Forums

SPICE gets it wrong

Started by Tim Williams May 7, 2017
Speaking of SPICE, here's a nice little circuit:
https://www.seventransistorlabs.com/Images/BakerDarlington.png

I discovered the oscillator on the left, when attempting to build a 
Darlington transistor pair with a Baker clamp.  (In practice, R10 comes from 
a logic voltage input, and R2 is a load, often inductive.  R14 is optional, 
but exacerbates the oscillation, making it reproducible.)

The model on the left, of course, doesn't produce any oscillation: it 
reaches equilibrium within a couple hundred nanoseconds.  (Which is kind of 
suspiciously fast for a TIP31 to do much of anything, I might add.)

The simulation on the right, showing semi-reasonable parasitics, was 
necessary to reproduce the oscillations.  The parasitics are in reasonable 
locations, but their values are completely unreasonable.

The left circuit appears to be a limit cycle of an chaotic system; adding C1 
to the real circuit introduces what looks like rising-edge crossover 
distortion, and adding a few other things causes all-out chaos.

(Incidentally, if you'd like to volunteer solutions that stabilize the right 
hand circuit -- other than reducing and removing the parasitics, which 
apparently isn't an option -- I'd be interested to hear them.)

I think the TL;DR is:
1. TIP31Cs suck as switching transistors to begin with,
2. The TIP31C SPICE model sucks, despite them having half a century to try 
and get it right.

Tim

-- 
Seven Transistor Labs, LLC
Electrical Engineering Consultation and Contract Design
Website: http://seventransistorlabs.com 

On Sun, 7 May 2017 19:14:26 -0500, "Tim Williams"
<tiwill@seventransistorlabs.com> wrote:

>Speaking of SPICE, here's a nice little circuit: >https://www.seventransistorlabs.com/Images/BakerDarlington.png > >I discovered the oscillator on the left, when attempting to build a >Darlington transistor pair with a Baker clamp. (In practice, R10 comes from >a logic voltage input, and R2 is a load, often inductive. R14 is optional, >but exacerbates the oscillation, making it reproducible.) > >The model on the left, of course, doesn't produce any oscillation: it >reaches equilibrium within a couple hundred nanoseconds. (Which is kind of >suspiciously fast for a TIP31 to do much of anything, I might add.) > >The simulation on the right, showing semi-reasonable parasitics, was >necessary to reproduce the oscillations. The parasitics are in reasonable >locations, but their values are completely unreasonable. > >The left circuit appears to be a limit cycle of an chaotic system; adding C1 >to the real circuit introduces what looks like rising-edge crossover >distortion, and adding a few other things causes all-out chaos. > >(Incidentally, if you'd like to volunteer solutions that stabilize the right >hand circuit -- other than reducing and removing the parasitics, which >apparently isn't an option -- I'd be interested to hear them.) > >I think the TL;DR is: >1. TIP31Cs suck as switching transistors to begin with, >2. The TIP31C SPICE model sucks, despite them having half a century to try >and get it right. > >Tim
Manufacturers of discrete devices hire newbie PhD's to make their models (if they can't find a monkey with better skills >:-} ...Jim Thompson -- | James E.Thompson | mens | | Analog Innovations | et | | Analog/Mixed-Signal ASIC's and Discrete Systems | manus | | STV, Queen Creek, AZ 85142 Skype: skypeanalog | | | Voice:(480)460-2350 Fax: Available upon request | Brass Rat | | E-mail Icon at http://www.analog-innovations.com | 1962 | Thinking outside the box... producing elegant solutions. "It is not in doing what you like, but in liking what you do that is the secret of happiness." -James Barrie
On Sun, 7 May 2017 19:14:26 -0500, "Tim Williams"
<tiwill@seventransistorlabs.com> wrote:

>Speaking of SPICE, here's a nice little circuit: >https://www.seventransistorlabs.com/Images/BakerDarlington.png > >I discovered the oscillator on the left, when attempting to build a >Darlington transistor pair with a Baker clamp. (In practice, R10 comes from >a logic voltage input, and R2 is a load, often inductive. R14 is optional, >but exacerbates the oscillation, making it reproducible.) > >The model on the left, of course, doesn't produce any oscillation: it >reaches equilibrium within a couple hundred nanoseconds. (Which is kind of >suspiciously fast for a TIP31 to do much of anything, I might add.) > >The simulation on the right, showing semi-reasonable parasitics, was >necessary to reproduce the oscillations. The parasitics are in reasonable >locations, but their values are completely unreasonable. > >The left circuit appears to be a limit cycle of an chaotic system; adding C1 >to the real circuit introduces what looks like rising-edge crossover >distortion, and adding a few other things causes all-out chaos. > >(Incidentally, if you'd like to volunteer solutions that stabilize the right >hand circuit -- other than reducing and removing the parasitics, which >apparently isn't an option -- I'd be interested to hear them.)
Use a mosfet? -- John Larkin Highland Technology, Inc lunatic fringe electronics
"John Larkin" <jjlarkin@highlandtechnology.com> wrote in message 
news:vmnvgcpmkl85uf7gb0ub1k680f2hlot825@4ax.com...
> Use a mosfet? >
No can do. Then it would use _eight_ transistors. :-) (I'm not actually kidding, the circuit snippet I've been entertaining myself with -- which this is a part of -- is at seven BJTs right now. It's kind of a lie, because it needs two or three more transistors to be a complete circuit, but oh well.) Tim -- Seven Transistor Labs, LLC Electrical Engineering Consultation and Contract Design Website: http://seventransistorlabs.com
"Jim Thompson" <To-Email-Use-The-Envelope-Icon@On-My-Web-Site.com> wrote in 
message news:05lvgc19bchncub8ais9n3cr0cnber9k55@4ax.com...
> Manufacturers of discrete devices hire newbie PhD's to make their > models (if they can't find a monkey with better skills >:-}
I'm thinking the biggest discrepancy may be a transport delay sort of thing. Which calls to mind some recent discussions here... think you're game to make one that works? ;-) Tim -- Seven Transistor Labs, LLC Electrical Engineering Consultation and Contract Design Website: http://seventransistorlabs.com
On a sunny day (Mon, 8 May 2017 00:15:33 -0500) it happened "Tim Williams"
<tiwill@seventransistorlabs.com> wrote in <oeouoi$j3i$1@dont-email.me>:

>"Jim Thompson" <To-Email-Use-The-Envelope-Icon@On-My-Web-Site.com> wrote in >message news:05lvgc19bchncub8ais9n3cr0cnber9k55@4ax.com... >> Manufacturers of discrete devices hire newbie PhD's to make their >> models (if they can't find a monkey with better skills >:-} > >I'm thinking the biggest discrepancy may be a transport delay sort of thing. >Which calls to mind some recent discussions here... think you're game to >make one that works? ;-) > >Tim
Not sure about what you want that thing to do, but spread in params for TIP<anything> is huge. Driving those in linear range between be like that with no resistor in e is asking for trouble. Emitter followers have many catches. I remember long long long time ago somebody designed an audio power amp with transistors like that, it oscillated all over the place, replaced transistors with a different manufacturer and it worked OK. MJE3055 versus 2N3055 or something.
On a sunny day (Sun, 07 May 2017 19:59:37 -0700) it happened John Larkin
<jjlarkin@highlandtechnology.com> wrote in
<vmnvgcpmkl85uf7gb0ub1k680f2hlot825@4ax.com>:

>On Sun, 7 May 2017 19:14:26 -0500, "Tim Williams" ><tiwill@seventransistorlabs.com> wrote: > >>Speaking of SPICE, here's a nice little circuit: >>https://www.seventransistorlabs.com/Images/BakerDarlington.png >> >>I discovered the oscillator on the left, when attempting to build a >>Darlington transistor pair with a Baker clamp. (In practice, R10 comes from >>a logic voltage input, and R2 is a load, often inductive. R14 is optional, >>but exacerbates the oscillation, making it reproducible.) >> >>The model on the left, of course, doesn't produce any oscillation: it >>reaches equilibrium within a couple hundred nanoseconds. (Which is kind of >>suspiciously fast for a TIP31 to do much of anything, I might add.) >> >>The simulation on the right, showing semi-reasonable parasitics, was >>necessary to reproduce the oscillations. The parasitics are in reasonable >>locations, but their values are completely unreasonable. >> >>The left circuit appears to be a limit cycle of an chaotic system; adding C1 >>to the real circuit introduces what looks like rising-edge crossover >>distortion, and adding a few other things causes all-out chaos. >> >>(Incidentally, if you'd like to volunteer solutions that stabilize the right >>hand circuit -- other than reducing and removing the parasitics, which >>apparently isn't an option -- I'd be interested to hear them.) > >Use a mosfet?
Indeed. Or drive it from a current source.
On 05/08/2017 01:15 AM, Tim Williams wrote:
> "Jim Thompson" <To-Email-Use-The-Envelope-Icon@On-My-Web-Site.com> wrote > in message news:05lvgc19bchncub8ais9n3cr0cnber9k55@4ax.com... >> Manufacturers of discrete devices hire newbie PhD's to make their >> models (if they can't find a monkey with better skills >:-} > > I'm thinking the biggest discrepancy may be a transport delay sort of > thing. Which calls to mind some recent discussions here... think you're > game to make one that works? ;-) > > Tim >
I think the circuit on the left could be at least shown to be unstable (though maybe not the complex chaotic behavior you're seeing) on paper if one took the emitter follower transconductances/beta to not be simple functions of emitter current, but a complex number that's also a function of frequency and fT. It doesn't matter so much with small signal transistors like the 2N etc. with fTs in the 100s of MHz, but the TIP31 is a real slowpoke with an fT of about 1 MHz probably. So the input and output ports are looking very reactive, and the base of Q3 is being fed thru the depletion capacitance of D2 when it's reversed-biased. There's probably still plenty of gain left when the inductive-looking output impedance of Q3 plus the input C of Q4 flips the loop gain thru 180 degrees. Single-ended emitter-followers are bad, ones made from power BJTs even worse...
On 05/08/2017 07:34 AM, bitrex wrote:

> It doesn't matter so much with small signal transistors like the 2N etc. > with fTs in the 100s of MHz, but the TIP31 is a real slowpoke with an fT > of about 1 MHz probably. So the input and output ports are looking very > reactive, and the base of Q3 is being fed thru the depletion capacitance > of D2 when it's reversed-biased.
Disregard, the first transistor in the stage isn't a power transistor. It's maybe not that one where the output impedance looking inductive is a problem.
On 05/07/2017 08:14 PM, Tim Williams wrote:

> I think the TL;DR is: > 1. TIP31Cs suck as switching transistors to begin with, > 2. The TIP31C SPICE model sucks, despite them having half a century to > try and get it right. > > Tim >
I found this interesting paper from back in the day about stability issues when using BJTs as emitter followers/switches in them newfangled computer circuits. Just digitized in 2013: <https://archive.org/stream/emitterfollowers134moto#page/n19/mode/1up>