MOSFETs don't have an intrinsic frequency limit like a bipolar. Which is also why good board layout and attention to parasitics is important because you can easily get VHF oscillations.?
But as Allison said the gate capacitance is the issue. Which in other words means it gets harder and harder to match the ever lower impedance efficiently as you go up in frequency.?
Even though the IRF510 is often accused of only being a car headlamp switching transistor... These capacitance and impedance issues affect even "real" RF MOSFETs as much or almost as much.?
With careful feedback and layout as in the 10W Linear amp kit the gain can be made quite flat across HF 1.8-30MHz to within a dB or two.?
The 50W PA kit was a much more basic design effort originally done for 40m where everything is easiest as Allison mentioned. Reasonable worthwhile performance obtained up to 20m made those acceptable options too. I'm not surprised 15m was even lower down.?
On Thu, Mar 14, 2024, 7:30?PM ajparent1/kb1gmx <kb1gmx@...> wrote:
I have a linear amp that gets over 40W at 6M using IRF510s. Wasn't hard to do that.
The characteristics are not great but its not about capacitance as a sole factor.? Then again input Z is a factor even MRF and BLF series MOSFETs.
The problem of wideband amps with MOSFETs is input and output matching. The gate is a capacitor and that makes input matching difficult.? Most designs using IRF510 or IRF530 40M is a sweet spot.? The problem being input impedance goes down with increasing frequency.
Develop voltage at the gate effectively you get power out.
-- Allison ------------------ Post online only,? direct email will go to a bit bucket.
