Re: Constant Current Source Design

From: Nicolai (nmahncke_at_tiscali.dk)
Date: 01/25/05 

Date: Tue, 25 Jan 2005 18:23:16 +0100

Have a look at the Fixed Current Source schematic on page 10 of
http://cache.national.com/ds/LM/LM124.pdf
Exchange the PNP transistor with a P-channel MOSFET with low Vgs threshold
and recalculate the resistor values for your own needs and it should do the
trick. If you need better accuracy just use a better OP-AMP and resistors.

The High Compliance Current Sink on page 12 will do the same trick if you
need to sink instead of source the current. Again just substitute the NPN
transistor with an N-channel MOSFET and recalculate for your needs. The
current sink circuit has the advantage that it can be made to work with any
voltage as long as you use the right MOSFET.

You can keep the transistors if you don't like MOSFETs but then you will
have a small error contribution from the base current. Old datasheets are
abundant with similar circuits.

HTH

"John Woodgate" <jmw@jmwa.demon.contraspam.yuk> wrote in message
news:SVfBDwGd4W9BFw1n@jmwa.demon.co.uk...
>I read in sci.electronics.design that confused soul
> <confused.mind@gmail.com> wrote (in <1106601781.005044.163500@z14g2000cw
> z.googlegroups.com>) about 'Constant Current Source Design', on Mon, 24
> Jan 2005:
>
>>Looking at the previous replies, I kinda feel dumb that I didnt even
>>think about the possible voltage required. 100 V seems impractical in
>>my case. It is possible for me to change the constant current value to
>>0.01 mA or even 1 uA instead of 0.1 mA. In that case, what kind of
>>circuits can be used for building a constant current source.
>>
> A few weeks ago we had a thread here in which a FET-based current source
> could give output source impedances of gigohms. That should be OK for
> you! If you can't search the group archive, maybe someone will give you
> a message ID to look up or a copy of one of the relevant posts.
> --
> Regards, John Woodgate, OOO - Own Opinions Only.
> The good news is that nothing is compulsory.
> The bad news is that everything is prohibited.
> http://www.jmwa.demon.co.uk Also see http://www.isce.org.uk

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