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Constant Current Source question
Constant Current Source question
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Old 27th November 2020, 03:13 PM   #1
rhafley is offline rhafley
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Default Constant Current Source question

I'm looking for helping choosing an appropriate constant current source for a discrete op-amp preamp I'm working on. It's a 4mA tail current source for a jfet pair that goes between the jfet source and the negative 15v rail. Obviously a resistor or current limiting diode is the simplest solution. I've also looked at some circuits using a TLV431/transistor/resistor combo but I would have to figure out how to implement it in a negative voltage circuit. I was hoping some of the wizards here can provide some direction.
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Old 27th November 2020, 03:30 PM   #2
nigelwright7557 is offline nigelwright7557  United Kingdom
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I always use a 2 bipolar transistor CCS.
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Old 27th November 2020, 04:30 PM   #3
basreflex is offline basreflex  Spain
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or a led as small zener NPN current source, if you need very high impedance you can stack a jfet on top of it
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Old 27th November 2020, 08:08 PM   #4
egellings is offline egellings  United States
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Use a depletion mode MOSFET, like the DN2540 in either a TO-92 or TO-220 case. A resistor sets it up, and it's a 2-terminal source or sink. If you have about 6V of headroom in the circuit, a cascode version can be made with better characteristics using 2 of the devices.
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Old 27th November 2020, 08:17 PM   #5
MarcelvdG is offline MarcelvdG  Netherlands
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A TLV431 generates quite a lot of noise (just over 200 nV/sqrt(Hz)). If noise matters, I'd rather go with Nigel's or basreflex's solution.
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Old 27th November 2020, 10:52 PM   #6
PRR is offline PRR  United States
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Constant Current Source question
It's from the last century but the biasing in the Jensen 990 still works OK. John Hardy wrote a good description:
http://www.johnhardyco.com/pdf/990.pdf
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Old 28th November 2020, 04:28 PM   #7
Fast Eddie D is offline Fast Eddie D  United States
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Quote:
John Hardy
He's only a couple of miles from my house! I never heard of him.

I would like to meet him. I need a local electronics buddy.
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Old 28th November 2020, 05:16 PM   #8
rhafley is offline rhafley
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I appreciate the guidance. It seems there a few good ways to get this done. Now I just need to pick one and go.
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Old 28th November 2020, 11:52 PM   #9
MarsBravo is offline MarsBravo  Netherlands
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Quote:
Originally Posted by egellings View Post
Use a depletion mode MOSFET, like the DN2540 in either a TO-92 or TO-220 case. A resistor sets it up, and it's a 2-terminal source or sink. If you have about 6V of headroom in the circuit, a cascode version can be made with better characteristics using 2 of the devices.
Very smart, I like this solution!
Is a Pch equivalent available?
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Old 29th November 2020, 03:02 PM   #10
MarcelvdG is offline MarcelvdG  Netherlands
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Why would you need a P-channel equivalent for a two-terminal circuit? If the polarity is wrong, just swap the two terminals.
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