Re: fast NPN in LT Spice

On 12/06/2024 1:27 am, legg wrote:

On Wed, 12 Jun 2024 00:35:59 +1000, Bill Sloman <bill.sloman@ieee.org wrote:

On 11/06/2024 7:27 am, legg wrote:

On Sat, 8 Jun 2024 03:29:11 +1000, Bill Sloman <bill.sloman@ieee.org wrote:

On 7/06/2024 11:24 pm, legg wrote:

On Fri, 7 Jun 2024 14:03:16 +1000, Bill Sloman <bill.sloman@ieee.org> wrote:

On 7/06/2024 4:05 am, legg wrote:

On Thu, 6 Jun 2024 23:18:12 +1000, Bill Sloman <bill.sloman@ieee.org> wrote:

On 6/06/2024 1:46 pm, legg wrote

  <snip>

Right, I see Q2 normally inverted.
>
C2 seems unneccessarily loaded.

>
If you are talking about the .asc file I posted on the 3rd June, Q2
isn't "normally inverted" - its mostly on, and it gets turned off for
the 30nsec in which the stretched pulse is being generated. Inverted
operation of a bipolar transistor is usually taken to mean running
current through them in the opposite direction than is seen in normal
operation.

 Just returning to this, trying to see what determines the switch from
non-switching to astable operation (ECLTN0E - ECLTN0D). Misreading a
current and voltage waveform with similar color coding.

>
C2 isn't carrying any current worth worrying about. It stabilises the
voltage at the base of Q3 against the base current drawn when the output
pulse is turned on and turned off, -6.7mA when it is turned on an +2mA
when it is turned off, and holds the voltage excursion down to 3.7mV,
not that I ever bothered working this out.
>
If you want to discuss when the circuit does and how it does it, feel
free, but this wasn't a good start.

 Not so much interested in the circuit, as its reaction to model
parameters presented by similar devices.

You need to work out how the emitter-couple monostable works.
https://www.daenotes.com/electronics/digital-electronics/monostable-multivibrators-working-construction-types
actually does try to spell this out (in it's second section on emitter-coupled monstables) . It doesn't do it well, but it does it well enough that you should be able to work out what is going on, and keep track of the base-emitter voltages across both transistors and their effect on the collector current.
The Gummel-Poon transistor model keeeps track of the various currents flowing in and out of each transistor junction while this is going on, and subtle differences  in the parameter values can give you different currents (and different trajectories)

Not Beta, Tr or Tf in this case.

If you don't know what the circuit is doing, speculation about what the model might be doing it is a bit pointless.
The emitter-coupled monstable isn't well understood here.
This is from the end of a long thread in 2013.
On Mar 9, 3:54=A0am,JohnLarki<jjlar...@highNOTlandTHIStechnologyPART.com> wrote:
 > On Fri, 08 Mar 2013 01:18:53 -0600, John_Fiel<jfie...@austininstruments.com>
     > wrote:
     >
     > >On Thu, 07 Mar 2013 16:02:49 -0800, John Larkin
     > ><jjlar...@highNOTlandTHIStechnologyPART.com> wrote:
     >
     > >>http://www.highlandtechnology.com/DSS/T240DS.shtml
     >
     > >---
     > >That's not a pulse stretcher, cheater, that's a puls_generator_.
     >
     > It's a one-shot. It has no internal trigger. It generates no pulses.
     >
     > And neither you nor Jim have a clue as to how this might be done.
     >
     > We have a customer who wants us to take this down to 10 ps pulses. At that point, I'm not sure that I know how that might be done. We're thinking about it.
http://books.google.co.nz/books?id=3D-pi4vP6xMOQC&pg=3DPA571&lpg=3DPA571&dq=
=3D%22emitter-coupled%22+monostable&source=3Dbl&ots=3DCFsGlVE2YN&sig=3DTUbj=
QhyQPk_cd5tj_UKlIhFVXt8&hl=3Den&sa=3DX&ei=3DTZw6UYmmPMeNyAHA4oHYAg&ved=3D0C=
EIQ6AEwAw#v=3Donepage&q=3D%22emitter-coupled%22%20monostable&f=3Dfalse
describes the emitter-coupled monostable. Put one together out of a
pair of wide-band transistors - BFR92 or better - with 33R up against
each base, and you can certainly get below 10nsec. Since the mechanism
depends on the change of base-emitter impedance with emitter current,
it isn't as easy as it might be to get a wide range of output pulse
widths.
Jim Thompson could probably remember a better solution for you. The
long-obsolete MC10198 ECL monostable
http://www.digchip.com/datasheets/parts/datasheet/343/MC10198-pdf.php
could just get down to 10nsec, but we used two of them when we wanted
to offer long pulses as well - being able to switch in bigger
capacitors put too much stray capacitance on the relevant input pin
for 10nsec operation.
Something boringly obvious with a constant current ramp and a fast
comparator would do the job, but - as with the MC10198, being able to
switch in bigger capacitors to generate much longer pulses is probably
incompatible with a 10nsec pulse width.
--
Bill Sloman, Sydney
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