will this work?

[tempus fugit]

Hey all;

I'm trying to design a circuit that will ground a signal when the control
voltage is applied, like this:




signal

o
|
|
|
o
|
||-+
||<- N-channel MOSFET
control voltage o--------------|--------o-||-+
.-. |
| | o
| | 10K |
'-' |
| o
| |
=== ===
GND GND



So, when I apply a logic hi (5v) to the gate, the signal should be muted
because it is grounded. This part I'm pretty sure I got OK. Now, when I
remove the control signal, will the signal be un-muted? I would think so,
but I have tried this with a J111, and I need to actually apply a negative
voltage to the gate to get it to turn off. Is it different in the case of a
MOSFET?

Thanks

 

[Eeyore]

Hey all;

I'm trying to design a circuit that will ground a signal when the control
voltage is applied, like this:

signal

o
|
|
|
o
|
||-+
||<- N-channel MOSFET
control voltage o--------------|--------o-||-+
.-. |
| | o
| | 10K |
'-' |
| o
| |
=== ===
GND GND

So, when I apply a logic hi (5v) to the gate, the signal should be muted
because it is grounded. This part I'm pretty sure I got OK. Now, when I
remove the control signal, will the signal be un-muted? I would think so,
but I have tried this with a J111, and I need to actually apply a negative
voltage to the gate to get it to turn off. Is it different in the case of a
MOSFET?

Yes.

A jfet is the correct one to use. And yes a J111 will need negative volts on the
gate to turn off. I suggest that if you only have postive volts available then
you use a p-channel jfet like one from the J175 family. Choose according to your
available cut-off voltage.

Graham

 

[tempus fugit]

Yes.

A jfet is the correct one to use. And yes a J111 will need negative volts on the
gate to turn off. I suggest that if you only have postive volts available then
you use a p-channel jfet like one from the J175 family. Choose according to your
available cut-off voltage.

Graham

Thanks again for the reply Graham (one of many of my posts of late!). Why
would a JFET be better to use? I have successfully done this with a 2n2222,
but it doesn't quite kill the whole signal (you can still hear it a bit). I
was looking at a MOSFET like BS270 because it has a lower Ron than a BJT,
and of course for the low turn on current.

 

[John Larkin]

Hey all;

I'm trying to design a circuit that will ground a signal when the control
voltage is applied, like this:




signal

o
|
|
|
o
|
||-+
||<- N-channel MOSFET
control voltage o--------------|--------o-||-+
.-. |
| | o
| | 10K |
'-' |
| o
| |
=== ===
GND GND



So, when I apply a logic hi (5v) to the gate, the signal should be muted
because it is grounded. This part I'm pretty sure I got OK. Now, when I
remove the control signal, will the signal be un-muted? I would think so,
but I have tried this with a J111, and I need to actually apply a negative
voltage to the gate to get it to turn off. Is it different in the case of a
MOSFET?

Thanks

The mosfet will work, 0 or +5 on the gate for off/on, but be aware
that in the fet OFF state, the D-S substrate diode will conduct if the
drain tries to swing any more than a few tenths of a volt negative.
Jfets don't have this problem.

You could use a CMOS analog switch, 74HC4066 or something. Make a tee
switch for really good on/off ratios.

Opto solid-state relays can be interesting, too.

John

 

[Eeyore]

Thanks again for the reply Graham (one of many of my posts of late!). Why
would a JFET be better to use? I have successfully done this with a 2n2222,
but it doesn't quite kill the whole signal (you can still hear it a bit). I
was looking at a MOSFET like BS270 because it has a lower Ron than a BJT,
and of course for the low turn on current.

You *can* do it with a bipolar device but as soon as the signal amplitude
exceeds ~ 400mV the transistor will conduct in the reverse direction. Also, as
you've noticed, it doesn't clamp the signal totally. Neither will a jfet totally
since it'll simply become a small resistance. The very best technique is to have
a series and shunt fet. I'd choose a series fet alone for best results but it
requires being a bit more 'clever' with driving the gate since there's signal on
it.

I've never used a mosfet for this application and since no pro-audio company I
know does, I suspect there may be a good reason for that but I've never
considered the reason why. Asymmetric characteristics strikes me as possible.

Graham

 

[tempus fugit]

You *can* do it with a bipolar device but as soon as the signal amplitude
exceeds ~ 400mV the transistor will conduct in the reverse direction. Also, as
you've noticed, it doesn't clamp the signal totally. Neither will a jfet totally
since it'll simply become a small resistance. The very best technique is to have
a series and shunt fet. I'd choose a series fet alone for best results but it
requires being a bit more 'clever' with driving the gate since there's signal on
it.

I've never used a mosfet for this application and since no pro-audio company I
know does, I suspect there may be a good reason for that but I've never
considered the reason why. Asymmetric characteristics strikes me as possible.

Graham

Hmmm.....

Your point is well taken Graham. Maybe I'll have to try one and see if there
are any problems. The signal is not actually going to pass through the
MOSFET. I plan to connect both output (from guitar) and input (to amp) to
the drain. SO when the MOSFET is off, there will be a direct connection from
in to out, but when it is on, both will be grounded.

Thanks again

 

[tempus fugit]

but


Hmmm.....

Your point is well taken Graham. Maybe I'll have to try one and see if there
are any problems. The signal is not actually going to pass through the
MOSFET. I plan to connect both output (from guitar) and input (to amp) to
the drain. SO when the MOSFET is off, there will be a direct connection from
in to out, but when it is on, both will be grounded.

Thanks again

Just had a thought....


What type of material is used in an analog switch (like
http://www.maxim-ic.com/quick_view2.cfm/qv_pk/2072)? I have been messing
around with one of these, and it works nicely. I had originally planned to
use this device, but the way I am switching has changed, so I can go to a
simpler (and smaller) device.

I'm just wondering if it uses MOSFETs for switching.

 

[tempus fugit]

The mosfet will work, 0 or +5 on the gate for off/on, but be aware
that in the fet OFF state, the D-S substrate diode will conduct if the
drain tries to swing any more than a few tenths of a volt negative.
Jfets don't have this problem.

You could use a CMOS analog switch, 74HC4066 or something. Make a tee
switch for really good on/off ratios.

Opto solid-state relays can be interesting, too.

John

Thanks for the reply John. I wonder what the output voltage of a guitar
pickup is.....
So a MOSFET should be fine for switching DC if the voltage is positive then?

THanks

 

[John Popelish]

Thanks for the reply John. I wonder what the output voltage of a guitar
pickup is.....
So a MOSFET should be fine for switching DC if the voltage is positive then?

You might also add a second N-channel MOSFET in series with
the first, but with its source and drain reversed. You
connect the gates together. This puts a second body diode
in series with the first that prevents either from
conducting when the gate is low. When the gate is high,
both channels have low resistance. The limitation on
negative off state signal voltage is then that which begins
to turn the reversed device on with zero gate voltage and
negative source voltage. But even with a logic level
MOSFET the turn on threshold is normally above a volt. A
big improvement on the .3 or .4 volts it takes to turn the
body diode on. And that turn on threshold is added to the
body diode turn on voltage before significant current passes.

 

[niftydog]

Thanks for the reply John. I wonder what the output voltage of a guitar
pickup is...

Typically around 1V peaks as an absolute maximum for passive guitars,
larger for actives. Average voltage is much lower, maybe 0.1V - hard to
say because it depends on a lot of factors.

nifty