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OT: SOT-523 PNP Question

'Tain't hard to calculate. At constant V_BE, a junction temperature rise of 1 degree produces a current increase of 9%. Assuming a low-Z collector load, device dissipation increases by 9% as well. Instability will occur if that 9% causes a temperature change of another degree, i.e.

0.09 I_C V_CE theta_JA > 1 K.

The general derivation isn't much harder, even with a more complicated load.

If you bias the output transistor near its maximum-power point, you can even use this effect as a temperature controller:

formatting link

Cheers

Phil Hobbs

--
Dr Philip C D Hobbs 
Principal Consultant 
ElectroOptical Innovations LLC 
Optics, Electro-optics, Photonics, Analog Electronics 

160 North State Road #203 
Briarcliff Manor NY 10510 

hobbs at electrooptical dot net 
http://electrooptical.net
Reply to
Phil Hobbs
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"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

--
Seven Transistor Labs 
Electrical Engineering Consultation 
Website: http://seventransistorlabs.com
Reply to
Tim Williams

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

--
Seven Transistor Labs 
Electrical Engineering Consultation 
Website: http://seventransistorlabs.com
Reply to
Tim Williams

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...

--

Tim Wescott 
Wescott Design Services 
http://www.wescottdesign.com
Reply to
Tim Wescott

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

Tim

--
Seven Transistor Labs 
Electrical Engineering Consultation 
Website: http://seventransistorlabs.com
Reply to
Tim Williams

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.

--
Anyone wanting to run for any political office in the US should have to 
have a DD214, and a honorable discharge.
Reply to
Michael A. Terrell

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.

--

John Larkin         Highland Technology, Inc 

jlarkin at highlandtechnology dot com 
http://www.highlandtechnology.com 

Precision electronic instrumentation 
Picosecond-resolution Digital Delay and Pulse generators 
Custom laser drivers and controllers 
Photonics and fiberoptic TTL data links 
VME thermocouple, LVDT, synchro   acquisition and simulation
Reply to
John Larkin

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

Cheers

Phil Hobbs

--
Dr Philip C D Hobbs 
Principal Consultant 
ElectroOptical Innovations LLC 
Optics, Electro-optics, Photonics, Analog Electronics 

160 North State Road #203 
Briarcliff Manor NY 10510 

hobbs at electrooptical dot net 
http://electrooptical.net
Reply to
Phil Hobbs

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

--
Dr Philip C D Hobbs 
Principal Consultant 
ElectroOptical Innovations LLC 
Optics, Electro-optics, Photonics, Analog Electronics 

160 North State Road #203 
Briarcliff Manor NY 10510 

hobbs at electrooptical dot net 
http://electrooptical.net
Reply to
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

--
Dr Philip C D Hobbs 
Principal Consultant 
ElectroOptical Innovations LLC 
Optics, Electro-optics, Photonics, Analog Electronics 

160 North State Road #203 
Briarcliff Manor NY 10510 

hobbs at electrooptical dot net 
http://electrooptical.net
Reply to
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

--
| James E.Thompson                                 |    mens     | 
| Analog Innovations                               |     et      | 
| Analog/Mixed-Signal ASIC's and Discrete Systems  |    manus    | 
| San Tan Valley, AZ 85142   Skype: Contacts Only  |             | 
| Voice:(480)460-2350  Fax: Available upon request |  Brass Rat  | 
| E-mail Icon at http://www.analog-innovations.com |    1962     | 
              
I love to cook with wine.     Sometimes I even put it in the food.
Reply to
Jim Thompson

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

--
| James E.Thompson                                 |    mens     | 
| Analog Innovations                               |     et      | 
| Analog/Mixed-Signal ASIC's and Discrete Systems  |    manus    | 
| San Tan Valley, AZ 85142   Skype: Contacts Only  |             | 
| Voice:(480)460-2350  Fax: Available upon request |  Brass Rat  | 
| E-mail Icon at http://www.analog-innovations.com |    1962     | 
              
I love to cook with wine.     Sometimes I even put it in the food.
Reply to
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

--
| James E.Thompson                                 |    mens     | 
| Analog Innovations                               |     et      | 
| Analog/Mixed-Signal ASIC's and Discrete Systems  |    manus    | 
| San Tan Valley, AZ 85142   Skype: Contacts Only  |             | 
| Voice:(480)460-2350  Fax: Available upon request |  Brass Rat  | 
| E-mail Icon at http://www.analog-innovations.com |    1962     | 
              
I love to cook with wine.     Sometimes I even put it in the food.
Reply to
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

--
Dr Philip C D Hobbs 
Principal Consultant 
ElectroOptical Innovations LLC 
Optics, Electro-optics, Photonics, Analog Electronics 

160 North State Road #203 
Briarcliff Manor NY 10510 

hobbs at electrooptical dot net 
http://electrooptical.net
Reply to
Phil Hobbs

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

--
| James E.Thompson                                 |    mens     | 
| Analog Innovations                               |     et      | 
| Analog/Mixed-Signal ASIC's and Discrete Systems  |    manus    | 
| San Tan Valley, AZ 85142   Skype: Contacts Only  |             | 
| Voice:(480)460-2350  Fax: Available upon request |  Brass Rat  | 
| E-mail Icon at http://www.analog-innovations.com |    1962     | 
              
I love to cook with wine.     Sometimes I even put it in the food.
Reply to
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

--
Dr Philip C D Hobbs 
Principal Consultant 
ElectroOptical Innovations LLC 
Optics, Electro-optics, Photonics, Analog Electronics 

160 North State Road #203 
Briarcliff Manor NY 10510 

hobbs at electrooptical dot net 
http://electrooptical.net
Reply to
Phil Hobbs

Or two.

--

John Larkin                  Highland Technology Inc 
www.highlandtechnology.com   jlarkin at highlandtechnology dot com    

Precision electronic instrumentation 
Picosecond-resolution Digital Delay and Pulse generators 
Custom timing and laser controllers 
Photonics and fiberoptic TTL data links 
VME  analog, thermocouple, LVDT, synchro, tachometer 
Multichannel arbitrary waveform generators
Reply to
John Larkin

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

--
Dr Philip C D Hobbs 
Principal Consultant 
ElectroOptical Innovations LLC 
Optics, Electro-optics, Photonics, Analog Electronics 

160 North State Road #203 
Briarcliff Manor NY 10510 

hobbs at electrooptical dot net 
http://electrooptical.net
Reply to
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

--
Dr Philip C D Hobbs 
Principal Consultant 
ElectroOptical Innovations LLC 
Optics, Electro-optics, Photonics, Analog Electronics 

160 North State Road #203 
Briarcliff Manor NY 10510 

hobbs at electrooptical dot net 
http://electrooptical.net
Reply to
Phil Hobbs

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

--
| James E.Thompson                                 |    mens     | 
| Analog Innovations                               |     et      | 
| Analog/Mixed-Signal ASIC's and Discrete Systems  |    manus    | 
| San Tan Valley, AZ 85142   Skype: Contacts Only  |             | 
| Voice:(480)460-2350  Fax: Available upon request |  Brass Rat  | 
| E-mail Icon at http://www.analog-innovations.com |    1962     | 
              
I love to cook with wine.     Sometimes I even put it in the food.
Reply to
Jim Thompson

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