How does a pair of transistors cascoded like the Bottlehead C4S perform?
It uses 2 red LEDs and 2 transistors in a cascode.
It uses 2 red LEDs and 2 transistors in a cascode.
It would appear to be pretty much the same as the diyAudio CCS boards offered a few years ago and the CCS shown in Morgan Jones's "Valve Amplifiers." Performance is very good- not as good as cascoded DN2540, but good enough that it won't be the limitation in any circuit you're likely to build.
You can approximate them from the published curves, but there's enough batch-to-batch variation that you might want to set up a test jig to determine the resistor value (IME, the resistor value will be pretty consistent device-to-device in the same batch). For 10mA, a good starting point will be about 300R. For 20mA, about 120R. For 5mA, about 560R.
Are they good as current sources in the range of 0.5mA to 2mA?
Some say a gate resistor should be used, or can I just ground the gate to the source resistor?
Some say a gate resistor should be used, or can I just ground the gate to the source resistor?
You absolutely want a gate-stopper resistor. Anything from 300R-3k will work, but you want the resistor body as close to the gate as possible. The work very well at low currents, but I would definitely cascode them.
Yes, you can cascade the IXYS part; I use the 10M45S cascaded in my preamp as a plate load, with SiC Schottky diode bias. It sounds excellent.
The cascoded DN2540 is insanely better than you'd ever need (>100M impedance, noise in the pA range). I don't know if the IXYS part is any better, but you're already well past the point of diminishing returns, so use whichever you can get easiest/cheapest.
Absolutely, but you have to pick a bottom FET with a lowish pinch off voltage. Fortunately, the DNs are only a buck or so, so it hardly seems worthwhile to use something else.
I buy two on eBay from UK about 3 months ago, I paid 8,30$ CAN (£5,21 GBP) shipping included.
But I don't try them yet, it is for my winter experimental circuits.
Alain. 🙂
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hey-Hey!!!,
On the cascoding I think it would be useful to run one of the IXYS bits on top. They have about double the gate-source voltage and would allow more d-s voltage for the lower one to work in. The larger voltage is ov primary concern cause gate-srain capacitance drops so rapidly at these d-s voltages. This would allow use of the higher than 400V parts here too( to do the 'heavy lifting' ).
cheers,
Douglas
On the cascoding I think it would be useful to run one of the IXYS bits on top. They have about double the gate-source voltage and would allow more d-s voltage for the lower one to work in. The larger voltage is ov primary concern cause gate-srain capacitance drops so rapidly at these d-s voltages. This would allow use of the higher than 400V parts here too( to do the 'heavy lifting' ).
cheers,
Douglas
Using the DN2540 cascode as an active load I sill get hum through, but not if 2SK170 is used. What gives?
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