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Hi;
Is there a way to figure out the R(on) of a JFET when this parameter isn't
specified in the data sheet?
Thanks
Measure one ?Hi;
Is there a way to figure out the R(on) of a JFET when this parameter isn't
specified in the data sheet?
Can't you get samples ? I've rarely had a problem for a genuine enquiry. The"Eeyore" <rabbitsfriendsandrelations@hotmail.com> wrote in message
tempus fugit wrote:
Is there a way to figure out the R(on) of a JFET when this parameter
isn't specified in the data sheet?
Measure one ?
I mean before I place the order.
On resistance is measured at Vgs = 0. Which gives @ 5V Id = ~ 0.07mA ( Fig 3 )Thanks John. I was thinking there must be some way using Idss, but I wasnt
sure which V to use. So for example from this data sheet:
http://www.fairchildsemi.com/ds/FJ/FJN598J.pdf
it would be 1.5V/350uA = roughly 4.3 megohms.
I must be doing something wrong. Can you set me straight?
1.5v/350 ua = 4.3KThanks John. I was thinking there must be some way using Idss, but I wasnt
sure which V to use. So for example from this data sheet:
http://www.fairchildsemi.com/ds/FJ/FJN598J.pdf
it would be 1.5V/350uA = roughly 4.3 megohms.
I must be doing something wrong. Can you set me straight?
Thanks
"John Larkin" <jjlarkin@highNOTlandTHIStechnologyPART.com> wrote in message
news:31a0u4hb3i92u8m8vuhv0tigpji99cchvd@4ax.com...
On Sat, 11 Apr 2009 23:18:50 -0400, "tempus fugit"
toccata@quitspammingme.ciaccess.com> wrote:
Hi;
Is there a way to figure out the R(on) of a JFET when this parameter
isn't
specified in the data sheet?
Thanks
Depends on what other data is available. Got a link to the datasheet?
It's in the rough ballpark of Vpinchoff/Idss.
Easily derived from drain curves, if provided.
John
Yup, he did make a truly bad guess, off by roughly 50:1.Wow thats awful..
Why not use a cmos analog switch like an HC4066?I'm looking for a JFET to switch a guitar signal in and out of effects
pedals. I'll need 8 in total, but the signal will only ever be passing
through 4 JFETS at any 1 time. I am going to bias the drain and source at
1/2 V+ (4.5v in this case) and would like to switch the JFETs on and off
using a PIC. Since the PIC runs at 5v, I'm assuming it's control outputs
will also be 5v, so I'll need a fairly low Vgs off to switch the JFETs
completely off to isolate the effects from the signal chain when I want them
bypassed. I realize the Ron doesn't need to be extremely low (although the
lower the better, I suppose) but I also need the lower Vgs off. I'm trying
to find a nice balance between those parameters.
Thanks
turn it off. If you can't find what your looking for, look for the pinchHi;
Is there a way to figure out the R(on) of a JFET when this parameter isn't
specified in the data sheet?
Thanks
Well maybe you're looking at a generic JFET which is on and you must
OK, you can slow down the gate drive and de-click the switching, but"John Larkin" <jjlarkin@highNOTlandTHIStechnologyPART.com> wrote in message
news:tbj1u41pm09rjphl1odd4230qvmb1vl2om@4ax.com...
On Sun, 12 Apr 2009 10:50:21 -0400, "tempus fugit"
toccata@quitspammingme.ciaccess.com> wrote:
Wow thats awful..
Yup, he did make a truly bad guess, off by roughly 50:1.
I'm looking for a JFET to switch a guitar signal in and out of effects
pedals. I'll need 8 in total, but the signal will only ever be passing
through 4 JFETS at any 1 time. I am going to bias the drain and source at
1/2 V+ (4.5v in this case) and would like to switch the JFETs on and off
using a PIC. Since the PIC runs at 5v, I'm assuming it's control outputs
will also be 5v, so I'll need a fairly low Vgs off to switch the JFETs
completely off to isolate the effects from the signal chain when I want
them
bypassed. I realize the Ron doesn't need to be extremely low (although
the
lower the better, I suppose) but I also need the lower Vgs off. I'm
trying
to find a nice balance between those parameters.
Thanks
Why not use a cmos analog switch like an HC4066?
Why not bottom post?
John
I'm trying to get the switching to be click free, and I've tried a MAX4622
CMOS switch, with no success. I tried using an MPF 102 and the switching was
much quieter (only a tiny thump in one direction), and I've read that it's
easier to ramp up the control voltage on a JFET than it is on CMOS.
Which has led me to try ordering up a bunch of suitable JFETs to see if I
can get them to behave.
Thanks
I mean before I place the order.tempus fugit wrote:
Hi;
Is there a way to figure out the R(on) of a JFET when this parameter
isn't
specified in the data sheet?
Measure one ?
Graham
tempus fugit wrote:
Thanks John. I was thinking there must be some way using Idss, but I
wasnt
sure which V to use. So for example from this data sheet:
http://www.fairchildsemi.com/ds/FJ/FJN598J.pdf
it would be 1.5V/350uA = roughly 4.3 megohms.
I must be doing something wrong. Can you set me straight?
On resistance is measured at Vgs = 0. Which gives @ 5V Id = ~ 0.07mA ( Fig
3 )
Roughly 70 k ohms.
What do want it for ? I can get you 8 ohms if you want.
Graham
On Sun, 12 Apr 2009 10:50:21 -0400, "tempus fugit"
toccata@quitspammingme.ciaccess.com> wrote:
Wow thats awful..
Yup, he did make a truly bad guess, off by roughly 50:1.
I'm looking for a JFET to switch a guitar signal in and out of effects
pedals. I'll need 8 in total, but the signal will only ever be passing
through 4 JFETS at any 1 time. I am going to bias the drain and source at
1/2 V+ (4.5v in this case) and would like to switch the JFETs on and off
using a PIC. Since the PIC runs at 5v, I'm assuming it's control outputs
will also be 5v, so I'll need a fairly low Vgs off to switch the JFETs
completely off to isolate the effects from the signal chain when I want
them
bypassed. I realize the Ron doesn't need to be extremely low (although
the
lower the better, I suppose) but I also need the lower Vgs off. I'm
trying
to find a nice balance between those parameters.
Thanks
Why not use a cmos analog switch like an HC4066?
Why not bottom post?
John
On Sat, 11 Apr 2009 23:18:50 -0400, "tempus fugit"
toccata@quitspammingme.ciaccess.com> wrote:
Hi;
Is there a way to figure out the R(on) of a JFET when this parameter
isn't
specified in the data sheet?
Thanks
Depends on what other data is available. Got a link to the datasheet?
It's in the rough ballpark of Vpinchoff/Idss.
Easily derived from drain curves, if provided.
John
"John Larkin" <jjlarkin@highNOTlandTHIStechnologyPART.com> wrote in message
news:tbj1u41pm09rjphl1odd4230qvmb1vl2om@4ax.com...
On Sun, 12 Apr 2009 10:50:21 -0400, "tempus fugit"
toccata@quitspammingme.ciaccess.com> wrote:
Wow thats awful..
Yup, he did make a truly bad guess, off by roughly 50:1.
I'm looking for a JFET to switch a guitar signal in and out of effects
pedals. I'll need 8 in total, but the signal will only ever be passing
through 4 JFETS at any 1 time. I am going to bias the drain and source at
1/2 V+ (4.5v in this case) and would like to switch the JFETs on and off
using a PIC. Since the PIC runs at 5v, I'm assuming it's control outputs
will also be 5v, so I'll need a fairly low Vgs off to switch the JFETs
completely off to isolate the effects from the signal chain when I want
them
bypassed. I realize the Ron doesn't need to be extremely low (although
the
lower the better, I suppose) but I also need the lower Vgs off. I'm
trying
to find a nice balance between those parameters.
Thanks
Why not use a cmos analog switch like an HC4066?
Why not bottom post?
John
I'm trying to get the switching to be click free, and I've tried a MAX4622
CMOS switch, with no success. I tried using an MPF 102 and the switching was
much quieter (only a tiny thump in one direction), and I've read that it's
easier to ramp up the control voltage on a JFET than it is on CMOS.
Which has led me to try ordering up a bunch of suitable JFETs to see if I
can get them to behave.
Thanks
So what's the matter with an MPF-102? Not available?
5V is really too low a pinch-off voltage when you consider some guitar pickupsWow thats awful..
I'm looking for a JFET to switch a guitar signal in and out of effects
pedals. I'll need 8 in total, but the signal will only ever be passing
through 4 JFETS at any 1 time. I am going to bias the drain and source at
1/2 V+ (4.5v in this case) and would like to switch the JFETs on and off
using a PIC. Since the PIC runs at 5v, I'm assuming it's control outputs
will also be 5v, so I'll need a fairly low Vgs off to switch the JFETs
completely off to isolate the effects from the signal chain when I want them
bypassed. I realize the Ron doesn't need to be extremely low (although the
lower the better, I suppose) but I also need the lower Vgs off. I'm trying
to find a nice balance between those parameters.
Actually I did seem I dropped a decimal place somehow. I get ~ 6k now. With IdssOn Sun, 12 Apr 2009 10:50:21 -0400, "tempus fugit"
toccata@quitspammingme.ciaccess.com> wrote:
Wow thats awful..
Yup, he did make a truly bad guess, off by roughly 50:1.
Possibly, at insane prices.Don't people already make thumpless analog switch chips?
Turn-off time is quite long.Led-photoresistor things could be interesting.
You'll almost certainly need a seperate supply to drive the things off or theActually I breadboarded my design using an MPF 102, since it was all I had
lying around. Unfortunately, it has a Vgs off of 8V, so it didnt provide
great isolation when it was off. You're suggesting a MOSFET also? I've got
some BS170s lying around so I'll do some work with them.
shorts out the signal when it's on -- these will handle higher voltages,tempus fugit wrote:
Wow thats awful..
I'm looking for a JFET to switch a guitar signal in and out of effects
pedals. I'll need 8 in total, but the signal will only ever be passing
through 4 JFETS at any 1 time. I am going to bias the drain and source at
1/2 V+ (4.5v in this case) and would like to switch the JFETs on and off
using a PIC. Since the PIC runs at 5v, I'm assuming it's control outputs
will also be 5v, so I'll need a fairly low Vgs off to switch the JFETs
completely off to isolate the effects from the signal chain when I want them
bypassed. I realize the Ron doesn't need to be extremely low (although the
lower the better, I suppose) but I also need the lower Vgs off. I'm trying
to find a nice balance between those parameters.
5V is really too low a pinch-off voltage when you consider some guitar pickups
can deliver volts of output.
Also, there tends to be a relationship between a higher pinch-off voltage and a
lower on-resistance.
Typical jfets used for switching audio are the J09 and J175 families.
Graham
I've seen circuits that start with a series resistance and a switch that
The shunt / series configuration is very good but if you happen to have a higherEeyore wrote:
tempus fugit wrote:
Wow thats awful..
I'm looking for a JFET to switch a guitar signal in and out of effects
pedals. I'll need 8 in total, but the signal will only ever be passing
through 4 JFETS at any 1 time. I am going to bias the drain and source at
1/2 V+ (4.5v in this case) and would like to switch the JFETs on and off
using a PIC. Since the PIC runs at 5v, I'm assuming it's control outputs
will also be 5v, so I'll need a fairly low Vgs off to switch the JFETs
completely off to isolate the effects from the signal chain when I want them
bypassed. I realize the Ron doesn't need to be extremely low (although the
lower the better, I suppose) but I also need the lower Vgs off. I'm trying
to find a nice balance between those parameters.
5V is really too low a pinch-off voltage when you consider some guitar pickups
can deliver volts of output.
Also, there tends to be a relationship between a higher pinch-off voltage and a
lower on-resistance.
Typical jfets used for switching audio are the J09 and J175 families.
I've seen circuits that start with a series resistance and a switch that
shorts out the signal when it's on -- these will handle higher voltages,
and you can follow that with another switch that turns on/off to finish
the job (although you'll then have to make sure that you've turned it on
enough for that several-volt swing). Such things are fairly typical of
RF circuits that use PIN diodes for switching, although I think I've
seen it in audio paths.
I'd consider attenuating the guitar input to something guaranteed to be
manageable by the switch, but I don't know if it'd get past the "better
make sure this won't cause problems" stage of the design process.