Looked at that and the output stage is the Softrock-Ensamble amp a proven
design for 3-30.
Also Hans QRPlabs used the BS170 (*2 P-P) as driver for IRF510s to get near flat 3-30mhz.
I used a 2n5109 single ended driving IRF510s also remarkably flat.
The real trick is the driver as if that not flat the amp is then degraded.
Thats a variation of Miller calculation for triodes from way back and applied
to modern devices.? The rough outline of the calculation is input C (c gate to source)
and the feedback C? (c gate to drain) and the other effector is gain.? more gain
more the effect of feedback C and of course the frequency.? ?Its why pushing
the IRF510 or any MOSFET for more gain at high impedance (input and output)
results in poor gain with increasing frequency or even instability.
The use of a bifiler choke at the output at 1W is likely of little value.
It does allow for other transformer possibilities that might not provide
a convenient center tap such as transmission line transformers.
In short you want the output as a current source driving a low impedance load
and the input as a low impedance voltage driver.? ?Then the MOSFET gain
and usable frequency range is limited by other factors (lead length and
operating parameters).? As always the trade is gain vs bandwidth.
Allison
