Re: simple 150uA constant current supply


"Fred Bloggs" <nospam@xxxxxxxxxx> schreef in bericht
news:47F03544.3070408@xxxxxxxxxxxxx

Thanks Martin but, it's not homework, I don't have any space for an
IC and this is battery powered. I can only spare about 30uA to
'generate' this supply.

There are microscopic nanopower opamps from Linear and others. Since all
you want is nominal +/-20% , and this is the same as Vbatt ranging from
2-3V, then something simple that does not require much battery current
would be like so, total bias around 16ua, compliance should be to within a
few hundred millivolts of the rail, and you could do even better with a
PFET:

A simple asymmetric current mirror built with a dual PNP transistor - as
drawn below - or an NPN part would have much the same accuracy (depending on
the Vbe matching between the two transistors in the dual) would probably be
cheaper and would use fewer parts, and is unlikely to oscillate.

Farnell stock four NPN and five PNP duals from NXP (was Philips) as well as
five current mirrors - three NPN and two PNP three of them from NXP and and
two from Infineon (was Siemens).
You'd need to look at the data sheets to work out which would suit you best

View in a fixed-width font such as Courier.

BATT---------------+-----------.
. | |
. | |
. [3K9] |
. | |
. | |
. / /
. |< |<
. | |
. +---------+-----------|
. | |\ |\
. | | |
. | | |
. | | Iout
. | |
. [166k] |
. | |
. | |
. COM---+-----------+-------

The 10:1 asymmetry is a bit on the high side - your spare 30uA would
accomodate 5:1 - 80k and 1k4

--
Bill Sloman, Nijmegen


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