ORS87 with different FETs
Here are some measurements with a variety of (through-hole) FETs. As in earlier posts, these were sourced as follows:
- J113: OnSemi, CPC (Farnell)
- 2N3819: bought in 1990's, probably from Maplin or Farnell, Fairchild marking
- BF256B: OnSemi, CPC (Farnell)
- J305: InterFET, Mouser
- 2SK170: bought > 5 years ago, small UK supplier, can't remember
- 2SK117: eBay, Chinese seller
- 2SK30A: eBay, Chinese seller
Test conditions: C4 = 10pF, Rload = 47K, C12/C13/R14/R15 not fitted, 1KHz 100mV sine wave in. Measurements from REW.
Gain vs bias current
View attachment 135978
THD vs bias current
View attachment 135979
Summary table
Includes the gate-source (Vgs), and the calculated value of the R7 trimmer.
| J113 | 2N3819 | BF256B | J305 | 2SK170 | 2SK117 | 2SK30A | 2N3819 (SGD) |
---|
Id (mA) | 0.37 | 0.41 | 0.43 | 0.40 | 0.36 | 0.36 | 0.45 | 0.40 |
Vd (V) | 12.7 | 10.5 | 9.2 | 10.8 | 13.3 | 13.0 | 7.9 | 10.7 |
Vgs (V) | -1.64 | -3.32 | -2.10 | -1.70 | -0.63 | -0.91 | -1.49 | -3.33 |
R7 (Kohm) | 4.47 | 8.19 | 4.92 | 4.26 | 1.77 | 2.51 | 3.29 | 8.25 |
Gain | 4.81 | 4.61 | 4.72 | 4.71 | 3.79 | 4.64 | 4.3 | 4.55 |
THD (%, 100mV in) | 0.12 | 0.17 | 0.15 | 0.14 | 0.07 | 0.078 | 0.17 | 0.17 |
Is the 2N3819 reversible?
The Internet (and various datasheets) can't agree on whether pin 1 on the 2N3819 is source or drain (and pin 3 is, respectively, drain or source). I have been using pin 1 = drain ("DGS", when looking at the flat side of the package) for most measurements, as per the Fairchild datasheet, but thought I'd try with it reversed ("SGD"). Results are in the final column above.
Basically, there's no meaningful difference, within the repeatability of the measurement setup. The gain difference (0.1dB) might be some change in drain-gate capacitance, but unlikely to make any meaningful difference to circuit performance, and almost certainly smaller that the difference between individual FETs.