Re: simple 150uA constant current supply

On Mar 30, 6:24 pm, John Larkin
<jjlar...@xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx> wrote:
On Sun, 30 Mar 2008 21:09:15 -0400, Fred Bloggs <nos...@xxxxxxxxxx>
wrote:





John Larkin 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.

John

Okay, very cool- so he needs to rectify and smooth the output <wink>, do
you have a problem with that?
Seriously, the GBW is like 25KHz...

If the opamp is very slow, as most micropower amps are, the main loop
will be stable. But the 3904 may, usually does, oscillate at a couple
hundred MHz on its own. A bit of resistance in series with the base,
33 ohms or so, will fix that. Strangely, the Zout of most opamps is
not enough to kill the Q of the base circuit.

I've found that on some of the micro power ones it is.

.

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