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Transistor Beta Minimum and Maximum

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vladtess

Joined Jan 5, 2011
43
Hi there!!

I have a question about hfe of transistors. Lets take for example a low power npn transistor, 2N2222A. On 2nd page, there is a section of table where various conditions are stated and various hfe values are presented. On the right, there is also a max and min values of hfe.

I know the single formula (\(\beta\) = \(\frac{Ic}{Ib}\)) that is used to property use the transistor in combination with ohm's law. Can someone please explain the meaning behind the minimum hfe and maximum as well as those test conditions and how they can be useful.

Thanks much!!!
 
S

someonesdad

Joined Jul 7, 2009
1,583
It means it's a random variable and the highs and lows tell you something about the range of parts' values you can expect (either due to process variations, inspection culling, or both). hfe is a notoriously poorly-controlled quality parameter.
 
ErnieM

ErnieM

Joined Apr 24, 2011
8,415
Way back when I took transistor theory our book was "Wave generation and shaping" by Leonard Strauss, and our professor (who by a curious coincidence was also named Leonard Strauss) taught us to just assume the beta of a transistor was 30 minimum, as that was the criteria for an IC wafer acceptance.
 
Wendy

Wendy

Joined Mar 24, 2008
23,797
The values in the datasheet set upper and lower limits, transistors are notoriously fickle. Depending on how they biased, the exact value can vary even on the same transistor.

There are many ways to overcome this, mostly through various biasing techniques that set the gain of the circuit, which will always be less than the gain of the transistor. If not, then the transistors gain is the limiting factor.
 
ErnieM

ErnieM

Joined Apr 24, 2011
8,415
And perhaps one of us should note that while:

\( \beta = \fra {I_B}{I_C}\)

is a gross oversimplification it is a quite useful simplification when used correctly.
 
Ron H

Ron H

Joined Apr 14, 2005
7,063
ErnieM said:
And perhaps one of us should note that while:

\( \beta = \fra {I_B}{I_C}\)

is a gross oversimplification it is a quite useful simplification when used correctly.
Click to expand...
\( \beta = \fra {I_C}{I_B}\)
is the exact definition. No oversimplification. As others have mentioned, beta varies from one device to the next, and with current and voltage. But the ratio Ic/Ib is always beta.
 
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Thread Starter

vladtess

Joined Jan 5, 2011
43
Well then, when I insert my PNP into a multi meter, I get a single value, not mins or max. Is that the value that I should use in my calculations? I have got to read that chapter... Thanks you all!
 
t_n_k

t_n_k

Joined Mar 6, 2009
5,455
vladtess said:
Well then, when I insert my PNP into a multi meter, I get a single value, not mins or max. Is that the value that I should use in my calculations?
Click to expand...
That's true but if you were designing something that was going to be produced on a mass scale you'd want to be sure your design worked for the range of β that might be expected for that device type - i.e. from minimum to maximum.
 
SgtWookie

SgtWookie

Joined Jul 17, 2007
22,230
Your meter just gives an hFE for a fixed low current value; perhaps a milliampere or few.

If you are using a transistor as a saturated switch, then use:
Ib = Ic/10
where Ic is the desired collector current.
If you try to use the minimum hFE in the datasheet, the transistor may not be in saturation, with resulting power dissipation in the transistor due to a high Vce.
 
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