Low-distortion Audio-range Oscillator

[davada]

Originally Posted by RNMarsh

Chicken ! I dont have those same devices but I think I might have some J111 if they are same/similar? I'm not sqeemish about cutting a trace - isnt a problem for me. I dont faint at the sight of a cut. Have No Fear. -RNMarsh

Well if there is promising results I don't have a problem. One thing at a time though.
I did the dual Jfet late night. I have another idea to try similar to the degeneration but active.
Or you could call it active degeneration.

[john curl]

app note

[john curl]

more app note

[john curl]

and more:

[richiem]

John -- can you make those into a PDF and zip it, so we can download the whole thing at once?

[john curl]

No, I can't, sorry.

[1audio]

The note mentions that the lowest distortion is when the fet is most on and has the lowest Vds. The next real question is how much gain range is necessary for an AGC to stabilize an oscillator and what tradeoffs are there? I believe a range from 1.1 to .9 is more than enough (may take a while to settle or start). If Rds on is the important factor 20 Ohm FETs are quite available. Lowering the impedance of the circuit at that point would only help anyway.

[davada]

Originally Posted by 1audio

The note mentions that the lowest distortion is when the fet is most on and has the lowest Vds. The next real question is how much gain range is necessary for an AGC to stabilize an oscillator and what tradeoffs are there? I believe a range from 1.1 to .9 is more than enough (may take a while to settle or start). If Rds on is the important factor 20 Ohm FETs are quite available. Lowering the impedance of the circuit at that point would only help anyway.

More gain may be needed to get the oscillator started and to it's reference level.
There is no reason why we could'nt switch from a start-up to a run mode.
Fast response to low distortion as it was done in the ST1700 series.

My experience has been that just enough gain for running at say 1KHz is insufficient for start up at other higher frequencies. The gains are different at 100, 1KHz 10KHz and 100KHz. The oscillator may run in a narrow gain range but it start.

[1audio]

I'll scan that portion of the ST1700 oscillator this evening and publis it. They used both an LDR and a FET. Get all options involved. . .

[davada]

Originally Posted by 1audio

I'll scan that portion of the ST1700 oscillator this evening and publis it. They used both an LDR and a FET. Get all options involved. . .

I used to have one of these. I have the manual in PDF for the 1700B.
I can post it if you want to save the time scanning it. If the files are small enough to post.

Up to you.