analogjfetswitches
This is a circuit I've found online and apparently it is taken from the very rare audio book by National semi. The circuit is used to switch a signal on and off at a transition time that the user sets using the RC input. The problem is that I can't seem to get this to work in simulation. Any ideas what I might be missing out of?
In order to turn on the MOSFET, a certain voltage between its gate and source must be attained. In your case, you have an n-channel MOSFET, meaning that once the gate is driven to a higher voltage than the source, it will begin to conduct current between its source and drain.1
In order to do this the 12V circuit must not float with respect to the control circuit. You'll need the 12V circuit to share a ground with the Raspberry Pi in this case:
1 There's also a parasitic diode that allows conduction in the other direction regardless of gate voltage, but it's not relevant to this circuit as it is currently designed.
You can't switch voltages higher than the gate voltage, no. You must always have the gate-to-source voltage (Vgs) greater than the transistor's threshold voltage if you want it to conduct well. If you want "pass a 10V signal" then you want the source and drain to be at 10V, which means that the voltage on the gate must be (at least) 10V plus Vth. Lower values of Vgs will result in higher drain-to-source resistance, so you really want the gate voltage to be much higher than the voltage you want to switch.
Best Answer
The circuit you posted can work. These are the results of a hastily conceived LTSpice simulation with some guesswork values for the components:
Therefore there could be something wrong in the simulation parameters or the choice of the values of the components.
As I said in a comment, you should post additional information for us to be able to troubleshoot the problem.
Note: this circuit works (i.e. switch opens) under the assumption that the control signal's amplitude is much higher than the amplitude of the controlled signal, here 10V and 100mV respectively. Well, actually the assumption is that the maximum positive value of \$V_{ctrl}\$ must be greater than the maximum positive \$V_{sig}\$ value + \$V_{GS(off)}\$ of the JFET, but since in practice \$V_{GS(off)}\$ is not very predictable it's better to err on the safe side.
This is the easiest mistake I guess you could have done (insufficient amplitude of control signal), but this is wild guessing in absence of more data from you.