Viva Analog tube emulation

[slacker]

Where I am struggling is that I don't quite understand what is happening when the output gets near the positive rail (I not proportional to v anymore), as it seems to cause the source voltage to drop.

If you're talking about the 2 opamp version then don't forget the LM358 isn't rail to rail, the output will only go up to about 1 Volt below the positive supply, I don't know if that explains anything.

[PBE6]

Thanks slacker, the 1V bias may do the trick for me.

Out of curiosity how close to the rail does the LM358 go? I didn't see it on the data sheet unless I'm reading it wrong/looking for the wrong parameter.

EDIT: Oops, it does say in the plain-language description that the LM358 can swing to ground when using a single supply. I guess that's why it's a good choice in the original circuit.

[PBE6]

Turns out I had some LM358P's lurking in the bottom of my opamp bin..mwahahah!! Ok time to breadboard..

[samhay]

Turns out I had some LM358P's lurking in the bottom of my opamp bin..mwahahah!! Ok time to breadboard..

That's why the 358 was suggested - they tend to show up in the dusty corners of your parts bin.

[samhay]

Where I am struggling is that I don't quite understand what is happening when the output gets near the positive rail (I not proportional to v anymore), as it seems to cause the source voltage to drop.

If you're talking about the 2 opamp version then don't forget the LM358 isn't rail to rail, the output will only go up to about 1 Volt below the positive supply, I don't know if that explains anything.

No, I think I am just trying too hard to understand the freak-out behaviour of the op-amp when the output hits the rails.

So here is the real quesiton. What's the best way to trim the two op-amp? I guess I am trying to understand theoretically where to set the top op-amp relative to the source voltage, which dictates how much higher the FET drain voltage is relative to source.