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design | Current mirror with gain?


There are 50 messages in this thread.

You are currently looking at messages 1 to 10.

Current mirror with gain? - Jan Panteltje - 2011-09-25 14:46:00

For some application I need a constant current drive of a few mA,
but I only can supply some uA from the opposite rail from a high impedance DAC output.
A single transistor would do, with a base resistor as drive,
but the gain depends on beta, and that drifts with temperature,
and from part to part.
Any ideas?






Re: Current mirror with gain? - John Larkin - 2011-09-25 14:57:00

On Sun, 25 Sep 2011 18:46:12 GMT, Jan Panteltje
<p...@yahoo.com> wrote:

>For some application I need a constant current drive of a few mA,
>but I only can supply some uA from the opposite rail from a high impedance DAC output.
>A single transistor would do, with a base resistor as drive,
>but the gain depends on beta, and that drifts with temperature,
>and from part to part.
>Any ideas?
>
>
>
>

ftp://jjlarkin.lmi.net/Ipump.JPG

John


Re: Current mirror with gain? - Phil Hobbs - 2011-09-25 15:06:00

On 09/25/2011 02:46 PM, Jan Panteltje wrote:
> For some application I need a constant current drive of a few mA,
> but I only can supply some uA from the opposite rail from a high impedance DAC output.
> A single transistor would do, with a base resistor as drive,
> but the gain depends on beta, and that drifts with temperature,
> and from part to part.
> Any ideas?

Well, the default approach would be an op amp driving a BJT current 
source--you get good high frequency performance (from the BJT) and good 
accuracy (from the op amp).  Any special constraints?

Cheers

Phil Hobbs


-- 
Dr Philip C D Hobbs
Principal Consultant
ElectroOptical Innovations LLC
Optics, Electro-optics, Photonics, Analog Electronics

160 North State Road #203
Briarcliff Manor NY 10510
845-480-2058

hobbs at electrooptical dot net
http://electrooptical.net

Re: Current mirror with gain? - John Fields - 2011-09-25 15:44:00

On Sun, 25 Sep 2011 18:46:12 GMT, Jan Panteltje
<p...@yahoo.com> wrote:

>For some application I need a constant current drive of a few mA,
>but I only can supply some uA from the opposite rail from a high impedance DAC output.
>A single transistor would do, with a base resistor as drive,
>but the gain depends on beta, and that drifts with temperature,
>and from part to part.
>Any ideas?

---
Like this?
                +V
                |
             [LOAD]
                |
Vin>----|+\     C
        |  >--B
     +--|-/     E
     |          |
     +----------+
                |
               [R]
                |
               GND

 
-- 
JF

Re: Current mirror with gain? - Jan Panteltje - 2011-09-25 16:14:00

On a sunny day (Sun, 25 Sep 2011 15:06:19 -0400) it happened Phil Hobbs
<p...@electrooptical.net> wrote in
<4...@electrooptical.net>:

>On 09/25/2011 02:46 PM, Jan Panteltje wrote:
>> For some application I need a constant current drive of a few mA,
>> but I only can supply some uA from the opposite rail from a high impedance DAC output.
>> A single transistor would do, with a base resistor as drive,
>> but the gain depends on beta, and that drifts with temperature,
>> and from part to part.
>> Any ideas?
>
>Well, the default approach would be an op amp driving a BJT current 
>source--you get good high frequency performance (from the BJT) and good 
>accuracy (from the op amp).  Any special constraints?

Yes, maximum 2 transistors, no opamsp, only 5V supply, very liitle current.



Re: Current mirror with gain? - Jan Panteltje - 2011-09-25 16:17:00

On a sunny day (Sun, 25 Sep 2011 11:57:46 -0700) it happened John Larkin
<j...@highNOTlandTHIStechnologyPART.com> wrote in
<j...@4ax.com>:

>On Sun, 25 Sep 2011 18:46:12 GMT, Jan Panteltje
><p...@yahoo.com> wrote:
>
>>For some application I need a constant current drive of a few mA,
>>but I only can supply some uA from the opposite rail from a high impedance DAC output.
>>A single transistor would do, with a base resistor as drive,
>>but the gain depends on beta, and that drifts with temperature,
>>and from part to part.
>>Any ideas?
>>
>>
>>
>>
>
>ftp://jjlarkin.lmi.net/Ipump.JPG
>
>John

John, that is a very nice circuit, but I do not have the space for opamps
and stuff, max 2 transistors and some R I would think.

Re: Current mirror with gain? - 2011-09-25 16:47:00

On Sep 25, 4:14 pm, Jan Panteltje <pNaonStpealm...@yahoo.com> wrote:
> On a sunny day (Sun, 25 Sep 2011 15:06:19 -0400) it happened Phil Hobbs
> <pcdhSpamMeSensel...@electrooptical.net> wrote in
> <4E7F7BAB.4070...@electrooptical.net>:
>
> >On 09/25/2011 02:46 PM, Jan Panteltje wrote:
> >> For some application I need a constant current drive of a few mA,
> >> but I only can supply some uA from the opposite rail from a high impedance DAC output.
> >> A single transistor would do, with a base resistor as drive,
> >> but the gain depends on beta, and that drifts with temperature,
> >> and from part to part.
> >> Any ideas?
>
> >Well, the default approach would be an op amp driving a BJT current
> >source--you get good high frequency performance (from the BJT) and good
> >accuracy (from the op amp).  Any special constraints?
>
> Yes, maximum 2 transistors, no opamsp, only 5V supply, very liitle current.

An SOT-23-5 op amp is the same size, runs on +5v, and it's accurate.

If that's really unacceptable, you could do the "old PNP follower -->
NPN stage with an emitter resistor," or the complement of that.

Not as accurate, but it fits your constraints.

--
Cheers,
James Arthur

Re: Current mirror with gain? - John Larkin - 2011-09-25 17:12:00

On Sun, 25 Sep 2011 20:17:51 GMT, Jan Panteltje
<p...@yahoo.com> wrote:

>On a sunny day (Sun, 25 Sep 2011 11:57:46 -0700) it happened John Larkin
><j...@highNOTlandTHIStechnologyPART.com> wrote in
><j...@4ax.com>:
>
>>On Sun, 25 Sep 2011 18:46:12 GMT, Jan Panteltje
>><p...@yahoo.com> wrote:
>>
>>>For some application I need a constant current drive of a few mA,
>>>but I only can supply some uA from the opposite rail from a high impedance DAC output.
>>>A single transistor would do, with a base resistor as drive,
>>>but the gain depends on beta, and that drifts with temperature,
>>>and from part to part.
>>>Any ideas?
>>>
>>>
>>>
>>>
>>
>>ftp://jjlarkin.lmi.net/Ipump.JPG
>>
>>John
>
>John, that is a very nice circuit, but I do not have the space for opamps
>and stuff, max 2 transistors and some R I would think.

It could be simpler if you don't need a bunch of accuracy and can
afford some nonlinearity, some deadband maybe.

The extreme of hackedness would involve two transistors and one
resistor, and would be beta dependent on one of them. Use a
beta-graded part on the high side, like BCX71 maybe.

It would be an NPN current driver on the dac side and a single PNP on
the high side. That would be fine if, say, there's some overall
software feedback somewhere around the whole loop.

John




Re: Current mirror with gain? - John S - 2011-09-25 17:33:00

On 9/25/2011 1:46 PM, Jan Panteltje wrote:
> For some application I need a constant current drive of a few mA,
> but I only can supply some uA from the opposite rail from a high impedance DAC output.
> A single transistor would do, with a base resistor as drive,
> but the gain depends on beta, and that drifts with temperature,
> and from part to part.
> Any ideas?
>

Use a classic current mirror:

http://en.wikipedia.org/wiki/Current_mirror

except with two PNPs in the same package such as NXP's PMBT3906YS,115 
which is available from Mouser for 5 cents in a quantity of one.

If you don't like those, NXP makes a lot of duals.

No guarantee that the two devices are matched, but at least they are 
reasonably coupled thermally.

Cheers,
John S


Re: Current mirror with gain? - Fred Bartoli - 2011-09-25 17:54:00

Jan Panteltje a écrit :
> For some application I need a constant current drive of a few mA,
> but I only can supply some uA from the opposite rail from a high impedance DAC output.
> A single transistor would do, with a base resistor as drive,
> but the gain depends on beta, and that drifts with temperature,
> and from part to part.
> Any ideas?
> 
> 
> 
> 
> 

If you can cope with the opamp minimum supply voltage (as low as 1V for 
some models), the min supply quiescent current and don't need a too high 
frequency response then that one is simple enough with a RR opamp:

                       v
                       |
                       |
                       |
                     |\|    Ro
          >----+-----|+\    ___
               |     |  >--|___|-.
               |   .-|-/         |
               |   | |/|         |
               |   '---+---------'
               |       |
              .-.     .-.
              | |     | |
              | |     | |
              '-'     '-'
               |       |
              ===     ==              GND     GND


And be careful with the stability issues... and Ro is there have a 
manageable loop gain.



-- 
Thanks,
Fred.

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