SImplest way to get a negative voltage to turn a JFET off?

[K3mHtH]

Hey everyone, I have a 9V battery powered circuit for a guitar effect.
I'd like to use a JFET to mute something in the audio path, but I'm already using the 9V battery as a positive power supply for the rest of the circuit. My JFET is across my audio path as a "mute" with the Source at audio-ground, which is also power-ground for the +9V supply. So I'm wondering how I can get at least -6.0V at the JFET's gate.

I see that there are charge-pump IC's like LT1054 that can obtain a negative voltage from a positive one.. awesome. But I really don't need any current here, just enough to turn the the JFET off... is there an easier way that maybe I'm missing? Are there 1:1 transformers maybe where the primary can go across my +9V supply and the secondary winding be used in the reverse direction for a negative voltage to the JFET's gate?

[MathWizard]

I think a lot of common JFET's work pretty good with drain and source flipped. But do you have any BJT's ? What's the circuit ? There might be other easy things to tie to GND or there about's, with a BJT, that would mute the output audio.

[David Hess]

Biasing the source positive will also turn the JFET off, but you already assigned signal ground and the negative of the battery to ground.

A charge pump can be made with diodes and capacitors if you have a square wave source.

A photovoltaic optoisolator will produce plenty of voltage to bias a JFET off with some current left over:

https://www.digikey.com/en/products/detail/toshiba-semiconductor-and-storage/TLP591B-C-F/2310604

[David Hess]

I think a lot of common JFET's work pretty good with drain and source flipped. But do you have any BJT's ? What's the circuit ? There might be other easy things to tie to GND or there about's, with a BJT, that would mute the output audio.

They used to make "chopper" bipolar transistors that could also be used for mute circuits, but a normal bipolar transistor can be used also.

[K3mHtH]

I think a lot of common JFET's work pretty good with drain and source flipped. But do you have any BJT's ? What's the circuit ? There might be other easy things to tie to GND or there about's, with a BJT, that would mute the output audio.

They used to make "chopper" bipolar transistors that could also be used for mute circuits, but a normal bipolar transistor can be used also.

I'm open to using other parts to mute the circuit that might not require a negative voltage, but I need the closed resistance to be 22 ohms or less and the device can't draw much power since this is battery powered.

[JXstaystonight]

It sounds like you're trying to bias the JFET to achieve a negative voltage at its gate relative to its source. Since you're already using a 9V battery for your positive supply, you'll need to generate a negative voltage to bias the JFET properly.

Using a charge pump IC like the LT1054 is indeed a common approach to generate a negative voltage from a positive one. However, if you're looking for a simpler solution without needing much current, you might consider using a voltage inverter or a voltage divider.

[dobsonr741]

How about this for muting - cutting the audio path instead of grounding: https://www.premierguitar.com/jfet-switching

No negative supply needed.

[K3mHtH]

It sounds like you're trying to bias the JFET to achieve a negative voltage at its gate relative to its source. Since you're already using a 9V battery for your positive supply, you'll need to generate a negative voltage to bias the JFET properly.

Using a charge pump IC like the LT1054 is indeed a common approach to generate a negative voltage from a positive one. However, if you're looking for a simpler solution without needing much current, you might consider using a voltage inverter or a voltage divider.

Thanks JX, what's a voltage inverter in this case?

[K3mHtH]

How about this for muting - cutting the audio path instead of grounding: https://www.premierguitar.com/jfet-switching

No negative supply needed.

I wish dobs! I can't interrupt the audio path (it's too complicated to explain why.. ) Nice article though, thanks for posting!

[pqass]

555 is cheaper than a LT1054; with just 3 more parts.   Simulation here.
Or substitute any CMOS inverter oscillator.

[magic]

ICL7660 is the jellybean voltage inverter, often found in battery powered devices.
Supposedly 80μA supply current with no load.

Note that most JFETs are quite symmetric and the source is simply whichever terminal happens to have lower potential at the time. So your gate voltage must be not only lower than ground, but also lower than the signal when the signal goes negative.

[Zero999]

How about this for muting - cutting the audio path instead of grounding: https://www.premierguitar.com/jfet-switching

No negative supply needed.

I wish dobs! I can't interrupt the audio path (it's too complicated to explain why.. ) Nice article though, thanks for posting!

Why not simply use a small MOSFET instead, such as the good old 2N7000?

Another, very simple way to negative bias a J-FET is to use a photovoltaic output opto-coupler, such as the TLP3905. It also has the advantage of being able to electrically isolate your control circuit.

[magic]

MOSFETs have body diodes so negative signals will be clamped to -0.6V or thereabouts and somewhat soft-clipped even earlier.

A pair of MOSFETs connected in series by the sources may work, but their source voltage will follow negative signal half-cycles and therefore gate voltage must be negative to keep them turned off at all times. So no easier to use than a single JFET.

[Zero999]

MOSFETs have body diodes so negative signals will be clamped to -0.6V or thereabouts and somewhat soft-clipped even earlier.

A pair of MOSFETs connected in series by the sources may work, but their source voltage will follow negative signal half-cycles and therefore gate voltage must be negative to keep them turned off at all times. So no easier to use than a single JFET.

It depends on the peak-to-peak voltage of the signal.

I've used a single MOSFET to mute microphone level signals before. Two back-to-back also works, so long as the peak signal voltage isn't high enough to turn it on. If it's more than 1V or so, then yes, you need a negative voltage and you're back to the same issue a J-FET.

[oPossum]

A PhotoMOS relay may work for muting.

https://www.mouser.com/c/electromechanical/relays-contactors-solenoids/solid-state-relays-ssr/solid-state-relays-pcb-mount/?output%20type=Photo%20MOSFET&sort=load%20voltage%20rating

[shapirus]

It sounds like you're trying to bias the JFET to achieve a negative voltage at its gate relative to its source. Since you're already using a 9V battery for your positive supply, you'll need to generate a negative voltage to bias the JFET properly.

Using a charge pump IC like the LT1054 is indeed a common approach to generate a negative voltage from a positive one. However, if you're looking for a simpler solution without needing much current, you might consider using a voltage inverter or a voltage divider.

I may be too paranoid, but this text looks absolutely like ChatGPT-generated to me. Same pattern, same reiteration of the original question, same near-uselessness.

[Zero999]

I've just had a look and there are analogue switches which can deal with negative voltages. They typically don't work up to 9V, so would need a lower voltage, but the current is tiny, so a zener diode will do.

ISL54059
https://www.renesas.com/us/en/document/dst/isl54059-datasheet
https://www.digikey.com/en/products/detail/renesas-electronics-corporation/ISL54059IRTZ/2000555

TS5A22364
https://www.ti.com/lit/ds/symlink/ts5a22364.pdf?ts=1708951514746
https://www.mouser.co.uk/ProductDetail/Texas-Instruments/TS5A22364DGSR?qs=9F2IyUQaA%2FhbjQmUI4VB%2FQ%3D%3D

[MrAl]

Hey everyone, I have a 9V battery powered circuit for a guitar effect.
I'd like to use a JFET to mute something in the audio path, but I'm already using the 9V battery as a positive power supply for the rest of the circuit. My JFET is across my audio path as a "mute" with the Source at audio-ground, which is also power-ground for the +9V supply. So I'm wondering how I can get at least -6.0V at the JFET's gate.

I see that there are charge-pump IC's like LT1054 that can obtain a negative voltage from a positive one.. awesome. But I really don't need any current here, just enough to turn the the JFET off... is there an easier way that maybe I'm missing? Are there 1:1 transformers maybe where the primary can go across my +9V supply and the secondary winding be used in the reverse direction for a negative voltage to the JFET's gate?

Hi,

9 volt batteries are pretty easy to add to a circuit if you have room all you need is a battery connector and a place to stick it.  JFET's also require very little gate current so a 9v battery would last for a long time.  That means if you added a 2nd 9v battery for the negative supply it would last for a long time if you made the circuit draw very little current.  It would most likely outlast the main battery by 10 times or more.
There are lots of ways to generate a negative voltage from a positive voltage, but they will use much more current which would drain the main battery faster.

[ejeffrey]

A charge pump doesn't need to use much current at all. It also only needs to run when the signal is muted so it shouldn't be a major factor in battery life.

[mikerj]

If you go for a charge pump like the ICL7660 be aware that they can be quite electrically noisy, ensure plenty of decoupling, solid grounding and avoid routing any high impedance signals nearby.

[Xena E]

Alternative approach.

P ch jfet J175 or similar.

1M from control input to gate, B+ for channel open, take to ground to mute.

[BrianHG]

Hey everyone, I have a 9V battery powered circuit for a guitar effect.
I'd like to use a JFET to mute something in the audio path, but I'm already using the 9V battery as a positive power supply for the rest of the circuit. My JFET is across my audio path as a "mute" with the Source at audio-ground, which is also power-ground for the +9V supply. So I'm wondering how I can get at least -6.0V at the JFET's gate.

I see that there are charge-pump IC's like LT1054 that can obtain a negative voltage from a positive one.. awesome. But I really don't need any current here, just enough to turn the the JFET off... is there an easier way that maybe I'm missing? Are there 1:1 transformers maybe where the primary can go across my +9V supply and the secondary winding be used in the reverse direction for a negative voltage to the JFET's gate?

Use the exact same trick we use in some modern high end vacuum tube amps.
Add a 3v lithium coin cell in series with the gate.  (Make sure the Jfet you choose switches completely off at around 2.5v-2.8v, you may need to bin test them, place a small cap in parallel with the lithium cell...)
The coin cell should last at least 10 years.

[ejeffrey]

If you do that, make sure that when unmuted you don't forward bias the gate junction.

[MathWizard]

Are modern coin cells really that good that after 10yrs even on the shelf, they are still ok ? IDK how bad they used to be, like in a digital wrist watch, but I'm guessing after 10yrs on the shelf, they would not be much good. IDK maybe they were tho.

[BrianHG]

Are modern coin cells really that good that after 10yrs even on the shelf, they are still ok ? IDK how bad they used to be, like in a digital wrist watch, but I'm guessing after 10yrs on the shelf, they would not be much good. IDK maybe they were tho.

Digital wrist watches used 1.5v cells, not the 3v lithium.  With no load, IE: J-Fet gate current, they will last 10 years unless you are seriously temperature cycling them.  Then expect at least 5 years.

If you do that, make sure that when unmuted you don't forward bias the gate junction.

In this possible case, assuming the OP wants the J-Fet as a switch, just add a 10-22 mega-ohm resistor in series to the gate.  Though, make sure the resistor is right up at the gate, touching it without any trace length to prevent any pickup of stray EM signals.  Again, typical vacuum tube amp design precautions.