sstc changing frequency issue

[MarkE]

Jeg, you can make your life slightly easier by lifting the 0.22uF capacitor from ground and inserting a choke there.  You can then inject your audio at the junction of the capacitor and the choke.

[Jeg]

I really like when my life is getting easier!

Is there any way to calculate this choke before to start winding?
Do i have to make a resonance between choke and cap at these low audio freqs?

Tnks
Jeg

[MarkE]

Pick the choke inductance based on the impedance of your audio source and the lower frequency limit that you want to go down to:

L ~= 0.025/(Fcutoff^2*0.22E-6)

Once you know the inductance, you can buy an inductor, or use the tables for whatever core you choose to determine the number of turns.

Another way to do it would be to replace the 0.22uF cap with something smaller like 0.022uF, and then just use a 2.2uF or larger nonpolar capacitor to couple your signal source at the junction of the 1K and 15K resistors.  A 0.022uF capacitor will roll off high frequencies at about 8kHz.  A 2.2uF coupling capacitor from your source will roll off on the low end at about 80Hz.  If you want to go lower than that, just use a bigger coupling capacitor.

[Jeg]

Tnks MarkE... i hope i'll do it tonight so to discuss it tomorrow.

[Jeg]

Hi again
At the above self oscillating mosfet circuit, when I matched the two coils in terms of wavelength, the oscillation became a clear sinus which immediately started to grow. After 2 seconds the oscillation became so big that mosfet explode. I think because of overdriving the gate above this 20V limit even I had a 18v zener between gate and sourse. I am thinking to substitute mosfet with a very tough IGbt. Does anyone know the way? I test it like that but it didn't work. Is there any other self oscillating circuit appropriate for IGBT?

STGW40N120KD