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From: Phil Hobbs <pcdhSpamMeSenseless@electrooptical.net>
Newsgroups: sci.electronics.design
Subject: Re: Curve Tracer
Date: Fri, 7 Feb 2025 14:18:48 -0000 (UTC)
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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 
-- 
Dr Philip C D Hobbs  Principal Consultant  ElectroOptical Innovations LLC /
Hobbs ElectroOptics  Optics, Electro-optics, Photonics, Analog Electronics