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Old 25th May 2008, 12:58 PM   #1
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Default Replacing a cathode resistor with a CCS

I haven't played with CCS's before so I need some guidance so I can learn. I will try to attach a schematic.

I would like to replace the 54.9K cathode resistor that is being shared by the 6SL7 with something better. I have 71 volts between ground and pins 3 and 6 of the 6sl7. I believe this will result in .0012932mA

Will this primative transistor,zenier and 2 resistor combination work or is there something better out there that will provide better results?
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Old 25th May 2008, 01:12 PM   #2
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Original circuit
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Old 25th May 2008, 03:08 PM   #3
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You've got some volts to work with, so that's a good thing. Your scheme will work, but to make it optimal you want the Zener voltage to be fairly large so that the emitter resistor is also large- that increases the source impedance by roughly hfe times Re.

One neat trick is to tap the DC voltage feeding the reference off the plates of the diff amp through symmetrical resistors - that way, common-mode stuff is partially canceled.

I think you mean 1.29mA, not 0.00129mA.
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Old 25th May 2008, 03:21 PM   #4
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SY

Thank you for your reply.

Would the HV NPN CCS circuit work here? If so would it be better?

Yes, I meant 1.29mA
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Old 25th May 2008, 03:33 PM   #5
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Do you mean the cascode? If so, yes, but you can easily hit diminishing returns. If we take a single NPN with an hfe of 150, and we use a couple of LM329s in series as a reference, the emitter resistor will be about 13k. So the source impedance will be roughly (150)(13k) = 1.9M. Not bad, especially compared to the cathode impedance.
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Old 26th May 2008, 12:31 PM   #6
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Are there any PCB available from anyone? I really would like to assemble something that looks neet instead of having a cluster farce of flying components.

SY,

Thanks for the response. I appreciate the fact that no one else would respond and you took the time.
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Old 26th May 2008, 02:10 PM   #7
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Quote:
Originally #5 posted by SY
If we take a single NPN with an hfe of 150, and we use a couple of LM329s in series as a reference, the emitter resistor will be about 13k. So the source impedance will be roughly (150)(13k) = 1.9M. Not bad ...
Sorry SY,
you should correct this one!

Kind regards,
Darius
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Old 26th May 2008, 08:00 PM   #8
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Anyone notice there's no DC return for the grids in that schematic?
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Old 26th May 2008, 08:05 PM   #9
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Quote:
Originally posted by oldeurope


Sorry SY,
you should correct this one!

Kind regards,
Darius
You're right, it's a bit higher than that since I neglected the 1/hoe term. But hfe time Re dominates.
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Old 26th May 2008, 08:07 PM   #10
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Quote:
Originally posted by Geek
Anyone notice there's no DC return for the grids in that schematic?
It's a fragment. The grid is direct coupled from the plate of a voltage amplifier.
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