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Old 18th May 2008, 01:45 PM   #1
regal is offline regal  United States
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Default TubeCad Journal FET CCS

In the TubeCad journal his designs use an FET CCS. All he says is "FET constant current sources are readily available in two-lead packages up to 100V's and in increments up to 4.7 mA"

I searched digikey and mouser and found nothing like this.

Anyone have an idea what he is talking about ?
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Old 18th May 2008, 02:06 PM   #2
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Search for constant current diode or CCD.
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Old 18th May 2008, 02:38 PM   #3
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An example is the Siliconix J500.

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Old 18th May 2008, 04:29 PM   #4
oshifis is offline oshifis  Hungary
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I use the 1N5283 ... 1N5314 series current regulator diode. A 3.9 mA and a 4.3 mA diode is connected parallel in the cathodes of a long-tailed pair phase splitter (E88CC), and they are connected to - 50 V. I think Motorola, ON Semi, Knox and others produce this range of CCS diodes.
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Old 18th May 2008, 04:35 PM   #5
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I imagine they can be paralleled to go higher than 4.7 mA ?
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Old 18th May 2008, 04:45 PM   #6
Sheldon is offline Sheldon  United States
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Careful now, SY hates those things. They are basically N- channel JFET's with the gate tied to the source, and sorted for IDSS, or limiting current. For some purposes they are probably quite adequate, but if they are directly in the signal path, you can do much better. A simple N-channel FET with a source resistor is better, but still simple and adjustable by changing the source resistor. Cascoded N-channel JFET add one more part and increase impedance and noise rejection further. Full explanation provided here: http://www.audioxpress.com/magsdirx/...a/jung2778.pdf

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Old 18th May 2008, 04:51 PM   #7
regal is offline regal  United States
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Thanks,


What sort of tax to the power supply do these add ?


I am trying to convert a SRPP to this:


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Old 18th May 2008, 05:36 PM   #8
SY is offline SY  United States
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Quote:
Anyone have an idea what he is talking about ?
I hate those things.

They are current sources with mediocre source impedance andmoderate voltage ratings. They aren't cheap. For the cathode CCS in your second stage, they'll be adequate since it is shunted by the load that you're using the preamp to drive. The plate CCS ought to be something better. I'd use either cascoded DN2450 MOSFETs (easy to self-bias) or cascoded bipolars. They won't add much to your power supply load (the self-biased MOSFET won't increase power supply load at all), they are cheap and simple, and the performance will easily exceed the two-terminal devices.

You can also get rid of the resistors you show in series with the CCSs. If the CCS voltage rating is sufficient, the resistors are redundant.
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Old 18th May 2008, 05:43 PM   #9
regal is offline regal  United States
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Originally posted by SY


IYou can also get rid of the resistors you show in series with the CCSs. If the CCS voltage rating is sufficient, the resistors are redundant.

Thankyou, I figured those resistors were just there because he was using a 100V device.


My B+ is only 180V, if I understand this circuit all I have to do is make sure the CCS and the cathode resistor are balanced (same current draw) ?
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Old 18th May 2008, 07:59 PM   #10
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The CCS will set the current through the tube and cathode resistor. The size of the cathode resistor is chosen to provide the desired cathode bias voltage, E=IR.

There are three parameters, plate current, plate voltage, and bias voltage. You get to pick two and the tube picks the third. The plate voltage is really plate-to-cathode voltage, but I usually think of it as plate voltage because the difference is only a couple of volts.

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