OT: SOT-523 PNP Question

In a built-up circuit? I can see it in an IC, provided the switching frequency is sufficiently far out of band. (There's a trick used in high power audio amps, to equalize the supplies: you ground one end of an inductor, and switch the other end from VCC to VEE. Any DC imbalance goes away magically, and the switcher doesn't have to supply the whole load.)

My interest is primarily in making accurate replicas of photocurrents for use in bandaging nonlinearities elsewhere in the system, e.g. Figure

12 of
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. So switching noise is a no-no. (As I say to myself over and over, there's SMPS quiet and then there's instrument quiet.)

I've been working for awhile, on and off, to make a noise canceller good to an honest 70 dB from DC to 10 MHz. The analogue accuracy and noise requirements are pretty stringent, and it has to work over about a

1000:1 photocurrent range. The combination makes it very interesting. Even with gussied-up mirrors, it needs three automatic tweaks per photodiode--R_ee' at DC, and AC tweaks of j omega and omega**2.

It simulates great, and I have some PDs that by my measurements should be adequate, but we'll see when the boards are done. My beautiful PCB hunchback and opera singer got chucked in the deep end on that one--over

200 parts, all connected to each other. She learned to swim pretty well, and is now doing her first client work. The analogue board will go out to fab later this month.

Hopefully Firmware Hunchback and I can get the CPU board done soon, and it'll be a product this year. (I'm like butter spread over too much bread at the moment.)

Cheers

Phil Hobbs

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Phil Hobbs
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I rarely do "built-up" circuits, though that kind of project is in the works.

Have fun ;-) ...Jim Thompson

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| James E.Thompson                                 |    mens     | 
| Analog Innovations                               |     et      | 
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Jim Thompson

Yes, an opamp could take out the relatively slow thermal errors.

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John Larkin                  Highland Technology Inc 
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John Larkin

Since the collector and emitter are swapped (right?), it's possible that the transistor could "just work", depending on the circuit.

See this electronics.stackexchange.com question:

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"Q: What will happen if for a BJT transistor it's Emitter terminal is treated as collector and collector as Emitter in a common Emitter amplifier circuit?"

"A: (Short Answer) It will work but will have a lower ? (beta)"

Perhaps the circuit can work with a lower beta. Or, if not, perhaps some component values can be modified to aid that.

Reply to
Kaz Kylheku

It does work. I've built choppers that way (for lower offset). But, the base-emitter junction goes into zener breakdown somewhere around 6 or 7 volts.

Joe Gwinn

Reply to
Joe Gwinn

(...)

"Low saturation" transistors (usually switching types in the 20-100Vceo,

1-10A range) have surprisingly good inverted hFE, especially in regards to the spice models, which almost unanimously give erroneous fractional BR parameters (presumably, to improve modeling of the forward region only). Veb is still limited though.

Tim

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Tim Williams

Interesting. Any favourites?

Cheers

Phil Hobbs

(Whose flight to LAX via DTW got cancelled this afternoon.)

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Dr Philip C D Hobbs 
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Phil Hobbs

Yep. The circuit did work... only it was marginal on breakdown. Problem was solved by eliminating need for PNP. ...Jim Thompson

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| James E.Thompson                                 |    mens     | 
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Jim Thompson

I used 2N3904, but it was the jellybean I had, and it was adequate as a chopper.

Joe Gwinn

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Joe Gwinn

There are a few transistors around that have high Vebo. For example,

2SA1252/2SC3134 (PNP/NPN), which are guaranteed to 15V.

If I had any, I'd measure the reverse beta.

Best regards, Spehro Pefhany

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"it's the network..."                          "The Journey is the reward" 
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Spehro Pefhany

Others have menti (printed) Mouser P/N:

771-PBSS4540X135 Mfg P/N: PBSS4540X,135 Desc: TRANS BISS TAPE-13 NXP Bipolar Small Signal

(scrawled) SOT89 NPN

0.2928A @ 1.78m = 164 hFE ~~ 300 reversed: V_EC = 2.5V I_B = 100uA I_E = 33.4mA V_EB < 5V (and other bits scratched out...)

Not sure if the "164" was forward or reverse, or at what voltage.

...Since it's reversed hFE, should it be hEF? ;-)

These I think are obsolete now (the baggie is also empty, oh noes!), but I imagine the newer ones (PBSS303NX,115, etc.) have similar behavior. Haven't measured more, haven't needed the behavior either.

Tim

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Tim Williams

Wow, datasheet shows some really weird kinks in the saturation curve. Multi-level Early effect or something like that, especially on the PNP.

I've got some 2SC2878 pulls laying around somewhere. 50V collector, 25V emitter, yet good hFE. Made for switching/muting applications.

Tim

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Seven Transistor Labs 
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Tim Williams

BR might be quite respectable. BR is higher the lower the doping density of the (effective) collector (the emitter when operated in reverse), but the current handling before crowding will likely be lower. ...Jim Thompson

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| James E.Thompson                                 |    mens     | 
| Analog Innovations                               |     et      | 
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Reply to
Jim Thompson

Thanks, that could come in handy.

Cheers

Phil Hobbs

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Phil Hobbs

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

It's weird, because the doping is apparently pretty similar between C and E. Yet the breakdown voltages aren't the same, so either the profile or the doping is different.

Probably, the base is also very thin (I don't have data on Early effect on these..), but it's probably also nicely interdigitated so the frequency response isn't bad (fT ~ 50MHz and up).

Tim

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Tim Williams

Even with equal doping, there would probably be 3D effects making the E field in the emitter (so labelled) higher than in the collector of the normally biased device.

Cheers

Phil Hobbs

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Dr Philip C D Hobbs 
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Reply to
Phil Hobbs

Naaaah! The diffusions are "graded". Remember, all dopants are introduced from the _top_ surface. ...Jim Thompson

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| James E.Thompson                                 |    mens     | 
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Reply to
Jim Thompson

In planar processes, sure. Not so much with ion implant and epi. (I haven't looked up the datasheet--is that one a planar device?)

Anyway, even in a planar device the gradient will be sharper in the emitter diffusion, because it doesn't go as far. That will reduce the breakdown voltage by making the depletion layer thinner.

Cheers

Phil Hobbs

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Phil Hobbs

Off the top of my head I don't know of any ion-implanted _discrete_ devices. I only see it in fine-definition chips. ...Jim Thompson

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| James E.Thompson                                 |    mens     | 
| Analog Innovations                               |     et      | 
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Jim Thompson

And: The depth to E-B and the depth to B-C are almost the same, differing only by the base width. The main asymmetry of the structure is that the collector is generally (modern day processes) an epitaxial layer... uniformly doped, while the base and emitter are diffusions following Fick's equation ;-) ...Jim Thompson

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| James E.Thompson                                 |    mens     | 
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Jim Thompson

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