I’ve been playing for some weeks the marvellous UV-201a stage. The addition of the source follower brings a new dimension to this stage. However, I experienced random oscillation on this setup. I knew it was the MOSFET, but I wanted to pin it down.
I replaced the SiC version with the famous AOT1N60. That cure the issue, but I wanted to find out what was going on, of course blaming my selection of gate stoppers.
In case you were asking, how does the follower oscillation manifests? Well, simply like a random harsh noise on the background. You can tame it by restarting the HT supply, but that is a temporary workaround.
The guilty bastard was the 1KΩ carbon composition gate stopper. Here is what you get typically:
A nice crest around 5-6Mhz. And this is the most stable, try the SiC MOSFET instead of the AOT1N60, it will go wild!
So if you change the stoppers from 1KΩ to simply 100Ω, this is what you get:
And surprisingly, this is what you get with the C3M028009 SiC used before:
My FR system can do better than this at 10Mz. A flat response all the way up, I like that.
Lesson learned: change the MOSFET enhancement mode stopper from 1KΩ down to 100Ω in a follower mode. Otherwise, it will go crazy!
I will update the source follower documentation…