I think that wirebond inductances are the big contributor to transistor oscillation.
-- John Larkin Highland Technology, Inc picosecond timing precision measurement jlarkin att highlandtechnology dott com http://www.highlandtechnology.com
I suspect the steadily rising impedance of the inductive load in the TIP31's collector plus the increasing value of its already large C-B capacitance with frequency is the primary culprit here, getting fed back as positive feedback through the depletion capacitance of the clamp diode to the input transistor
Cool 5 MHz power oscillator, bro
Just had a little look so far...
What is the tf in the model?
What is unreasonable about the 100p?
I checked the device data sheet. Its ft=3MHz so tf needs to be around 50n. This is a large value
Cbe = 40.IC/2pi.ft
so, at 100ma, its 0.2uf or so! 100p is small in comparison.
I will look in more detail later, once I get time...
-- Kevin Aylward
BF is stupendously unreasonable as well, but I forget how the other parameters work off against that.
Eugh, VAF is probably better than that, too.
Physically speaking, I can't account for more than 10pF and 20nH in any connections on this circuit.
Ah, the Miller equivalent input, more or less?
Yeah, that would be quite large.
Cheers!
Progress: PBSS303NX works /fantastically/ well, being both fast and well-behaved. In fact, with 470p + 100R from Q3 base to GND, it's quite stable, just a few blips at the beginning of a cycle.
Tim
-- Seven Transistor Labs, LLC Electrical Engineering Consultation and Contract Design Website: http://seventransistorlabs.com
At 5MHz?
With a TIP31C?
Tim
-- Seven Transistor Labs, LLC Electrical Engineering Consultation and Contract Design Website: http://seventransistorlabs.com
They're rather hard to find nowadays, outside of scrapped CRT monitors, which is where I found them.
Rest in peace, little transistors.
Tim
-- Seven Transistor Labs, LLC Electrical Engineering Consultation and Contract Design Website: http://seventransistorlabs.com
Yes, combine the fact that beta is in reality a complex-valued function of frequency (that giant capacitor), the C-B Miller effect, and the inductive load, and a transistor like the TIP31 in that structure seems to pretty much oscillate on principle. ;-)
Then why doesn't it oscillate when I strap a schottky from its own B to C? :^)
It's very interesting that even the stupid-fast transistor oscillated at only a modest frequency.
Tim
-- Seven Transistor Labs, LLC Electrical Engineering Consultation and Contract Design Website: http://seventransistorlabs.com
Probably not, although they might contribute at high currents. Didn't the sim need nanohenry inductances to get it to oscillate? The base lead of even a big transistor can have a skinny wirebond.
-- John Larkin Highland Technology, Inc picosecond timing precision measurement jlarkin att highlandtechnology dott com http://www.highlandtechnology.com
That's what I meant about "physically realistic" -- wire bonds are a couple nH, and even with the loose-wired layout, I can't blame more than about 20nH on the circuit itself.
More rather, it shows that the simulation is pretty close to instability. We're not talking much phase shift at this frequency, just a little is enough.
Bitrex's hybrid-pi approximation also shows a peak at a modest frequency, though it doesn't seem quite close enough to outright oscillation. For both models, a little delay or phase shift would seem to do the job.
Tim
-- Seven Transistor Labs, LLC Electrical Engineering Consultation and Contract Design Website: http://seventransistorlabs.com
The Z_o of the 3904 into the C_pi of the TIP31 plus the 3904's current gain is probably enough to push it over. It's oscillating as a unit, right? The collector of Q1/Q4 feeding back to the base of Q2/Q3 thru the diode capacitance or power supply leads or something?
It's certainly something about the 3904 having some delay or phase shift or funny impedances, and wrapping that around another transistor.
After some more playing around, I've concluded that:
- Q3 collector circuit doesn't play much role.
- An R+C from Q3 base to GND helps a lot. In some cases, it can fully stabilize the circuit (MJ15028 and 2SC4821 use around 100R + 470pF).
- I seem to have found unconditional stability: an R1 || L || (R2 + C) between Q3 emitter and Q4 base. Values are R1 = 220R, L = 8.7uH, R2 = 22R, C = 220pF. This, and 220R + 220pF from Q3 base to GND, seems to work on every transistor I try!
I've taken over Q3 collector as a "sense" for what's going on, since Q3 current is modulated heavily when oscillating, meanwhile all the other node voltage changes are fairly modest. I'm using 10uH || 470R there (between Q3 collector and VCC).
Tim
-- Seven Transistor Labs, LLC Electrical Engineering Consultation and Contract Design Website: http://seventransistorlabs.com
In related news, I've discovered this circuit drives Darlington transistors well beyond ludicrous speed: we're into plaid speed!
I have a 2SD560 (NEC, probably 80s or 90s) which claims 1/1.2us (rise/fall) at hFE(sat) = 1000.
In comparison, the fastest transistor (2SC4821) does: turn-off delay 125ns rise time 30ns turn-on delay 614ns* fall time 59ns
(*A consequence of the Q3 base damper, see last response to bitrex.)
The 2SD560 does: turn-off delay 160ns rise time 160ns turn-on delay 750ns fall time 230ns
Tim
-- Seven Transistor Labs, LLC Electrical Engineering Consultation and Contract Design Website: http://seventransistorlabs.com "Tim Williams" wrote in message news:oeod3g$id1$1@dont-email.me... > Speaking of SPICE, here's a nice little circuit: > https://www.seventransistorlabs.com/Images/BakerDarlington.png >
On a sunny day (Mon, 8 May 2017 16:25:09 -0500) it happened "Tim Williams" wrote in :
From memory, the discussion at that time was that Texas Instruments uses a different fabrication process than Motorola (MJE3055), but that is from hear-say. Have not used TIP ever since then though, why take chances.
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.
Why do you call it Darlington? I thought Darlington meant collector to collector. The second transistor in a Darlington is already saturation clamped - only the first transistor could be clamped.
Without your clamp diode there is unlimited base current in the second transistor - perhaps that is upsetting things? Insert resistance in the collector of the first transistor? Or wire it collector-collector as a true Darlington.
piglet
Wouldn't be surprised; 3055s were /terrifyingly/ bad.
By the 80s, most people were selling "okay" dies as 3055s. Dies that would've been used in MJ15020s and such. I have a few "2N3055"s, dated late
80s, that are downright reasonable in switching speed and Vce(sat).AFAIK, the same is true of TIPs, and pretty much every other early crap part that survived multiple generations of fabs.
I do have a 2SD880 that seems to perform even worse than the 2SD401. Very slow. I suspect it's closer to the original TIP31, or 2N3055 for that matter.
Tim
-- Seven Transistor Labs, LLC Electrical Engineering Consultation and Contract Design Website: http://seventransistorlabs.com
The emitter follower action is the first feature of the configuration that comes to mind, but it's true the collector clamping action is also inherent, so it's a misnomer to call it that.
The "unlimited" current seems to help more than it hurts, while the clamp is doing its job -- at turn-on, it's goosed with enough extra charge to swing the output node down /and/ stay fully saturated for a few hundred nanoseconds.
Here's the full circuit, adjustments included:
BTW, the ferrite bead L2 is a reasonable model (though a linear one), and C4 has a tinge of ESR and ESL. No supply R or L, and no other stray L beyond what's shown. (L3 really should have a model, since it's on a somewhat lossy ferrite core.)
Tim
-- Seven Transistor Labs, LLC Electrical Engineering Consultation and Contract Design Website: http://seventransistorlabs.com
The Radio Shack near me went out of business (again) and I bought up all the bagged 2N3055s they had sitting in their bin for about 2 bucks for
PS: need any twist-on BNC connectors? Nickel-plated brass with gold contacts. I get you good price. Oh wait I already asked ;-)
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