Re: Curve Tracer

On Fri, 7 Feb 2025 14:18:48 -0000 (UTC), Phil Hobbs
<pcdhSpamMeSenseless@electrooptical.net> wrote:

piglet <erichpwagner@hotmail.com> wrote:

Bill Sloman <bill.sloman@ieee.org> wrote:

On 7/02/2025 8:18 am, john larkin wrote:

On Thu, 6 Feb 2025 23:11:58 +1100, Chris Jones
<lugnut808@spam.yahoo.com> wrote:
 

On 6/02/2025 2:45 pm, Bill Sloman wrote:

On-Semi makes two monolithic duals, the NST45010 and the NST45011
 
https://www.onsemi.com/pdf/datasheet/nst45010mw6-d.pdf
https://www.onsemi.com/download/data-sheet/pdf/nst45011mw6-d.pdf
 

 
What makes you think those are monolithic? I think they are separate
chips, but measured to have similar parameters, like the BCM846BS.
 
The thermal coupling between the chips will be poor, so they will no
longer be matched if the dissipation is not the same between them. You
could cascode a current mirror to fix that, but if you are trying to
make an exponentiator (as used in analogue synth VCOs) then you are
stuffed, because you need to operate the two transistors at different
currents, that being the whole point of the circuit.
 
You will know if they are monolithic because it will have a pin called
"substrate" or a note saying one of the pins is the substrate, and there
will be a spec pointing out that the voltage between the two devices
must be kept below some lowish value.

 
You would know if they were monolithic if they did have a substrate pin.
 
The fact that they haven't got one isn't proof that they aren't
monolithic. A stronger argument is that they haven't put any limits on
device-to-device voltages.
 
My reason for thinking that they were monolithic was the 2mV worst case
and the 1mVB typical difference in Vbe at 2mA.
 
Monolithic does seem to offer the cheapest route to get that.
 

They are two similar chips, not monolithic. Thermals will be awful.

 
Prove it.
 
They may be two separate close-to-identical chips. There isn't room in
the package to mount them far apart, and the chip to chip thermal
resistance can't be large, and has to be much smaller than the package
to ambient thermal resistance, which is 328C/Watt.
 
Thermals won't be awful. Somebody who doesn't know about Wilson current
mirrors isn't going to be a particularly reliable source of information
about that kind of subject.
 
Interdigitated monolithic is hard to beat for thermal matching but
side-by-side devices on the same subtrate aren't going to be any better
than  devices on separate substrates if the substrates are mounted
back-to-back.
 
--
Bill Sloman, Sydney
 
 

 
I seem to remember someone here has an xray machine which could answer the
question?
 

>
It’s a FAQ that we’ve gone through many times, including my doing a bit of
math on the datasheet for the BCV61 current mirror that used its thermal
runaway spec to estimate the die-to-die thermal resistance.
>
Turns out to be about the same as the die-to-ambient, 300-500 K/W.
>
They really aren’t monolithic.
>
Cheers
>
Phil Hobbs
There

Right. Those dual-chip things are not much better thermally then two
SOT-23s mounted close on a board.

I make current mirrors now and then, with an opamp and a mosfet. Or,
sometimes, just an opamp. That's way more accurate than the transistor
versions.

Hey, Dr Hobbs, what is the attraction that PhDs seem to have for
current mirrors?