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Measuring hfe of "P2N2222A" varies wildly
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IanB:

--- Quote from: CatalinaWOW on November 08, 2021, 01:17:45 am ---You are absolutely and pedantically correct.  Though I don't know when this distinction was made.  When I was in university (during the transition from vacuum tubes to transistors) I don't recall both terms being identified.  And there is much useful (but approximate) analysis that can be accomplished without making this distinction.

If you check data sheets it is somewhat uncommon to find both specified.  And when they are it is common to find a wide range of spec values for both, with a very large overlap in the ranges.  So it would be unusual to make a production design that depended on a difference in the values. 

Of course, when it matters you need to be aware of the difference.  And you can then buy large numbers of transistors and select the ones that meet the needs of your circuit.  Or spend the time to measure each transistor and adjust the circuit to match the transistor.  Not the usual way to build things, but for certain applications might make sense.

--- End quote ---

Surely a comparison can be made here with diodes, where "resistance" could be measured as either V/I or as dV/dI, and clearly they have different values for any given value of I or V.

Similarly, if Ic/Ib is more or less constant for varying values of Ic, then dIc/dIb would have the same value as Ic/Ib. However, if this is not the case, and if dIc/dIb could have significant variation compared to Ic/Ib, then the distinction matters.

I imagine if you were doing a "small signal" design around a particular bias point on the transistor, then dIc/dIb would be the parameter of most interest?
TimFox:
To first approximation, given the wide range of hFE and hfe from unit to unit for a given part number, the numerical values for both will be similar (within the spec range).  However, the "fe" values are for AC small signal analysis and the "FE" values are for switching large-signal analysis.
CatalinaWOW:

--- Quote from: IanB on November 08, 2021, 02:05:27 am ---
--- Quote from: CatalinaWOW on November 08, 2021, 01:17:45 am ---You are absolutely and pedantically correct.  Though I don't know when this distinction was made.  When I was in university (during the transition from vacuum tubes to transistors) I don't recall both terms being identified.  And there is much useful (but approximate) analysis that can be accomplished without making this distinction.

If you check data sheets it is somewhat uncommon to find both specified.  And when they are it is common to find a wide range of spec values for both, with a very large overlap in the ranges.  So it would be unusual to make a production design that depended on a difference in the values. 

Of course, when it matters you need to be aware of the difference.  And you can then buy large numbers of transistors and select the ones that meet the needs of your circuit.  Or spend the time to measure each transistor and adjust the circuit to match the transistor.  Not the usual way to build things, but for certain applications might make sense.

--- End quote ---

Surely a comparison can be made here with diodes, where "resistance" could be measured as either V/I or as dV/dI, and clearly they have different values for any given value of I or V.

Similarly, if Ic/Ib is more or less constant for varying values of Ic, then dIc/dIb would have the same value as Ic/Ib. However, if this is not the case, and if dIc/dIb could have significant variation compared to Ic/Ib, then the distinction matters.

I imagine if you were doing a "small signal" design around a particular bias point on the transistor, then dIc/dIb would be the parameter of most interest?

--- End quote ---

You are right.  There can be factors of difference between the two.  But I think the real point many are trying to get to is that designs which depend on hfe or hFE being within a factor of three to five of a specific value are unlikely to be successful.  The variation from transistor to transistor is likely to be as large or larger than the difference between hFE and hfe. 

My design courses in school emphasized techniques to make the circuit insensitive to the specific value of these parameters. The work I have seen and done since then continues this emphasis.   And in designs which are insensitive to large differences in the value there is usually little sensitivity to the difference between the two parameters.

To me the right analogy with respect to the subject of this thread are those printed battery testers seen on one brand of battery.   The one that has a heating element and a liquid crystal coating over a printed red-yellow-green strip.  Shorting it across the battery terminals heats the display changing the opacity of the liquid crystal and revealing a good/bad indication for the battery.  It can be argued that this is a crude current meter or a crude voltmeter, though it properly does neither.  But it does provide a useful indication of the battery condition.  The hfe test in these DMMs does not technically measure either hFE or hfe correctly, but it does give a number in the neighborhood of both, which is good enough for some purposes.
Zero999:

--- Quote from: CatalinaWOW on November 08, 2021, 04:26:11 am --- The variation from transistor to transistor is likely to be as large or larger than the difference between hFE and hfe. 
--- End quote ---
That depends on the frequency and GBWP of the transistor. At higher frequencies, there will be a significant difference between the hFE and hfe.


--- Quote ---To me the right analogy with respect to the subject of this thread are those printed battery testers seen on one brand of battery.   The one that has a heating element and a liquid crystal coating over a printed red-yellow-green strip.  Shorting it across the battery terminals heats the display changing the opacity of the liquid crystal and revealing a good/bad indication for the battery.  It can be argued that this is a crude current meter or a crude voltmeter, though it properly does neither.  But it does provide a useful indication of the battery condition.  The hfe test in these DMMs does not technically measure either hFE or hfe correctly, but it does give a number in the neighborhood of both, which is good enough for some purposes.

--- End quote ---
Yes, the hFE test on a DMM is good enough to check whether a transistor works, or not. Unfortunately it tends to be the cheaper meters which have it, probably because it compromises the CAT rating, which isn't important if you're only using it on extra low voltage circuits.

Those battery test strips are often better, than a proper multimeter, because they actually provide a load for the battery. Measuring the voltage with a meter is often a poor indication of whether the battery is good, or not. It's not unusual for an AA cell to measure 1.5V, but have a high impedance, causing the voltage to drop under load.
Cerebus:

--- Quote from: Zero999 on November 08, 2021, 02:37:32 pm ---That depends on the frequency and GBWP of the transistor. At higher frequencies, there will be a significant difference between the hFE and hfe.

--- End quote ---

If we're all going to get pedantic about it then the significant difference is between the hFE at DC and the hfe at higher frequencies.  :)
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