Electronic – JFET Audio switch, does not work in simulation


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?

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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:

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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.