Re: simple 150uA constant current supply

On Mon, 31 Mar 2008 06:11:11 -0700 (PDT), MooseFET
<kensmith@xxxxxxxxx> wrote:

On Mar 30, 5:59 pm, John Larkin
<jjlar...@xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx> wrote:
On Sun, 30 Mar 2008 20:50:12 -0400, Fred Bloggs <nos...@xxxxxxxxxx>
wrote:





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:
View in a fixed-width font such as Courier.

.
.
.
. BATT---+-----------+-----------.
. | | |
. | | |
. [3.9k] | [390]
. | | |
. | .----|-----------+
. | | | |
. | | |\ | |
. | '-|-\| |<
. | | >---------| 2N3904
. +--------|+/| |\
. | |/ | |
. | | |
. | | Iout
. | |
. [166k] |
. | |
. | |
. COM---+-----------+-------
.
.
.

That circuit will usually oscillate, sometimes at two different
frequencies simultaneously.

Not if you use a really-really slow op-amp. The real part of the Zout
of the op amp will be high enough to prevent the 2N3904 from tuning up
and the 2N3904 is so fast compared to the op-amp that it adds no
noticable phase shift.

If you are using a non-really slow op-amp:
Modified version:
.
.
.
. BATT---+-----------+-----------.
. | | |
. | | |
. [3.9k] | [390]
. | | |
. | .----|--+--[R]---+
. | | | ! |
. | | |\ | [C] |
. | '-|-\| ! |<
. | | >--+-[R]--| 2N3904
. +--------|+/| |\
. | |/ | |
. | | |
. | | Iout
. | |
. [166k] |
. | |
. | |
. COM---+-----------+-------
.


Now add a ferrite bead in the collector. That doesn't affect the
oscillation situation much, but it does improve the
constantness-of-the-current at higher frequencies.

And bypass the 3.9K, for Pete's sake.

John

.