cascoded bjt ccs - design check please

aardvarkash10 · 17th December 2010, 06:01 AM

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...

SemperFi · 17th December 2010, 07:26 AM

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...)

aardvarkash10 · 07:54 AM

thanks semper - all things being equal, around 2Vdc. Its the common cathode of a 1st stage ltp.

SemperFi · 17th December 2010, 08:10 AM

Interesting, the CCS will have very limited headroom, but it may not need any more than 2V?

aardvarkash10 · 17th December 2010, 08:12 AM

yeah, wondered about that - should I put a negative rail in to give it a longer tail?

adamus · 17th December 2010, 08:13 AM

see the baby huey design and copy the ccs in the ltp

aardvarkash10 · 08:20 AM

aaaah true, but that's cheating - I don't learn as much...

OK, I cheated - negative rail advisable.

SemperFi · 17th December 2010, 08:43 AM

absolutely, you will have some voltage swing at the node where the cathodes meet the CCS.

aardvarkash10 · 08:16 PM

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?

Sheldon · 17th December 2010, 08:40 PM

Quote:

Originally Posted by aardvarkash10

- 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.

Sheldon

17th December 2010, 07:48 AM	#3
aardvarkash10 diyAudio Member Join Date: Mar 2007 Location: Auckland, NZ	thanks semper - all things being equal, around 2Vdc. Its the common cathode of a 1st stage ltp. __________________ Yes, conservatism thrives on low intelligence and poor information. But the liberals in politics... continue to back off, yielding to the supremacy of the stupid. It's turkeys all the way down. - George Monbiot, guardian.co.uk, 6 Feb 2012 Last edited by aardvarkash10; 17th December 2010 at 07:54 AM.

17th December 2010, 08:12 AM	#5
aardvarkash10 diyAudio Member Join Date: Mar 2007 Location: Auckland, NZ	yeah, wondered about that - should I put a negative rail in to give it a longer tail? __________________ Yes, conservatism thrives on low intelligence and poor information. But the liberals in politics... continue to back off, yielding to the supremacy of the stupid. It's turkeys all the way down. - George Monbiot, guardian.co.uk, 6 Feb 2012

17th December 2010, 08:16 AM	#7
aardvarkash10 diyAudio Member Join Date: Mar 2007 Location: Auckland, NZ	aaaah true, but that's cheating - I don't learn as much... OK, I cheated - negative rail advisable. __________________ Yes, conservatism thrives on low intelligence and poor information. But the liberals in politics... continue to back off, yielding to the supremacy of the stupid. It's turkeys all the way down. - George Monbiot, guardian.co.uk, 6 Feb 2012 Last edited by aardvarkash10; 17th December 2010 at 08:20 AM.

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17th December 2010, 07:26 AM	#2
SemperFi diyAudio Member Join Date: Jan 2002 Location: nowhere	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...)

17th December 2010, 08:10 AM	#4
SemperFi diyAudio Member Join Date: Jan 2002 Location: nowhere	Interesting, the CCS will have very limited headroom, but it may not need any more than 2V?

17th December 2010, 08:13 AM	#6
adamus diyAudio Member Join Date: Apr 2008	see the baby huey design and copy the ccs in the ltp

17th December 2010, 08:43 AM	#8
SemperFi diyAudio Member Join Date: Jan 2002 Location: nowhere	absolutely, you will have some voltage swing at the node where the cathodes meet the CCS.

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