OT: SOT-523 PNP Question

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

Sure, but BSS84s are 4c/ea in 1000s, versus MMBT3906s for 2.2c. And they probably aren't as well matched. Actually, I haven't compared FETs on tape... hmm...

Tim

Yup -- still fine when a wimp is all you need. And in the grander scheme of things, 0.5mA is hardly useless.

Any clue how the Rth(j-j) is between them? Twin packs are notoriously weak, but I wonder if having common metal makes them any better, even if it's two bond wires.

Tim

OK, for that you need to charge a fee.

$0.xx for chalking a mark on a pipe. $XXX,XXX.xx for knowing where to make a chalk mark on a pipe...

I have some germanium transistors which act like really slow latches. Once turned on, they bias themselves at half supply voltage, go figure!

Tim

GE built some radios and phono amps with crap like that in the '60. No bias resistors, just the internal leakage. Probably fallout from the military & commercial product lines.

The Infineon ones seem to be a bit better--they claim somewhat higher maximum dissipation before instability.

Cheers

Phil Hobbs

In the classic low-level audio amp stage bias scheme (divider to bias up the base, resistor in the source) the base divider often took current *out* of the base!

One occasionally saw an amp circuit with the base of a Ge transistor driven through a cap... and nothing else.

The synchronized-runaway trick can be used to enable multiple transistor array chips to be used much like a single larger array. That's potentially quite useful in my business.

Cheers

Phil Hobbs

I believe that. CMOS tends to be crappy that way, but OTOH you can bandage it with a whole lot of auxiliary transistors.

Cheers

Phil Hobbs

Theta_JA isn't the pertinent number, it's Theta_JC (junction to case), or even more accurately junction-to-mounting-slug. What did you get you PhD in... exaggeration ?>:-}

More in a couple of days... off to Yuma to the inauguration of the son-in-law. ...Jim Thompson

Yep, that's the (golden) rule >:-} ...Jim Thompson

The best of all worlds is BiCMOS. But CMOS isn't "crappy"... if you know how to design with it. With my proprietary cascode I can easily get to 0.2V of rail before it sags. ...Jim Thompson

So you believe that heat disappears once it gets to the case? I'm talking about the real theta_JA in the real system, not the marketing number. You can't usefully write equations containing marketing numbers.

Cheers

Phil Hobbs

You're missing the point. I'll address the math on Sunday+ ...Jim Thompson

Maybe, but I don't think that I am. The two dice are not meaningfully coupled thermally--the lead frames are completely isolated, and the plastic isn't much better at heat conduction than air is.

Cheers

Phil Hobbs

Or two.

There's a speed vs. accuracy tradeoff, though. Slowish mirrors made with quick op amps are great, but trying to make a fast, accurate mirror that way is surprisingly difficult, mostly because the op amp needs a big input error to reach maximum slew.

Cheers

Phil Hobbs

One interesting possibility is to let the current mirror mostly do its own thing, but use an op amp to null out the average error in the emitter circuit via a voltage divider. Hmm, might be useful.

Cheers

Phil Hobbs

There are better ways, think charge pumps used in PFD's. ...Jim Thompson