| General > General Technical Chat |
| Measuring hfe of "P2N2222A" varies wildly |
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| TimFox:
By definition, the “FE” subscript denotes the (DC) ratio of IC/IB, while the “fe” subscript denotes the (AC) derivative dIC/dIB, both evaluated at a given value of VC and IC. |
| armandine2:
open circuit output? or something in between? |
| armandine2:
no reply -probably not a great question - but it looks like hFE matches up well with a simple test: |
| Benta:
--- Quote from: CatalinaWOW on November 07, 2021, 03:19:10 am ---The data sheet tells the current at which the manufacturer specifies hfe. But hfe is a parameter that exists at all currents and temperatures. And what the DMM reports is a very good approximation of hfe at one operating point. --- End quote --- The DMM does NOT report hfe. That's an AC value. It leads you to think it's reporting hFE (which is a DC value), but in the wrong way. Uppercase and lowercase play a very large role in science and engineering. The almost correct way of measuring hFE is this: |
| CatalinaWOW:
--- Quote from: Benta on November 07, 2021, 10:24:18 pm --- --- Quote from: CatalinaWOW on November 07, 2021, 03:19:10 am ---The data sheet tells the current at which the manufacturer specifies hfe. But hfe is a parameter that exists at all currents and temperatures. And what the DMM reports is a very good approximation of hfe at one operating point. --- End quote --- The DMM does NOT report hfe. That's an AC value. It leads you to think it's reporting hFE (which is a DC value), but in the wrong way. Uppercase and lowercase play a very large role in science and engineering. The almost correct way of measuring hFE is this: --- End quote --- 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. |
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