Re: Curve Tracer

On 2025-02-07 10:52, john larkin wrote:
 > 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
 >>>>>>>
 >>>>>>
 >>>>>>
 >>>>>>> Whatmakes 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 betweenthem. 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 pincalled>>>>>> "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
 >>>> substratepin.>>>>
 >>>> 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\u2019s a FAQ that we\u2019ve gone through many times, including
 >> mydoing 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\u2019t monolithic.
 >>
 >> Cheers
 >>
 >> Phil Hobbs
 >
 > 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?
The "Dr Hobbs" business is strictly reserved for opposing counsel. ;)
The basic reason is to be able to use current-mode circuitry.
I use current mirrors in some designs, e.g. my best laser noise canceller, where I need replicas of the currents in the main diff pair to cancel the effects of R_ee' and R_bb'.
My fave is the switcheroo Wilson, which is a double-cascode design (two
transistors per side), where the diode-connected transistors are in opposite corners, like this:
  O OUT         O  IN
  |  |          |  |
  |  V   *------*  V
  |      |      |
   \|    |    |/
    |----*----|
   V| Q1   Q2 |V
  |             |
  *------*      |
  |   Q3 | Q4   |
   \|    |    |/
    |----*----|
   V|         |V
   |           |
   |           |
  GND         GND
This actually has three distinct feedback loops (two transdiodes and the big loop), and cancels beta error to leading order.
Since all of the transistors are operating at the same collector
current, you can apply a couple of small emitter resistors and a dual op amp to null out the V_BE offsets.  The dissipation in Q2-Q4 is generally small enough that the thermals don't hurt much. (The V_BE error of Q1, which potentially dissipates much more, gets taken out in the main loop--it appears between the collector of Q4 and the emitter of Q2.)
Larger amounts of emitter degeneration will also equalize the currents in a restricted current range, at the price of trashing the bandwidth (nobody talks about that).
There are a few devices, AFAIK only SiGe, that have high enough Early
voltages that you can equalize the dissipation in the two sides by cascoding them and dorking one of the V_CEs so that I_C*V_CE is the same.
That's magic--you can basically reproduce the performance of a 40 GHz
monolithic diff pair using two separate transistors.
(All of which takes several extra parts, but you can do some pretty stunning things, for sufficiently permissive definitions of 'stunning'.)
For lower performance things, a bunch of outfits will make semicustom
ICs on an old-fashioned planar NPN process (300 MHz-ish) for a few $k of NRE.  I've often thought of making a laser noise canceller ASIC, but the performance isn't good enough--betas of 50, no fast PNPs, and so on.
Cheers
Phil Hobbs