Problem with current mirrors

Early effect. Change of collector current with collector voltage.

Cure it by dropping several volts in the emitter path... add R's in series to Q1:E, Q2:E, Q13:E, Q12:E, etc. ...Jim Thompson

Emitter resistors help, or you can cascode it (Wilson mirror, etc.), or both.

Why use Q8, 9, 12 and 13, why not run Q2 straight into R40?

Why all the resistors? You can remove more than half with no change in performance. R15, 16, 21, 22, 23, 37, 39 and 40, which basically leaves R12, 19, 27, 28 and 48.

Tim

What is the overall circuit supposed to do?

Discrete-transistor current mirrors like this tend to be very inaccurate.

John

How about something simpler? Try this for starters [1] maybe:

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

John

Not if you know what you're doing. See my post...

From: Jim Thompson Newsgroups: sci.electronics.design Subject: Re: Problem with current mirrors Date: Tue, 23 Feb 2010 07:53:39 -0700 Message-ID: ...Jim Thompson

This circuit won't work without global feedback keeping it in place. Current mirrors are not going to be any accurate.

VLV

The OP is referencing to the Vbe of Q7, probably stable enough if temperature range isn't an issue. Add a zener under the emitter of Q7 if you fret; or replace the Q5/Q7 assembly with a TL431. ...Jim Thompson

John Larkin a écrit :

That's fine but why the reference and biasing/depletion Fet? Loose them and just use the input logic levels. One might want to add some VGS clamping zener diodes to allow maximum FET drive in all temperature conditions.

"[1]" is called an ego protector in case John is wrong... as he often is ;-) ...Jim Thompson

That is a pretty elaborate cct. to just switch a Highside PFET and a low side NFET. If your willing to use all those components just to switch a HS PFET you might as well use a NFET on the HS and bootstrap the drive. The whole reason you would use a PFET on the HS is to reduce the component count and complexity you defiantly aren't doing that.

What is its purpose? Active inrush suppression?

Odd! I didn't find the OP's post to be "defiant" ;-)

But he indeed didn't ask for alternate circuits, he asked _why_ his mirrors were inaccurate. ...Jim Thompson

What are you really doing here? What's this circuit driving? How fast are you switching the FETs?

Whatever else, you have enough stuff tossed on the board that you may well be able to reach your end goal with an IC or three, with less board space used, and fewer manufacturing techs confused.

I know my spelling has really gone downhill. :-)

To much relying on software spell checkers.

If all he wants to do is switch the fets on and off, he doesn't need current mirrors at all. The whole thing can be done with 4 to 6 parts.

John

It looks like Q12, Q13, Q9 are all unnecessary; take collector current of Q2 instead directly to emitter of Q8, and ground the base of Q8. I'm also unclear on the expected behavior, but is R37 really the right size to carry the desired current? Your Q2 might just be saturating instead of being in the gain region.

Yep. Too many "fro" accepted for "for" :-( ...Jim Thompson

An extreme demonstration, I picked a transistor with VAF=20V, then showed mirrors with and without emitter "ballast" resistors...

...Jim Thompson

Well, it might help to use Vbe matched transistors so that the collector currents are reasonably matched. Also, i note there are about 3 times the transistors needed to do the job.

Cool! Not many use common base...