AoE x-Chapters, 4x.26, MOSFET current source, nodal analysis

On Sat, 10 Aug 2019 11:50:08 -0500, "Tim Williams"
<tiwill@seventransistorlabs.com> wrote:

>"Tom Gardner" <spamjunk@blueyonder.co.uk> wrote in message 
>news:PFA3F.982900$j33.789547@fx26.am4...
>> Nonetheless, an analytical solution to a /simplified/ model
>> can yield valuable insights. The classic simplified model
>> in physics is exemplified by "...assume a spherical cow...".
>>
>> There are many similar things in electronics, e.g. simple
>> model are used to estimate EMI/EMC between one comms system
>> and another. Imperfect? Of course; it never matches reality.
>> Useful? Yes.
>
>Don't read into it too deeply.  JL is not so talented at analysis, and 
>forgets not to project his capability onto others.

Not many people are talented at producing closed-form solutions to
nonlinear system response. Most interesting electronic systems are
nonlinear. What's a boy to do?

One big field where instinct+simulation usually (but not always) fails
is filter design. Things can get crazy and diverge fast.

I have managed up to 5th order, mostly luck. I've done some nice
one-side-absorptive lowpass filters by fiddling in Spice. The
criterion for absorbing cable reflections was fuzzy, what looks good.

"Modern filter design" is computer based anyhow. That's where the
tables in the books come from.


-- 

John Larkin   Highland Technology, Inc   trk

jlarkin att highlandtechnology dott com
http://www.highlandtechnology.com

On Saturday, August 10, 2019 at 12:54:06 PM UTC-7, tabb...@gmail.com wrote:
> On Saturday, 10 August 2019 18:55:51 UTC+1, Tim Williams  wrote:
> 
> > An interesting thing, as security goes: for all their problems, Apple has 
> > been quite staunch in their rejection of such softening measures.

> Was Apple's public stand for real, or was it showmanship when they complied behind closed doors? We'll probably never know.

Oh, we'll know.   This kind of thing is a very temporary secret, at best.

Cisco sales (and stock) plummeted when their shipped products were found to have
(official US government) tampering, and Apple's iPhone sales are too important
to risk, so the public stand is probably also the private one.

Cisco's recent packaging is tamper-evident and more secure against
counterfeiting than most currency.   It's a SERIOUS concern that Apple
and even Huawei must recognize, publicly and privately.

On Saturday, 10 August 2019 21:14:09 UTC+1, whit3rd  wrote:
> On Saturday, August 10, 2019 at 12:54:06 PM UTC-7, tabby wrote:
> > On Saturday, 10 August 2019 18:55:51 UTC+1, Tim Williams  wrote:
> > 
> > > An interesting thing, as security goes: for all their problems, Apple has 
> > > been quite staunch in their rejection of such softening measures.
> 
> > Was Apple's public stand for real, or was it showmanship when they complied behind closed doors? We'll probably never know.
> 
> Oh, we'll know.   This kind of thing is a very temporary secret, at best.
> 
> Cisco sales (and stock) plummeted when their shipped products were found to have
> (official US government) tampering, and Apple's iPhone sales are too important
> to risk, so the public stand is probably also the private one.
> 
> Cisco's recent packaging is tamper-evident and more secure against
> counterfeiting than most currency.   It's a SERIOUS concern that Apple
> and even Huawei must recognize, publicly and privately.

point taken :)


NT

On 8/10/19 4:01 PM, whit3rd wrote:
> On Friday, August 9, 2019 at 4:48:32 PM UTC-7, John Larkin wrote:
> 
>> Besides, I've forgotten most of that college math.
> 
> Math is universal.   College is just a friendly place to learn some of it
> Forgotten or not, math is omnipresent.
> 

"Very deep.  You should send that in to the Readers' Digest--they have a 
page for people like you."  --Ford Prefect

;)


Cheers

Phil Hobbs

On 10/08/2019 17:05, Phil Hobbs wrote:
> On 8/9/19 4:50 PM, Winfield Hill wrote:
>> Here's a new section I'm hoping to complete, so it can be added to the 
>> x-Chapter book before it goes to the printer in a few weeks. Please 
>> look it over, but don't be too harsh, about its lack of mathematical 
>> vigor.  It's closer to our usual back-of-the envelope approach to 
>> calculations. Fixes for errors, suggestions for clarification, 
>> improved accuracy, and comments welcome.
>>
>> https://www.dropbox.com/s/7zl3yi789idg3s8/4x.26_Loop%20%26%20Nodal%20Analysis.pdf?dl=1 
>>
>>
> 
> Nice.  I like your making a virtue out of a necessity (hand-drawn
> figures). ;)
> 
> One point that might be worth a footnote is that Kirchhoff's laws are a
> low-frequency approximation, applicable only when radiation and
> self-capacitance are negligible.

If your schematic is really complete, then I think that the laws apply 
usefully, at least until the point where radiation is efficient. I am 
assuming here that current through parasitic capacitances is counted 
just as much as if it were a current flowing through a terminal of an 
intentional capacitor. If, in your schematic and arithmetic, you leave 
out things like the inductance and self-capacitance of wires, (and in 
difficult cases, even the distributed capacitance at different points 
along the inductance of wires), then of course the result of applying 
Kirchoff's laws to the (incomplete) schematic won't predict the 
behaviour of the actual construction. I suspect that radiation could 
also be modelled in a way that allows Kirchoff's laws to be applied but 
that the resulting schematic would be too complicated.

In making spice models for leadframes and bondwires, it makes a 
significant difference whether or not one includes the self-capacitance 
of the conductors (in addition to the mutual capacitances). Fastcap can 
extract all of these. You can apportion the capacitances to different 
points along the inductances of the conductors (extracted with 
FastHenry). The resulting model is very useful when simulating packaged 
chips, and of course the simulator at least attempts to satisfy 
Kirchoff's laws.

On 8/12/19 9:11 AM, Chris Jones wrote:
> On 10/08/2019 17:05, Phil Hobbs wrote:
>> On 8/9/19 4:50 PM, Winfield Hill wrote:
>>> Here's a new section I'm hoping to complete, so it can be added to
>>> the x-Chapter book before it goes to the printer in a few weeks.
>>> Please look it over, but don't be too harsh, about its lack of
>>> mathematical vigor.  It's closer to our usual back-of-the envelope
>>> approach to calculations. Fixes for errors, suggestions for
>>> clarification, improved accuracy, and comments welcome.
>>>
>>> https://www.dropbox.com/s/7zl3yi789idg3s8/4x.26_Loop%20%26%20Nodal%20Analysis.pdf?dl=1
>>>
>>>
>>
>> Nice.  I like your making a virtue out of a necessity (hand-drawn
>> figures). ;)
>>
>> One point that might be worth a footnote is that Kirchhoff's laws are a
>> low-frequency approximation, applicable only when radiation and
>> self-capacitance are negligible.
> 
> If your schematic is really complete, then I think that the laws apply
> usefully, at least until the point where radiation is efficient. I am
> assuming here that current through parasitic capacitances is counted
> just as much as if it were a current flowing through a terminal of an
> intentional capacitor. If, in your schematic and arithmetic, you leave
> out things like the inductance and self-capacitance of wires, (and in
> difficult cases, even the distributed capacitance at different points
> along the inductance of wires), then of course the result of applying
> Kirchoff's laws to the (incomplete) schematic won't predict the
> behaviour of the actual construction. I suspect that radiation could
> also be modelled in a way that allows Kirchoff's laws to be applied but
> that the resulting schematic would be too complicated.

Nope.  Transmission lines at the schematic level are non-local, i.e. you
can't write a system of ODEs to describe a circuit with transmission
lines or significant radiation.  Kirchhoff's laws are derived from
Maxwell's equations in the limit of low frequency (or alternatively, of
small size for a fixed frequency).

And if you have to model the circuit "in a way that allows Kirchhoff's
laws to be applied", you've implicitly admitted that they don't apply to
the actual circuit.

Don't get me wrong--K's equations are useful and all, but they have
limits.  Being a physicist, I fully recognize the usefulness of sleazy
approximations, but you have to remember that that's what they are, or
you'll get snookered.

> 
> In making spice models for leadframes and bondwires, it makes a
> significant difference whether or not one includes the self-capacitance
> of the conductors (in addition to the mutual capacitances). Fastcap can
> extract all of these. You can apportion the capacitances to different
> points along the inductances of the conductors (extracted with
> FastHenry). The resulting model is very useful when simulating packaged
> chips, and of course the simulator at least attempts to satisfy
> Kirchoff's laws.

Yeah, a lot of times you can patch up approximations by hand like
that--in my business one of the major examples is scalar optics, where
you treat the EM field as though it had only one component.
Polarization gets put in by hand once the calculation is done.  Not the
absolute cleanest procedure conceptually, but it works great for almost
everything.

Another example is geometric optics, where you keep going back and forth
between plane waves and rays, i.e. between infinitely broad wavefronts
and infinitely narrow ones.  When a ray encounters a curved dielectric
surface, you assume that it bounced off the tangent plane at the point
of incidence.  That gives you the surface normal.  Then you switch to
the plane wave picture and apply Snell's law and the Fresnel formulae to
get the direction and amplitude of the refracted ray.  Both Snell and
Fresnel depend on the wave picture, specifically phase matching at the
boundary (Snell) and continuity of tangential E and perpendicular D
(Fresnel).  The derivation of Snell's law requires translational
invariance, i.e. it only works on a flat boundary.

So to have well-founded confidence in our tools, we have to know where
they break down and how to patch them up.

Cheers

Phil Hobbs

-- 
Dr Philip C D Hobbs
Principal Consultant
ElectroOptical Innovations LLC / Hobbs ElectroOptics
Optics, Electro-optics, Photonics, Analog Electronics
Briarcliff Manor NY 10510

http://electrooptical.net
https://hobbs-eo.com

On Monday, August 12, 2019 at 10:02:48 AM UTC-4, Phil Hobbs wrote:
> On 8/12/19 9:11 AM, Chris Jones wrote:
> > On 10/08/2019 17:05, Phil Hobbs wrote:
> >> On 8/9/19 4:50 PM, Winfield Hill wrote:
> >>> Here's a new section I'm hoping to complete, so it can be added to
> >>> the x-Chapter book before it goes to the printer in a few weeks.
> >>> Please look it over, but don't be too harsh, about its lack of
> >>> mathematical vigor.  It's closer to our usual back-of-the envelope
> >>> approach to calculations. Fixes for errors, suggestions for
> >>> clarification, improved accuracy, and comments welcome.
> >>>
> >>> https://www.dropbox.com/s/7zl3yi789idg3s8/4x.26_Loop%20%26%20Nodal%20Analysis.pdf?dl=1
> >>>
> >>>
> >>
> >> Nice.  I like your making a virtue out of a necessity (hand-drawn
> >> figures). ;)
> >>
> >> One point that might be worth a footnote is that Kirchhoff's laws are a
> >> low-frequency approximation, applicable only when radiation and
> >> self-capacitance are negligible.
> > 
> > If your schematic is really complete, then I think that the laws apply
> > usefully, at least until the point where radiation is efficient. I am
> > assuming here that current through parasitic capacitances is counted
> > just as much as if it were a current flowing through a terminal of an
> > intentional capacitor. If, in your schematic and arithmetic, you leave
> > out things like the inductance and self-capacitance of wires, (and in
> > difficult cases, even the distributed capacitance at different points
> > along the inductance of wires), then of course the result of applying
> > Kirchoff's laws to the (incomplete) schematic won't predict the
> > behaviour of the actual construction. I suspect that radiation could
> > also be modelled in a way that allows Kirchoff's laws to be applied but
> > that the resulting schematic would be too complicated.
> 
> Nope.  Transmission lines at the schematic level are non-local, i.e. you
> can't write a system of ODEs to describe a circuit with transmission
> lines or significant radiation.  Kirchhoff's laws are derived from
> Maxwell's equations in the limit of low frequency (or alternatively, of
> small size for a fixed frequency).
Hmm Phil, to put this in my own words... and please correct me if I'm wrong..
or I'm only part right.  I think a limitation of K's laws is that they 
treat V and I as instantaneously the same everywhere.  

George H. 
(every theory is an approximation at some level)   
> 
> And if you have to model the circuit "in a way that allows Kirchhoff's
> laws to be applied", you've implicitly admitted that they don't apply to
> the actual circuit.
> 
> Don't get me wrong--K's equations are useful and all, but they have
> limits.  Being a physicist, I fully recognize the usefulness of sleazy
> approximations, but you have to remember that that's what they are, or
> you'll get snookered.
> 
> > 
> > In making spice models for leadframes and bondwires, it makes a
> > significant difference whether or not one includes the self-capacitance
> > of the conductors (in addition to the mutual capacitances). Fastcap can
> > extract all of these. You can apportion the capacitances to different
> > points along the inductances of the conductors (extracted with
> > FastHenry). The resulting model is very useful when simulating packaged
> > chips, and of course the simulator at least attempts to satisfy
> > Kirchoff's laws.
> 
> Yeah, a lot of times you can patch up approximations by hand like
> that--in my business one of the major examples is scalar optics, where
> you treat the EM field as though it had only one component.
> Polarization gets put in by hand once the calculation is done.  Not the
> absolute cleanest procedure conceptually, but it works great for almost
> everything.
> 
> Another example is geometric optics, where you keep going back and forth
> between plane waves and rays, i.e. between infinitely broad wavefronts
> and infinitely narrow ones.  When a ray encounters a curved dielectric
> surface, you assume that it bounced off the tangent plane at the point
> of incidence.  That gives you the surface normal.  Then you switch to
> the plane wave picture and apply Snell's law and the Fresnel formulae to
> get the direction and amplitude of the refracted ray.  Both Snell and
> Fresnel depend on the wave picture, specifically phase matching at the
> boundary (Snell) and continuity of tangential E and perpendicular D
> (Fresnel).  The derivation of Snell's law requires translational
> invariance, i.e. it only works on a flat boundary.
> 
> So to have well-founded confidence in our tools, we have to know where
> they break down and how to patch them up.
> 
> Cheers
> 
> Phil Hobbs
> 
> -- 
> Dr Philip C D Hobbs
> Principal Consultant
> ElectroOptical Innovations LLC / Hobbs ElectroOptics
> Optics, Electro-optics, Photonics, Analog Electronics
> Briarcliff Manor NY 10510
> 
> http://electrooptical.net
> https://hobbs-eo.com

On 8/12/19 12:05 PM, George Herold wrote:
> On Monday, August 12, 2019 at 10:02:48 AM UTC-4, Phil Hobbs wrote:
>> On 8/12/19 9:11 AM, Chris Jones wrote:
>>> On 10/08/2019 17:05, Phil Hobbs wrote:
>>>> On 8/9/19 4:50 PM, Winfield Hill wrote:
>>>>> Here's a new section I'm hoping to complete, so it can be added to
>>>>> the x-Chapter book before it goes to the printer in a few weeks.
>>>>> Please look it over, but don't be too harsh, about its lack of
>>>>> mathematical vigor.  It's closer to our usual back-of-the envelope
>>>>> approach to calculations. Fixes for errors, suggestions for
>>>>> clarification, improved accuracy, and comments welcome.
>>>>>
>>>>> https://www.dropbox.com/s/7zl3yi789idg3s8/4x.26_Loop%20%26%20Nodal%20Analysis.pdf?dl=1
>>>>>
>>>>>
>>>>
>>>> Nice.  I like your making a virtue out of a necessity (hand-drawn
>>>> figures). ;)
>>>>
>>>> One point that might be worth a footnote is that Kirchhoff's laws are a
>>>> low-frequency approximation, applicable only when radiation and
>>>> self-capacitance are negligible.
>>>
>>> If your schematic is really complete, then I think that the laws apply
>>> usefully, at least until the point where radiation is efficient. I am
>>> assuming here that current through parasitic capacitances is counted
>>> just as much as if it were a current flowing through a terminal of an
>>> intentional capacitor. If, in your schematic and arithmetic, you leave
>>> out things like the inductance and self-capacitance of wires, (and in
>>> difficult cases, even the distributed capacitance at different points
>>> along the inductance of wires), then of course the result of applying
>>> Kirchoff's laws to the (incomplete) schematic won't predict the
>>> behaviour of the actual construction. I suspect that radiation could
>>> also be modelled in a way that allows Kirchoff's laws to be applied but
>>> that the resulting schematic would be too complicated.
>>
>> Nope.  Transmission lines at the schematic level are non-local, i.e. you
>> can't write a system of ODEs to describe a circuit with transmission
>> lines or significant radiation.  Kirchhoff's laws are derived from
>> Maxwell's equations in the limit of low frequency (or alternatively, of
>> small size for a fixed frequency).

> Hmm Phil, to put this in my own words... and please correct me if I'm wrong..
> or I'm only part right.  I think a limitation of K's laws is that they 
> treat V and I as instantaneously the same everywhere.  
> 
> George H. 
> (every theory is an approximation at some level)   

Everywhere on a given circuit node or loop, right.  Anything with
transmission-line behaviour can't be modelled as an ODE--the fields
inside the T-line can be modelled with PDEs (Maxwell), but circuits are
all ODEs.  The T-line has invisible internal state, so its circuit
behaviour is nonlocal.

Antennas have voltage nodes and antinodes on the same wire, so that K's
voltage law doesn't hold, and they have current nodes and antinodes as
well, so neither does the current law.

Cheers

Phil Hobbs

-- 
Dr Philip C D Hobbs
Principal Consultant
ElectroOptical Innovations LLC / Hobbs ElectroOptics
Optics, Electro-optics, Photonics, Analog Electronics
Briarcliff Manor NY 10510

http://electrooptical.net
https://hobbs-eo.com

On 13/08/2019 00:02, Phil Hobbs wrote:
> On 8/12/19 9:11 AM, Chris Jones wrote:
>> On 10/08/2019 17:05, Phil Hobbs wrote:
>>> On 8/9/19 4:50 PM, Winfield Hill wrote:
>>>> Here's a new section I'm hoping to complete, so it can be added to
>>>> the x-Chapter book before it goes to the printer in a few weeks.
>>>> Please look it over, but don't be too harsh, about its lack of
>>>> mathematical vigor.  It's closer to our usual back-of-the envelope
>>>> approach to calculations. Fixes for errors, suggestions for
>>>> clarification, improved accuracy, and comments welcome.
>>>>
>>>> https://www.dropbox.com/s/7zl3yi789idg3s8/4x.26_Loop%20%26%20Nodal%20Analysis.pdf?dl=1
>>>>
>>>>
>>>
>>> Nice.  I like your making a virtue out of a necessity (hand-drawn
>>> figures). ;)
>>>
>>> One point that might be worth a footnote is that Kirchhoff's laws are a
>>> low-frequency approximation, applicable only when radiation and
>>> self-capacitance are negligible.
>>
>> If your schematic is really complete, then I think that the laws apply
>> usefully, at least until the point where radiation is efficient. I am
>> assuming here that current through parasitic capacitances is counted
>> just as much as if it were a current flowing through a terminal of an
>> intentional capacitor. If, in your schematic and arithmetic, you leave
>> out things like the inductance and self-capacitance of wires, (and in
>> difficult cases, even the distributed capacitance at different points
>> along the inductance of wires), then of course the result of applying
>> Kirchoff's laws to the (incomplete) schematic won't predict the
>> behaviour of the actual construction. I suspect that radiation could
>> also be modelled in a way that allows Kirchoff's laws to be applied but
>> that the resulting schematic would be too complicated.
> 
> Nope.  Transmission lines at the schematic level are non-local, i.e. you
> can't write a system of ODEs to describe a circuit with transmission
> lines or significant radiation.  Kirchhoff's laws are derived from
> Maxwell's equations in the limit of low frequency (or alternatively, of
> small size for a fixed frequency).
I would say that that a wire on a schematic is not a valid 
representation of a transmission line, and if necessary I would 
approximate a transmission line as a ladder of (ideally infinitely) many 
series inductors and shunt capacitors. Of course very many components 
are required for this to be reasonably accurate.

At frequencies where the number of required components is excessive, I 
would then say that a schematic is not a good way to describe the 
physical system.
> And if you have to model the circuit "in a way that allows Kirchhoff's
> laws to be applied", you've implicitly admitted that they don't apply to
> the actual circuit.
If by actual circuit we mean the physical object, then really I only 
expect Maxwell's equations to describe it, and I'm not very good at 
solving those. In a completely general sense I'm not even sure how one 
would try to apply Kirchoff's laws to an arbitrary three dimensional 
piece of electronics.

> Don't get me wrong--K's equations are useful and all, but they have
> limits.  Being a physicist, I fully recognize the usefulness of sleazy
> approximations, but you have to remember that that's what they are, or
> you'll get snookered.
Agreed.

I guess I might have a rather unusual idea of what a schematic is, and 
this might be what causes me to take issue with what you said. To me, a 
schematic ought to be something that, when simulated (by some ideal 
simulator!), applying Kirchoff's laws, Ohm's law, i=C.dv/dt and so on, 
would sufficiently accurately predict the behavoiur of the real system.

To me, if the predictions are wrong, then I blame the schematic as being 
an inaccurate representation of the system, rather than blaming the 
equations used to simulate the behaviour of the schematic. Perhaps my 
philosophy on this topic comes from having had the job of making a 
schematic (sometimes pulling in netlists from field solvers) in order to 
simulate my design as implemented in a physical product. There was an 
expectation that I would use a circuit simulator provided to me, that 
did try to apply Kirchoff's laws (though not perfectly in the case of 
KCL). I did at least have the luxury that the physical dimensions of the 
system were a tiny fraction of a wavelength.

Chris Jones wrote...
>
> I guess I might have a rather unusual idea of what a schematic is, and 
> this might be what causes me to take issue with what you said. To me,
> a schematic ought to be something that, when simulated (by some ideal 
> simulator!), applying Kirchoff's laws, Ohm's law, i=C.dv/dt and so on, 
> would sufficiently accurately predict the behavior of the real system.
>
> To me, if the predictions are wrong, then I blame the schematic as
> being an inaccurate representation of the system, rather than blaming
> the equations used to simulate the behaviour of the schematic. 

 Yes, I agree in that, for example, if the build documentation
 schematic showed e.g., a resistor and a MOSFET, the simulation
 schematic should show the resistor's inductance or capacitance,
 as needed, and the MOSFET's Ciss, gm, and other aspects.  But
 things get more painful when the part is say a high-performance
 op-amp, and you don't know what's in the manufacturer's model.


-- 
 Thanks,
    - Win