Current mirror with gain?

Started by Jan Panteltje September 25, 2011
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?





On Sun, 25 Sep 2011 18:46:12 GMT, Jan Panteltje
<pNaonStpealmtje@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
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
On Sun, 25 Sep 2011 18:46:12 GMT, Jan Panteltje
<pNaonStpealmtje@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
On a sunny day (Sun, 25 Sep 2011 15:06:19 -0400) it happened Phil Hobbs
<pcdhSpamMeSenseless@electrooptical.net> wrote in
<4E7F7BAB.4070104@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.
On a sunny day (Sun, 25 Sep 2011 11:57:46 -0700) it happened John Larkin
<jjlarkin@highNOTlandTHIStechnologyPART.com> wrote in
<j6uu7792hh83j576qrbrv8ccevpsusam25@4ax.com>:

>On Sun, 25 Sep 2011 18:46:12 GMT, Jan Panteltje ><pNaonStpealmtje@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.
On Sep 25, 4:14=A0pm, 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 imped=
ance 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). =A0Any special constraints? > > Yes, maximum 2 transistors, no opamsp, only 5V supply, very liitle curren=
t. 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
On Sun, 25 Sep 2011 20:17:51 GMT, Jan Panteltje
<pNaonStpealmtje@yahoo.com> wrote:

>On a sunny day (Sun, 25 Sep 2011 11:57:46 -0700) it happened John Larkin ><jjlarkin@highNOTlandTHIStechnologyPART.com> wrote in ><j6uu7792hh83j576qrbrv8ccevpsusam25@4ax.com>: > >>On Sun, 25 Sep 2011 18:46:12 GMT, Jan Panteltje >><pNaonStpealmtje@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
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
Jan Panteltje a &#2013265929;crit :
> 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.