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Old 22nd December 2012, 07:36 PM   #1
Gost22 is offline Gost22  Bolivia
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Unhappy Circuit or schematic for current source of 7.5mA

Hi all!
I need a circuit or schematic with MOSFET for constant current source of 7.5 mA at the cathode for tube preamp with 6DJ8 (6922).
Thank you!

Last edited by Gost22; 22nd December 2012 at 07:43 PM.
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Old 22nd December 2012, 07:49 PM   #2
DUG is offline DUG  Canada
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What is the voltage (min and max) that it will see?
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Old 22nd December 2012, 07:56 PM   #3
Gost22 is offline Gost22  Bolivia
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What is the voltage (min and max) that it will see?
The voltage at the cathode -2V.
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Old 22nd December 2012, 07:57 PM   #4
Gost22 is offline Gost22  Bolivia
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The voltage at the cathode -2V.
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Old 22nd December 2012, 08:14 PM   #5
DUG is offline DUG  Canada
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Here is a basic current sink circuit.

Adjustable up to 10mA.
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Old 22nd December 2012, 08:20 PM   #6
DUG is offline DUG  Canada
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You could also use a 267R resistor...tubes tend to self regulate their bias point.

I know...OLD SCHOOL

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Old 22nd December 2012, 08:50 PM   #7
Gost22 is offline Gost22  Bolivia
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Originally Posted by DUG View Post
You could also use a 267R resistor...tubes tend to self regulate their bias point.

I know...OLD SCHOOL

I agree, but I want this to rid with electronic circuit with mosfet with a constant current of about 7.5mA.
thank you and cheers!
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Old 22nd December 2012, 09:41 PM   #8
Gost22 is offline Gost22  Bolivia
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I agree, but I want this to rid with electronic circuit with mosfet with a constant current of about 7.5mA.
thank you and cheers!
Here's the complit schematic!
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Old 22nd December 2012, 09:49 PM   #9
kevinkr is offline kevinkr  United States
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It makes much more sense to put the CCS in the plate circuit. (You'll get good linearity and gain approximating mu if the AC load impedance is high relative to rp.)

Placing an unbypassed CCS in the cathode circuit will result in much less than unity gain in a common cathode amplifier.

There is a voltage compliance issue as well, there are no commonly available CCS that perform well with just 2V across them. (incidentally +2V at cathode)

Depending on required gain you might want to consider a single red led for cathode bias and a CCS for plate load.
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Old 22nd December 2012, 11:05 PM   #10
DUG is offline DUG  Canada
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Originally Posted by kevinkr View Post
It makes much more sense to put the CCS in the plate circuit. (You'll get good linearity and gain approximating mu if the AC load impedance is high relative to rp.)

Placing an unbypassed CCS in the cathode circuit will result in much less than unity gain in a common cathode amplifier.

There is a voltage compliance issue as well, there are no commonly available CCS that perform well with just 2V across them. (incidentally +2V at cathode)

Depending on required gain you might want to consider a single red led for cathode bias and a CCS for plate load.
1. Gost22 wants to regulate current. Placing a CCS in the plate circuit is for a different purpose. The load line would be almost horizontal. Perhaps something to look at. (If you can get a very low capacitance CCS)

2. Yes, you are correct in that a CCS without a bypass cap will get zero gain to the plate.

Gost22, you do need to put a bypass cap in the cathode cct. (parallel with the CCS)

3. The CCS I have suggested will go down to almost 1V...Well within what Gost22 asked for.

4. The bias point is specified at 7.5mA not 2V. Depending on the tube it might be 1.9V or 2.1V but the CCS will compensate for that. LED would not. LED's also have temperature characteristics that are rather bad for a voltage reference.
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