Author Topic: Measuring hfe of "P2N2222A" varies wildly  (Read 5042 times)

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Offline IanB

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Re: Measuring hfe of "P2N2222A" varies wildly
« Reply #25 on: November 08, 2021, 02:05:27 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.

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?
 

Offline TimFox

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Re: Measuring hfe of "P2N2222A" varies wildly
« Reply #26 on: November 08, 2021, 04:11:10 am »
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.
 

Offline CatalinaWOW

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Re: Measuring hfe of "P2N2222A" varies wildly
« Reply #27 on: November 08, 2021, 04:26:11 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.

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?

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.
 

Online Zero999

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Re: Measuring hfe of "P2N2222A" varies wildly
« Reply #28 on: November 08, 2021, 02:37:32 pm »
The variation from transistor to transistor is likely to be as large or larger than the difference between hFE and hfe. 
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.
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.
 

Offline Cerebus

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Re: Measuring hfe of "P2N2222A" varies wildly
« Reply #29 on: November 08, 2021, 02:44:39 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.

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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Online Zero999

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Re: Measuring hfe of "P2N2222A" varies wildly
« Reply #30 on: November 08, 2021, 02:58:35 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.

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.  :)
Fair enough my wording could have been better, but I would have thought it would be obvious that hFE is nearly equal to hfe at low frequencies and there's only a big difference at higher frequencies. For the purpose of this exercise, what constitutes high and low frequencies, depends on the transistor.  A  big and slow 2N3055's hfe might drop significantly at the upper end of the audio band, whereas a PN2222A's hFE and hfe can be treated the same, up to a few MHz.
 

Offline Cerebus

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Re: Measuring hfe of "P2N2222A" varies wildly
« Reply #31 on: November 08, 2021, 03:27:10 pm »
A  big and slow 2N3055's hfe might drop significantly at the upper end of the audio band, whereas a PN2222A's hFE and hfe can be treated the same, up to a few MHz.

"Plummet" is the word you're looking for:



Yes, a guaranteed minimum hfe of 10 at 1kHz. There's a reason nobody uses 2N3055s in audio any more (if they have any sense).
« Last Edit: November 08, 2021, 03:29:30 pm by Cerebus »
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Offline SeanB

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Re: Measuring hfe of "P2N2222A" varies wildly
« Reply #32 on: November 08, 2021, 06:14:30 pm »
Only problem with that is that a part branded as 2N3055 might actually be a very fast part, not the original slower than paint drying planar device, but the modern part, while meeting or exceeding all the headline specs for a 2N3055, might actually have quite a bit of gain at 30MHz, and can bite you. After all, all you need to brand a device as 2N3055, is have a peak collector current of 15A, gain greater than 3 at that current, and low leakage at 60V, which is very easy today with most modern process plant. But the data sheet does not reflect this at all, as you do not get typical values for a lot of those parameters, just the minimum JEDEC spec for the part number.
 
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Offline Cerebus

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Re: Measuring hfe of "P2N2222A" varies wildly
« Reply #33 on: November 08, 2021, 06:25:32 pm »
That snippet is actually from the datasheet of a modern 2N3055 from OnSemi. That second column is a "Max" column, it was just too much faff to screen grab the table headers and integrate them into the upload.

Nevertheless, your point is well made.
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Offline CatalinaWOW

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Re: Measuring hfe of "P2N2222A" varies wildly
« Reply #34 on: November 08, 2021, 06:37:41 pm »
Just out of curiosity I went and looked at the current OnSemi 2N3055AG data sheet.  No mention of hfe at all.  Just a gain bandwdith product spec (.8 MHz min to 6 mHz max for base part, 2.2 MHz min and 18 MHz max for a select part)

As always parts and their data sheets are a moving target, with traps for the unwary in any on going production situation.

https://www.onsemi.com/pdf/datasheet/2n3055a-d.pdf
 

Offline TheBay

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Re: Measuring hfe of "P2N2222A" varies wildly
« Reply #35 on: November 10, 2021, 06:38:23 pm »
Lenovo and Dell use jellybean transistors as thermal sensors on their desktop PC's
 

Offline SeanB

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Re: Measuring hfe of "P2N2222A" varies wildly
« Reply #36 on: November 11, 2021, 06:44:14 am »
Lenovo and Dell use jellybean transistors as thermal sensors on their desktop PC's

Yes, the physics does not change at all for the part being cheap. Going to be accurate to under 5C straight out the box with only the reference voltage of the ADC chip, good enough for a PC, and with a little bit of correction per batch you can get to 1C easily.
 

Online Zero999

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Re: Measuring hfe of "P2N2222A" varies wildly
« Reply #37 on: November 11, 2021, 08:17:38 am »
Lenovo and Dell use jellybean transistors as thermal sensors on their desktop PC's

Yes, the physics does not change at all for the part being cheap. Going to be accurate to under 5C straight out the box with only the reference voltage of the ADC chip, good enough for a PC, and with a little bit of correction per batch you can get to 1C easily.
It's got nothing to do with the hFE though.
https://www.sensortips.com/featured/get-temperature-sensor-transistor/
 

Online David Hess

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Re: Measuring hfe of "P2N2222A" varies wildly
« Reply #38 on: November 11, 2021, 10:04:20 pm »
Yes, the physics does not change at all for the part being cheap. Going to be accurate to under 5C straight out the box with only the reference voltage of the ADC chip, good enough for a PC, and with a little bit of correction per batch you can get to 1C easily.

A good delta Vbe temperature measurement circuit can achieve better than 1C with even a cheap transistor and no calibration.  Accuracy is primarily limited by the accuracy of the current ratio used for measurement, and series resistance including base spreading resistance.  The error contribution from series resistance can be removed with a Vbe temperature measurement using three currents but this is rarely done because keeping the measurement current low has the same effect.
« Last Edit: November 11, 2021, 10:13:54 pm by David Hess »
 


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