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.
or a led as small zener NPN current source, if you need very high impedance you can stack a jfet on top of it
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.
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.
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
http://www.johnhardyco.com/pdf/990.pdf
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.
I appreciate the guidance. It seems there a few good ways to get this done. Now I just need to pick one and go.
Why would you need a P-channel equivalent for a two-terminal circuit? If the polarity is wrong, just swap the two terminals.
