Current mirror with gain?

output.

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

John

output.

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

output.

On a sunny day (Sun, 25 Sep 2011 15:06:19 -0400) it happened Phil Hobbs wrote in :

DAC output.

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 wrote in :

output.

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.

ance DAC output.

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

DAC output.

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

output.

Use a classic 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 écrit :

output.

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.

Fred Bartoli a écrit :

Given the number of 'and' I managed to put in two sentences, this one sure isn't a "low and" circuit...

nce DAC output.

Asymmetric current mirrors are fun; if you feed a low current into one side of a current mirror and jack up the emitter of the low-current carrying transistor by 60mV with respect to the other half of the current mirror, you get ten times as much current through the other side of the mirror, while 120mV gives you a hundred times as much (with a high-beta transistor). You usually need to compensate out the source resistance of the higher current side by giving the voltage generator an appreciable resistance - respectively 10 or 100 times the source resistance in these two examples.

The "60mV" is actually proportional to absolute temperature, so you may end up putting a metal resistance temperature sensor into the voltage generator.

-- Bill Sloman, Nijmegen

output.

Have you got room for an IC of matched transistors?

output.

Is this what you're looking for?

VCC + | Iout ~= In * R1/R2 | .-. | | | | RL '-' +---+----- + | | | | | In | | |/ Q1 |/ Q2 | |----|---------| | |> |>

| | | |+ | | / \ .-. .-. ( I ) | | | | \_/ | |R1 | |R2 | '-' '-' | | | | | | === === === GND GND GND

Best regards, Spehro Pefhany

DAC output.

The non-monolithic dual transistors I've tried are thermally a little better than two separate transistors, but not that much.

ftp://jjlarkin.lmi.net/UPA800_80mW_one-side.jpg

They probably *are* well matched, since they usually come from the same wafer.

John

output.

Defining character is '~' ...Jim Thompson

output.

He needs a high mirror ratio, like 1000:1 roughly. That's going to have beta problems. A PNP follower would help, powered from V+. Too hard to draw in ASCII.

It's not clear where he needs his load referenced to, or how accurate it needs to be.

John

output.

Well, is there any reason you can't use a 2N7000 MOSFET, source to GND, drain to sink the current, and feed a calibrated command voltage to its gate? It'd take calibration, of course. A source resistor would be nice, but of course that can hurt output compliance.

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Not if he needs 1000:1. There are lots of tiny tiny op-amps that will do the trick.

Best regards, Spehro Pefhany