Background
I’m a firm believer than sharing knowledge and experience is the best way forward to continue learning yourself. It always pay pack at some point. This time Paul Prinz, a fellow implementer of the 3B7 DHT Preamp using the gyrator PCB, came back with a great suggestion. He found a MOSFET which could do high drain currents, it has high transconductance and most importantly the parasitic capacitances were low even close to the BF862. Hooray, I thought. We may have a great solution here to use the gyrator load for currents above 25mA and with similar performance to the great BF862. There are some other depletion MOSFETs that can do high currents, however they all have relatively high capacitances and low transconductances when VDS is low, like in the cascoded gyrator circuit.
The BSH111BK is an enhancement MOSFET, so doesn’t have a “depletion” behaviour like the jFETs. This isn’t a problem as the bias voltage can be set by the reference CCS.
For comparison, here is a brief summary of the key characteristics of these three devices:
| BF862 | BSH111BK | MMBFJ310L | |
| Ptot (W) | 0.3 | 0.3 | 0.225 |
| VDSmax (V) | 20 | 55 | 25 |
| VGS off (V) | -1.2 | -4 | |
| IDSS (mA) | 25 | 210 | 60 |
| Gfs (mS) | 45 | 640 | 18 |
| Ciss (pF) | 10 | 19.1 | 5 |
| Crss (pF) | 1.9 | 1.5 | 2.5 |
| Coss (pF) | 2.7 |