inverse beta

Anybody know the the typical inverse beta (positive base current, negative collector current) of a typical NPN gumdrop transistor, like a BCX70 maybe? I suppose I could get off my swivel chair and measure it.

I want to be sure that a signal stays near ground during powerup, so an NPN with some positive base current could clamp it to ground. The signal source impedance could be high, 5 or 10K maybe, and it wants to swing from maybe -5 to +12 volts. After things settle I could pull the base to -5 to let things run.

I guess a jfet could do it, but we don't have a single jfet in stock.

Making sure that things power up safe can be tricky.

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John Larkin         Highland Technology, Inc 
picosecond timing   precision measurement  
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John Larkin
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My WAG is you should be able to get a beta of 10, probably more. You'd better measure it. Just stay away from the fast saturated switches - the gold doped bipolars have low inverse beta.

Careful, abs max Vebo is 5V for the BCX70.

Reply to
Frank Miles

Around 1/2, IIRCFALTA. But that may just be the inverse beta of the input transistor in TTL.

  • If I Recall Correctly From A Long Time Ago.
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Tim Wescott 
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Reply to
Tim Wescott

Between 3 and 10, based on modeling Br for NPNs in this range.

RL

Reply to
legg

Then it should work. I could design around 2 maybe. I suppose I should test a couple tomorrow, to be safe.

I may as well measure Vbe breakdown if I go to the trouble to set things up.

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John Larkin         Highland Technology, Inc 

lunatic fringe electronics
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John Larkin

How reliable is it to rely on an unspecified unmeasured inverse beta?

NT

Reply to
tabbypurr

Have a look at Rohm 2SD2704K and 2SD2114 they are designed for muting switch apps and have Veb ratings higher than the usual 6-8V. The

2SD2704K datasheet actually shows it in inverted mode, it's Veb rating of 25V would fit your application.

piglet

Reply to
piglet

What i have seen, is the _stated_ Vebo max for any silicon low-power NPN is ALWAYS 5V, and (so far) the fact is the measured value is in the

8V region.
Reply to
Robert Baer

How about 10 years testing of production product showing less than ten percent variation? Oh, sorry, than it would not be unmeasured... And there are number of Zetex transistors where inverse beta is specified, so i guess you do not want those either..

Reply to
Robert Baer

Aww...you gave up the secret...seemed someone loves unspecified unmeasured inverse beta..

Reply to
Robert Baer

Sounds good to me.

I don't, John might.

NT

Reply to
tabbypurr

I don't know about BCX70.

Piglet mentioned muting transistors, which are botique, but work well.

General purpose parts are in the 0.5-10 range. I'm not sure if any are really that low, aside from power transistors probably. General purpose parts are in the 5ish range.

"Low Vce(sat)" parts, like PBSS303NX, have extremely high BR, like 150 versus 300 BF. When they say they make good switches, they aren't lying.

The SPICE models rarely indicate this, by the way, so no help looking there. :(

Tim

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Seven Transistor Labs, LLC 
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Reply to
Tim Williams

+1 on the 2SD2704K. I use them for low-current cap multipliers--the high beta and amazing V_BEO are the bee's knees.

The beta doesn't hold up that well at high current, though.

Cheers

Phil Hobbs

Reply to
Phil Hobbs

Depending on the device, Br can vary between 0.1 and 300 in modeling. A typical value indicating that the parameter is unspecified, is 1.

Integrated circuit Br for venerable CA30xx parts is 0.1.

More recent low VCEsat switches show Br >50, in practice and in their models.

Bipolar modeling parameters for standard bjts were organized on a spreadsheat, that should be searchable on the LTspice Yahoo Group website, or try:

formatting link

RL

RL

Reply to
legg

Interesting thanks. What does it mean when Br is greater than Bf (as in Q2SA1302 or 1357) ?

piglet

Reply to
piglet

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Gold-doped switching transistors may be different. The (UK) Journal of Scie ntific Instruments published a short note back in the 1960's about using su ch a device - I think it was a 2N3906 but it's a long time ago - exploiting a very predictable 5.6V base-emitter reverse breakdown voltage and the col lector-base diode drop to make a very cheap 6.2V low-temperature coefficien t referene diode.

I've got a note of the exact literature reference back in Sydney - I think

- but since I'm in Provence, France at the moment, I can't quote it, though I seem to remember posting it here years ago.

You had to tweak the reverse current around the one to two mA level to get the lowest temperature coefficient, but the author seemed happy to claim th at the reverse voltage was always close to 5.6V about a milliamp.

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Bill Sloman, Sydney
Reply to
bill.sloman

Semiconductors seem to be very consistant. I would guardband it by some factor. There are tons of interesting and important characteristics that aren't specified in active and passive parts, so testing is useful, followed by some prudent estimation of risk.

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John Larkin         Highland Technology, Inc 

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John Larkin

Low Vsat parts indeed have high inverse beta. E.g., the large family of parts by Zetex, now Diodes, Inc. You can plot Vce vs Ic, where the current goes through zero and negative and get a nice continuous trace.

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 Thanks, 
    - Win
Reply to
Winfield Hill

OK, not wanting to be a PHB, I did some soldering and tested one.

Inverse beta is 16!

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John Larkin         Highland Technology, Inc 
picosecond timing   precision measurement  
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John Larkin

ts.JPG

You don't really need to know inverse beta with any precision. IIRC most ch opper apps drove the emitter/base current ratio to less than one by some in teger multiple. Looks like the JFETs replaced transistors for chopper appli cations and even they are being phased out now. Here's some first order mod eling:

Please view in a fixed-width font such as Courier.

. . . . . Ebers-Moll for forward n-p-n: . . . 1 1 IC . - + - x -- . alphaI betaI IB . VCE= V x LN( --------------------- ) . T 1 IC . 1 - - x -- . betaF IB . . . . alpha beta . beta= --------- alpha= -------- V = 0.026V . 1-alpha 1+beta T . . . . . I= inverse transfer F= forward transfer . . . . For inverse n-p-n: . . . . 1 1 IE . - + - x -- . alphaF betaF IB . VEC= V x LN( --------------------- ) . T 1 IE . 1 - - x -- . betaI IB . . . . . . 1 . VEC= V x LN( --------------------- ) for IB>>IE . T 1 IE . 1 - - x -- . betaI IB . . . . . VEC=10x IE . . . .

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bloggs.fredbloggs.fred

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