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SPICE gets it wrong

Speaking of SPICE, here's a nice little circuit:

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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

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Seven Transistor Labs, LLC 
Electrical Engineering Consultation and Contract Design 
Website: http://seventransistorlabs.com
Reply to
Tim Williams
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Manufacturers of discrete devices hire newbie PhD's to make their models (if they can't find a monkey with better skills >:-} ...Jim Thompson

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Reply to
Jim Thompson

Use a mosfet?

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John Larkin         Highland Technology, Inc 

lunatic fringe electronics
Reply to
John Larkin

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

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Seven Transistor Labs, LLC 
Electrical Engineering Consultation and Contract Design 
Website: http://seventransistorlabs.com
Reply to
Tim Williams

"Jim Thompson" wrote in message news: snipped-for-privacy@4ax.com...

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

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Seven Transistor Labs, LLC 
Electrical Engineering Consultation and Contract Design 
Website: http://seventransistorlabs.com
Reply to
Tim Williams

On a sunny day (Mon, 8 May 2017 00:15:33 -0500) it happened "Tim Williams" wrote in :

Not sure about what you want that thing to do, but spread in params for TIP 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.

Reply to
Jan Panteltje

On a sunny day (Sun, 07 May 2017 19:59:37 -0700) it happened John Larkin wrote in :

Indeed. Or drive it from a current source.

Reply to
Jan Panteltje

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...

Reply to
bitrex

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.

Reply to
bitrex

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:

Reply to
bitrex

Page 8 seems to have a pertinent section:

"In order to achieve some type of compensation network the effect of Re and Lc, the collector inductance, was studied. There is nearly no problem associated with the collector inductance. However, if Lc becomes large fast pulses will yield voltage drops, which in turn cause Cc (collector depletion layer capacitance) to be modulated. But this effect is beneficial, since Cc would tend to increase, resulting in a decrease of the negative real part magnitude. But a side effect also occurs: If several collectors are tied together through a common power supply wire and marginal stability exists, instability of one may cause instability of all.

...the possibility of relaxation oscillations exists and has been verified expeimentally"

Reply to
bitrex

Add 100 Ohms in series with the base of the 3904.

m
Reply to
makolber

I was going to say a ferrite bead "gate stopper" in the base lead of the TIP31

Reply to
bitrex

Thanks, I'll give that a try.

I'd dabbled a bit with adding collector impedance, but didn't seem to get far. Maybe some additional C-B impedance (R+C?) will help.

That also implies there's something in series, though: the base-to-output diode is a pretty low impedance when it pulls in.

Tim

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

That's (50 ohms) the classical fix. Note that ECL is *full* of emitter followers and works rather well.

Reply to
krw

It's not that simple: I already tried a couple of things there:

10 ohm R Ferrite bead (100 ohms @ 100MHz, about 0.6uH at low frequencies) 10uH || 100 ohm (and other R values)

The tell is this: the oscillation is low frequency, typically 5-10MHz. If it can be squeezed high enough that it goes away, that would work. Most attempts to stabilize it, cause the frequency to drop, and don't do much to the amplitude.

Varying the power transistor, has the most interesting effect. I've tried:

TIP31C (ST, early 2000s date): spiky oscillation. Reasonable switching performance, considering. FJP13009 (Fairchild, 2010s?): average oscillation, slow switching.

2SD401-Y (SEC, 1998?): somewhat worse oscillation, very slow. KTC4368 (KEC, 1999?): spikier than TIP31C, seems faster in the linear range (is actually rated fT 100MHz typ), slow turnoff.

And just for kicks, a 2SC4821, which oscillates massively in the 30MHz range, and switches as fast as I can push it (turn-off delay time 40ns, no drool). :)

Tim

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

I rigged up a sim in LTSpice using just a simple hybrid-pi model with your model of your collector load and parasitics, only including the current generator, C_pi, R_pi, emitter inductance, Miller capacitance, and Z_o of the previous stage, with values that seemed reasonable for a slowpoke BJT like the TIP31.

It was enough to show a huge spike in gain in the low MHz where the gain comes up from well below 0 dB to around 10dB, and the phase rockets down from around 160 degrees (where it hovered around for most of the Bode plot prior) to 0 degrees.

Reply to
bitrex

Needz a lil' negative Miller capacitance rather than positive

Reply to
bitrex

Interelectrode capacitances aren't enough to make it pop off? You need the lead inductances?

--

Tim Wescott 
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http://www.wescottdesign.com 

I'm looking for work -- see my website!
Reply to
Tim Wescott

Hey, that's a GHz power transistor! Or should I say, ex-transistor.

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Reply to
Winfield Hill

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