from a tube perspective).
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Old 11th March 2018, 07:56 PM   #33
john_tracy is offline john_tracy  United States
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I've used a LM334 with extra diode and resistor for zero tempco, cascoded with a DN2530 and a series resistor as a voltage reference. The LM334 is somewhat noisy (white noise) but the voltage reference is constant. Current consumed was 1mA.
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Old 13th March 2018, 05:06 PM   #34
Sodacose is offline Sodacose  United States
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Just going with a CCS + resistor (which several people are mentioning), I'm arriving at the following:

Click the image to open in full size.

I'm planning to have some boards made up for my use and appreciate any suggestions for changes.

Initially I'd like to use this in the front end of an amplifier (hence the differential question that started the thread) but wouldn't mind trying it in an RIAA at some point, too.
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Old 13th March 2018, 06:54 PM   #35
Rod Coleman is offline Rod Coleman  United Kingdom
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Folded/shunt cascode differential question
Quote:
Originally Posted by Sodacose View Post
Just going with a CCS + resistor (which several people are mentioning), I'm arriving at the following:

Click the image to open in full size.

I'm planning to have some boards made up for my use and appreciate any suggestions for changes.

Initially I'd like to use this in the front end of an amplifier (hence the differential question that started the thread) but wouldn't mind trying it in an RIAA at some point, too.
Should be worth some tests and measurements, at least! Measure the base when running large swings - getting the tightest grip on the base is important for the measured performance, and the sound. But it should be low enough noise.

One thing needs a correction: diode D1 is reversed. It needs to protect the PNP from violations of VEBO, which is usually only -5V or so. The circuit may well start up with more voltage on the base, and exceeding this rating has the annoying outcome of leaving the PNP functional, but with degraded noise and gain.
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Old 13th March 2018, 07:02 PM   #36
Rod Coleman is offline Rod Coleman  United Kingdom
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Folded/shunt cascode differential question
Quote:
Originally Posted by Hearinspace View Post
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.
Yes, the house gets a bit colder in my hifi den. It can't be helped - the house was designed for heating with open fires, and now it's in a statutory «Conservation Area» - so any construction-hacks have to be kept invisible. The amps are near a gapless 9 inch wall, and they sit on a ventilated wooden floor. Proper 19th century squalor!

6Э5П's emission is solid, with time or temperature. But like most MOSPower devices, the DN2540's Vgs running value shifts enough over 10°C to be measurable.
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Old 13th March 2018, 08:18 PM   #37
Sodacose is offline Sodacose  United States
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Here's a pass at a board. I have not made boards before, but once I started it was kind of intuitive (like virtual p2p). It it a little tight around Q6, the follower for the voltage reference. The emitter trace crosses under the base trace, but they are roughly the same voltage.

Click the image to open in full size.

Dimensions are 3.75" x 2". Big cap spacing is 27.5mm and most of the resistors are 1/2W (except v-ref, led dropper, output). Room for a large heatsink (based on Aavid/Ohmite datasheet dimens).
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Old 14th March 2018, 06:54 AM   #38
mogliaa is offline mogliaa  United Kingdom
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Folded/shunt cascode differential question
Looks good! As I said the LND150 tempo is poor even when run at 500uA) so best to change it for a J112 and run it at 1mA. It will provide a more stable voltage. Otherwise will drift when hot

Also the anode DN2540 on its own is not the best CCS when running hot. Again, best is to have a cascode pair. The device on the top will take the heavy load and the below device will set the current and run cooler, hence more stable with temperature
I’d rather use an IXTP08n100d at the top, will allow the lower device to operate in better conditions.

Ale
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Old 14th March 2018, 10:16 AM   #39
Sodacose is offline Sodacose  United States
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Thanks for the reminder on the j112. That should be an easy swap. I usually have IXYS depletion FETs around (10m45 and 10m90) and will check out the ixtp08n100d part for a cascode.

I really appreciate the help Rod and Ale.
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Old 14th March 2018, 10:32 AM   #40
Rod Coleman is offline Rod Coleman  United Kingdom
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Folded/shunt cascode differential question
One more practical suggestion:

Instead of using the raw TO-92 PCB footprint, which places the base and collector much too close for easy soldering without shorts, let alone the high-voltage swing across them (case of the cascode PNP): use an "ammo-pack" TO-92 pattern. These put the pads on a 0.1" [2.54mm] pitch, and make life much easier. Form the leads to suit when stuffing. Many transistors are available on cut strips of ammo tape, ready to go right into those holes.
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