Help! Trying to switch JFET at 12v from arduino at 5v

[JP19]

I'm trying to use a JFET to switch audio in this arrangement (see pic). It works fine if I trigger it manually.

I'd like to find a way to turn the JFET on or off using an arduino.

My circuit is at 12v though, so I need to find a way to toggle the point marked 'switch control signal' between >6v and ground (when my arduino can only put out 5v).

Any suggestions on how could I approach this?

Thanks!

[BJM]

Lower the bias voltage maybe? As I understand it the switching is done by applying a voltage higher or lower than Vbias.

[JP19]

I was under the impression that I would lose headroom if I lowered the bias voltage?? (but please do correct me if I'm wrong, I don't really know what I'm talking about).

[BJM]

Maybe someone alse can answer that, I don't know that either. But this switching also works with 9V (as f.e. in Boss pedals), so I assume it will also work with even lower voltages.

[niektb]

A possible option would be to use a small N-Channel MOSFET in open-drain configuration.



Don't quote me on the resistor values though
Not sure though if it's the most cost-efficient solution

[merlinb]

Yes you need an extra transistor or MOSFET to act as a middle-man, like niektb shows. It's called a 'level shifter'.

[iainpunk]

you could use a level shifter transistor configuration, or bias the switching bit lower.
a N-Channel Jfet is ON (conducting) with its Gate at the same or higher voltage than the Drain/Source. its OFF (not conducting) with a voltage lower than its threshold voltage. a good rule of thumb is to guesstimate 4.5v as the threshold voltage, so it needs to be atleast 4.5V lower than the Drain/Source voltage.
if the Base voltage is in between, it acts like a variable resistor.

IMHO, the best option is to bias the signal to 5V only for the switching, and re-bias it to whatever the bias was after the switching stages.
Jfet switches are already low-headroom devices, going over 1Vp-p makes the Jfet not conduct linearly anymore (asymmetric transfer), so i wouldn't worry about headroom loss when moving the bias off center.

cheers, Iain

[JP19]

Jfet switches are already low-headroom devices, going over 1Vp-p makes the Jfet not conduct linearly anymore (asymmetric transfer), so i wouldn't worry about headroom loss when moving the bias off center.

Ah ok that is interesting! Maybe I need to have a total re-think then as I'm keen to maintain the maximum possible headroom for synths etc.

Are relays the only option if I want to switch multiple channels at once? I've always run into switch pop problems when I've messed with relays before (which was what brought me to the JFET switches in the first place)!

[ElectricDruid]

Are relays the only option if I want to switch multiple channels at once? I've always run into switch pop problems when I've messed with relays before (which was what brought me to the JFET switches in the first place)!

No, you've got CMOS switch ICs as an option too. Either older ones like the 4016/4066/4053 series or more modern ones like the DG403/DG407/etc series. The modern ones run with synth-level power supplies (+/-15V is no problem) and have built-in level shifters on the control inputs (so you can switch it with Arduino 5V logic with no messing about).

HTH

[JP19]

Thanks, I'll look into it and probably give CMOS switches a go.

[merlinb]

IMHO, the best option is to bias the signal to 5V only for the switching, and re-bias it to whatever the bias was after the switching stages.

Just to fill in the blanks for the OP:
If you bias the FETs to 3.5V, and use a FET with a Vgs(off)=2V or smaller (J201 is ideal), then you will be able to handle a 3Vpp signal while switching the FETs directly from the 5V Arduino.
Larger audio signal will break through.

Jfet switches are already low-headroom devices, going over 1Vp-p makes the Jfet not conduct linearly anymore

Thats true when using them as variable resistors, but when used as switches they can be used with much larger signal voltages, if the control voltage is even larger. So the OP is right, if he lowers the reference to make everything fit into a 5V control window then he will be sacrificing headroom. But maybe he can work around that elsewhere.