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Old 17th December 2010, 06:01 AM   #1
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Default cascoded bjt ccs - design check please

Hi guys - before I let the smoke out testing it, comments.

Note - no, its not a depletion mode device or any other format, I'm running with what I have here, so looking for confirmation that I'm in the ballpark with my figures (my first time specifying this schematic). I figure its around a megaohm of impedance anyway which is plenty for my purpose.

LED string should be passing around 3.5ma, ccs is set to 1.65ma or around that...
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Old 17th December 2010, 07:26 AM   #2
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looks good to go What voltage will be at the Q1's collector? (In other wordds, beware not going over the transistor's Vc rating, which I'm sure you know...)
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Old 17th December 2010, 07:48 AM   #3
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thanks semper - all things being equal, around 2Vdc. Its the common cathode of a 1st stage ltp.
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Old 17th December 2010, 08:10 AM   #4
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Interesting, the CCS will have very limited headroom, but it may not need any more than 2V?
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Old 17th December 2010, 08:12 AM   #5
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yeah, wondered about that - should I put a negative rail in to give it a longer tail?
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Old 17th December 2010, 08:13 AM   #6
adamus is offline adamus  United Kingdom
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see the baby huey design and copy the ccs in the ltp
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Old 17th December 2010, 08:16 AM   #7
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aaaah true, but that's cheating - I don't learn as much...

OK, I cheated - negative rail advisable.
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Old 17th December 2010, 08:43 AM   #8
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absolutely, you will have some voltage swing at the node where the cathodes meet the CCS.
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Old 17th December 2010, 08:10 PM   #9
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version 2.

The negative rail voltage is not a given - I have a bias supply I can draw from with up to -50v the practical limit.

I can't get my head around the biasing (I'm looking for -1.5v relative to the grid which results in 0.825ma idle per device)

- does the ccs automatically bias the devices, or, with the lowered negative rail, do I have to bias the cathode relative to signal ground using the variable resistor shown in the new diagram?
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Old 17th December 2010, 08:40 PM   #10
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Quote:
Originally Posted by aardvarkash10 View Post
- does the ccs automatically bias the devices, or, with the lowered negative rail, do I have to bias the cathode relative to signal ground using the variable resistor shown in the new diagram?
The tube will seek its bias, based on the current drawn by the CCS. Adjusting the resistor only changes the voltage at the top of the CCS, it doesn't change the grid to cathode voltage, and it requires a coupling cap on the input. It will bias that way, but there is no benefit.

Simpler to dispense with the variable resistor and connect your grid leak resistors to ground.


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