Precision current mirror

Well, you can certainly make three transistors into a 1:2 current mirror, and generic transistors will go that fast. Tweaking the full 13+ bits accuracy out of it is your problem. ;-)

Tim

-- Deep Friar: a very philosophical monk. Website:

Tim Williams should know better than to top post.

A Howland current source could provide the accuracy, but you'd need a very fast amplifier and resistors with very low parallel capacitance to maintain the bandwidth.

L-trimmed surface mount precision resistors at the resistance levels you'd need should be good enough, and there are plenty of really fast op amps around.

Driving the desired current into the source of a fast FET and taking the constant current from the drain is another popular approach, but

40mA implies a biggish FET, and the capacitances between drain and source and drain and gate might be a bit too big for comfort at 50nsec. 40mA would not be a problem for most MOSFETs - even the SD214 will take 50mA of drain current, though you might need two in paralle to keep the gate-to-source voltage down to practical levels.

-- Bill Sloman, Nijmegen

What's that supposed to mean? It was a short (three line) reply to the post at large, no particular quote needed.

Tim

--
Deep Friar: a very philosophical monk.
Website: http://webpages.charter.net/dawill/tmoranwms

..err..all of the current going into a FET source would be seen at the drain (assuming zero gate current which in normal operation is valid). Therefore, it is useless by itself to "amplify" 20mA to 40mA. Try again.

post

It was a pretty silly reply which did invite further comment - it would have been tidier if your comment had ended up below the OP's request, rather than above it.

-- Bill Sloman, Nijmegen

r,

s .

So much should be obvious to the meanest intellect.

I'd started off by recommending the Howland current source, which allows you to scale a tolerably arbitary input to generate a current output with close to infinite output impedance. Discussions of the Howland circuit almost always point out that it is easier to make a unipolar current source or sink by relying or a FET or a BJT to isolate the driving amplifier from the load.

The OP's 13-bit precision requirement more or less cuts out BJT's - the base currents are a little too unpredictable and temperature dependent to make such a circuit practical - but FETs and MOSFET's are a real possibility.

I would have thought it was obvious to the meanest intellect that that the op amp drivng the FET/MOSFET would have to boost the OP's output up to the 40mA desired, but you have proved me wrong.

Try thinking about what has been posted before you start trying to teach your grandmother to suck eggs.

-- Bill Sloman, Nijmegen

Bill, Thanks for the suggestion. Entering "currents sources" in google produced a lot of faff, but entering "howland" was rather more specific. That was useful!

However, reading about Howland sources (and if I understand it right) it appears their precision is overly sensitive on component accuracy, and possibly unable to provide a 13 bit resolution unless I use incredibly accurate and matched resistors.

Are there any other current source architectures, names or keywords I could try?

TIA

[snip]

That's not what YOUR grandmother sucks ;-)

...Jim Thompson

-- | James E.Thompson, P.E. | mens | | Analog Innovations, Inc. | et | | Analog/Mixed-Signal ASIC's and Discrete Systems | manus | | Phoenix, Arizona 85048 Skype: Contacts Only | | | Voice:(480)460-2350 Fax: Available upon request | Brass Rat | | E-mail Icon at

| 1962 | Democrats, the party of NO

Both of them are dead, as someone with any grasp of reality would expect, and don't suck anything any more.

And I did have two of them - odd as this may seem to somebody with West Virginia hillbilly in his background.

-- Bill Sloman, Nijmegen

ror,

its

..

and

You can buy 0.1% 15ppm resitors off the shelf from Farnell and other broad-line distributors. 0.1% is 10-bits.

You can trim a Howland circuit with a strategically placed trim-pot to do better - 13-bits is probably attainable, though soldered-in parallel trimming resistors would be more stable.

Farnell also stocked really tight-tolerance resistor arrays with very close tolerances on the initial matching and the temperature matching. I've not got access to a Farnell catalogue at the moment and I can't be bothered searching their web-site.

You should have found Bob Pease's application note

gives two - very basic - op amp based current sources that don't depend on well-matched resistors, both of which use FETs as output devices. MOSFETs would also serve.

-- Bill Sloman, Nijmegen

Thanks Bill, Yes I am aware of the application notes, and breadboarded many of them with little success, hence my original posting. They are either not fast enough, or require too much fiddling and trimming with components.

However I have found a very neat circuit (fig 12 of Burr Brown application note AB-165) It only needs two matched resistors and a 1x gain amplifier.

snip

I guess Jim could answer this but, if you got 3 transistor in a package on a single die I assume, and made a one to two transistor in parallel mirror how well would they match and track?

-Lasse

The main problem with the simple 3 transistor current mirror is that it produces a current "sink" from the original current "source"

To produce a matching current "source" from another "source" you need a more complex arrangement.