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 Author Topic: Help reading current gain from BJT datasheet  (Read 1889 times) 0 Members and 1 Guest are viewing this topic.
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 « on: February 03, 2011, 04:58:59 pm » Bigger Smaller Reset

The bjt I'm working with is the 2N4401: http://www.onsemi.com/pub_link/Collateral/2N4401-D.PDF

On page 5 of the datasheet they have current gain vs Ic graph as seen below

If the top 2 curves are unit 1 and unit 2, then what are the bottom 2 curves for? Which curve should I use?
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 « Reply #1 on: February 03, 2011, 05:04:00 pm » Bigger Smaller Reset

The bottom 2 curves represent even more "units" (parts) that they tested. Basically, the curves show that the various parameters (hFE, hIE, hRE, hOE) are highly variable from part to part. You cannot just "use" one of the curves -- you have to design for worst-case values across all parts.

On page 2 of the datasheet when they say that hFE ranges from 100 to 300 at Ic=150mA, Vce=1V, they really mean it. It can vary anywhere in that range under those conditions, and as you see further up the table as you change Ic then hFE changes too.

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 « Reply #2 on: February 03, 2011, 05:18:33 pm » Bigger Smaller Reset

So if I want to design for a maximum Ic of 5 A, I should make sure that Ib >= 5/120 = 41 mA?
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 « Reply #3 on: February 03, 2011, 05:23:52 pm » Bigger Smaller Reset

If you want to design for a maximum Ic of 5A then you do not want a 2N4401! That's a 0.6A transistor and I wouldn't sustain that for any length of time.

For these currents I'd recommend using a MOSFET instead, e.g., the Fairchild RFP30N06LE as a starting point.

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Measurement changes behavior
 « Reply #4 on: February 03, 2011, 05:25:38 pm » Bigger Smaller Reset

So if I want to design for a maximum Ic of 5 A, I should make sure that Ib >= 5/120 = 41 mA?

5 amps of collector current for a 2N4401 ????

From the front page of the datasheet, Maximum ratings: Collector Current − Continuous IC 600 mAdc

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 « Reply #5 on: February 03, 2011, 05:29:46 pm » Bigger Smaller Reset

Oops my bad! Okay, if I want a max of 600 mA, I should make sure that Ib >= 600/43 = 14 mA?
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 « Reply #6 on: February 03, 2011, 05:33:52 pm » Bigger Smaller Reset

Correct.
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 « Reply #7 on: February 03, 2011, 05:48:18 pm » Bigger Smaller Reset

Also how would I figure out VCEsat? For example, I'm running Ib at around 14 mA, but only wanting to run 250mA from collector to emitter, how would I figure out VCEsat? I think I should be using Figure 16 on pg 6, but that doesn't give an graph for Ic at 250 mA. Figure 17 has VCEsat curve but that's for hfe = 10.
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