Constant Current Source question

rhafley · 27th November 2020, 03:13 PM

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.

nigelwright7557 · 27th November 2020, 03:30 PM

I always use a 2 bipolar transistor CCS.

basreflex · 27th November 2020, 04:30 PM

or a led as small zener NPN current source, if you need very high impedance you can stack a jfet on top of it

egellings · 27th November 2020, 08:08 PM

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.

MarcelvdG · 27th November 2020, 08:17 PM

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.

PRR · 27th November 2020, 10:52 PM

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

Fast Eddie D · 28th November 2020, 04:28 PM

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.

rhafley · 28th November 2020, 05:16 PM

I appreciate the guidance. It seems there a few good ways to get this done. Now I just need to pick one and go.

MarsBravo · 28th November 2020, 11:52 PM

Quote:

Originally Posted by egellings

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?

MarcelvdG · 29th November 2020, 03:02 PM

Why would you need a P-channel equivalent for a two-terminal circuit? If the polarity is wrong, just swap the two terminals.

27th November 2020, 03:13 PM	#1
rhafley diyAudio Member Join Date: Jan 2020	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.

27th November 2020, 03:30 PM	#2
nigelwright7557 diyAudio Member Join Date: Apr 2008 Location: Carlisle, England	I always use a 2 bipolar transistor CCS. __________________ 2021 versions of PCBCAD51 and PCBCAD720 out now >>> https://www.murtonpikesystems.co.uk

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27th November 2020, 04:30 PM	#3
basreflex diyAudio Member Join Date: Oct 2007	or a led as small zener NPN current source, if you need very high impedance you can stack a jfet on top of it

27th November 2020, 08:08 PM	#4
egellings diyAudio Member Join Date: Aug 2010 Location: CA	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.

27th November 2020, 08:17 PM	#5
MarcelvdG diyAudio Member Join Date: Mar 2003 Location: Haarlem, the Netherlands	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.

27th November 2020, 10:52 PM	#6
PRR diyAudio Member Join Date: Jun 2003 Location: Maine USA	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

28th November 2020, 05:16 PM	#8
rhafley diyAudio Member Join Date: Jan 2020	I appreciate the guidance. It seems there a few good ways to get this done. Now I just need to pick one and go.

29th November 2020, 03:02 PM	#10
MarcelvdG diyAudio Member Join Date: Mar 2003 Location: Haarlem, the Netherlands	Why would you need a P-channel equivalent for a two-terminal circuit? If the polarity is wrong, just swap the two terminals.