Re: Current source design (tricky?)

From: Fred Bloggs (nospam_at_nospam.com)
Date: 03/12/05 

Date: Sat, 12 Mar 2005 22:22:47 GMT

John Larkin wrote:
> On Sat, 12 Mar 2005 10:00:25 -0800, "Larry Brasfield"
> <donotspam_larry_brasfield@hotmail.com> wrote:
>
>
>>"Fred Bloggs" <nospam@nospam.com> wrote in message
>>news:4233017B.3010007@nospam.com...
>>
>>>
>>>Fred Bloggs wrote:
>>>[..rearrange that pull-up which may not even be necessary with these
>>>power PMOS with VGS,th > 2V minimum...]
>>>
>>> View in a fixed-width font such as Courier.
>>>
>>>.
>>>. +--------------------+-----+
>>>. | | | PMOS
>>>. | I-> | | +-------+
>>>.VBATT>---+------+---/\/\-----+---------------------|s d|---->
>>>. | | Rs 100m | | | | |
>>>. / / / | | | g |
>>>. 680 10k 10k | / +-------+
>>>. / / / | 4.7k |
>>>. \ \ \ LM741| / |
>>>. | | | |\ | \ |
>>>. | +------------|----|+\ | 330 |
>>>. | | | | >-----+---/\/\----+
>>>. | | +----|-/ |
>>>. | | | |/ | |
>>>. | | | | |
>>>. | | +--||---------+
>>>. | | | 1.5n |
>>>. | / / |
>>>. | 10k 10k |
>>>. | / / |
>>>. | \ \ |
>>>. | | 330 | |
>>>. Vd +-----------/\/\----+ |
>>>. | | | |
>>>. --- | / |
>>>. \ / | 100 |
>>>. --- | / |
>>>. 1n4148 | \ |
>>>. | | | |
>>>. | | | |
>>>. +------+------------+-------+
>>>.
>>
>>Do you believe that your diode forward drop used as
>>a reference will meet the OP's stated 5% accuracy
>>requirement over the voltage and temperature range
>>to be expected in an automotive application?
>>
>
>
> Cute circuit, but will probably have some resistor tolerance problems
> too.
>
>
> John
>

And don't forget offset- as I said- a few tens of milliamps here and
there makes no difference for this circuit. You go to any ss OA
data*** from National and you will find that darned "ground
referencing a voltage" example which has been there for the past 50
years- this is that ckt, with feedback loop closed by P-FET to balance
the inputs through Io*Rs drop instead of locally and an offset inserted.
You pick any group of 10KR out of the same package and they will all be
within 100 ohms even at 5%.