Gate to drain capacitor + source to ground cap = oscillation, apparently?

[midwayfair]

Was messing around on the breadboard last night with a MOSFET booster, just experimenting with some things.



I decided to try a smaller source cap and moved C2 to between the gate and source. When the source cap got really small -- 100nF -- and the gain was maxed, it started oscillating. When the cap was much bigger (1uF was the next size up that I tried), this didn't happen. Plenty of power filtering.

So what happened here? What turns this into a positive feedback loop, and why could increasing the cap size fix it?

I was hoping to find a nice elegant way to get a low-pass and the "Miller capacitance" has often done the trick on some other things, but here something goes horribly amiss.

[PRR]

> what happened here?

I have an idea, but it would take too long to teach it.

> "Miller capacitance" has often done the trick

This only low-passes in conjuction with a resistor. What resistor?

> a nice elegant way to get a low-pass

Put 0.1uFd across R4. That'll whack your highs. Trim to taste.

[midwayfair]

> what happened here?

I have an idea, but it would take too long to teach it.

> "Miller capacitance" has often done the trick

This only low-passes in conjuction with a resistor. What resistor?

> a nice elegant way to get a low-pass

Put 0.1uFd across R4. That'll whack your highs. Trim to taste.

Thanks, PRR, I missed your reply earlier.

I'm not sure how to answer the "what resistor" question. When I stick it in this circuit, it trims the highs similarly whether the circuit's hooked right up to a guitar (figure 5-7K) or after a buffer (figure ... 300Ohm). I thought it was simply bypassing them -- meaning they aren't amplified.

I don't expect you to teach me what happened, but is there a one-sentence word salad that would tell me what to look up on my own?