As a continuation to my previous article, I decided to make some more rigorous measuring. I grabbed 8 transistors from each type and measured hFE, and plotted all samples.
Tables
Raw data
| PN2222 | 2N3904 | BC547 | BC546 |
|---|---|---|---|
| 8 | 324 | 209 | 541 |
| 30 | 340 | 217 | 527 |
| 96 | 316 | 235 | 511 |
| 91 | 335 | 204 | 529 |
| 39 | 323 | 232 | 527 |
| 18 | 324 | 228 | 548 |
| 14 | 339 | 194 | 504 |
| 8 | 345 | 204 | 451 |
Statistics
| PN2222 | 2N3904 | BC547 | BC546 | |
|---|---|---|---|---|
| Samples | 8 | 8 | 8 | 8 |
| Min | 8 | 316 | 194 | 451 |
| 25% | 12 | 323 | 204 | 509 |
| Median | 24 | 329 | 213 | 527 |
| 75% | 52 | 339 | 229 | 532 |
| Max | 96 | 345 | 235 | 548 |
| Mean | 38 | 330 | 215 | 517 |
| Std | 35.885 | 10.306 | 15.023 | 30.335 |
| Variance | 1287.714 | 106.214 | 225.696 | 920.214 |
| CV | 0.9443 | 0.0312 | 0.0698 | 0.0586 |
| Skew | 1.1033 | 0.0059 | 0.0675 | -1.6881 |
| Kurtosis | -0.4912 | -1.5993 | -1.5749 | 3.418 |
Charts
Box plot
Here are the plots for each type of transistors as well
BC546
BC547
2N3904
PN2222
Conclusion
The PN2222 batch exhibits unacceptable hFE variation (CV = 94%), with a mean of 38, below typical application needs and near the lower spec limit. In contrast, the 2N3904, BC547, and BC546 lots show tight, centered distributions well within datasheet ranges.
While the suspected “fakes” (BC547, 2N3904) perform reliably in DC gain, hFE alone is insufficient. High-frequency behavior (ft), saturation voltage, and noise remain untested.
Recommendation: Source small-signal transistors from authorized distributors. The cents premium per part avoid hours of debugging.