Stop that oscillation please!

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.

Doggy bag

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…


2 Replies to “Stop that oscillation please!”

    1. Hi John,
      Unfortunately it varies on the MOSFET used. For example as the CCS devices, the 3NK60ZFP was ok with 1K stopper but the FQPF2N60 wasn’t and also needed 100R. Mind you that I also made another change to the CCS which was to include a BC456 type of bipolar instead of the HV version MPSA42.

      The bipolar transistor is in fact protected (both VCBO and VCEO) with the back to back zeners and the VBE diode junction to ensure there is no more than 16-20V at startup to avoid damage, so an MPSA42 is not rather needed. However, as the open loop feedback gain of the CCS is increased due to hFE difference, then I suspect that may also contribute to it.

      Bottom line, to be on the safe side, I’d rather change both to 100R. I haven’t tested myself values above 100R, but I’d guess that up to 300R should be acceptable.
      Hope this helps

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