JFET on resistance?

[Eeyore]

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

 

[Eeyore]

I mean before I place the order.

Can't you get samples ? I've rarely had a problem for a genuine enquiry. The
only company that gave me trouble actually was IR who said 10k devices p.a.
wasn't enough.

Graham

 

[Eeyore]

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

 

[Jamie]

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

Well maybe you're looking at a generic JFET which is on and you must
turn it off. If you can't find what your looking for, look for the pinch
off voltage.

Unless we're talking about enhanced mode here?

http://webpages.charter.net/jamie_5"

 

[tempus fugit]

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

 

[tempus fugit]

Measure one ?

Graham

I mean before I place the order.

Thanks

 

[tempus fugit]

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?

Thanks

 

[tempus fugit]

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.

Thanks

 

[tempus fugit]

Yup, he did make a truly bad guess, off by roughly 50:1.



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

 

[Eeyore]

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

 

[Eeyore]

Yup, he did make a truly bad guess, off by roughly 50:1.

Actually I did seem I dropped a decimal place somehow. I get ~ 6k now. With Idss
as low as 100uA it's always going to be a high resistance device. Note the
application. Capacitor mics etc. It's designed for low current consumption.

Graham

 

[Eeyore]

Don't people already make thumpless analog switch chips?

Possibly, at insane prices.

Led-photoresistor things could be interesting.

Turn-off time is quite long.

Graham

 

[Eeyore]

Actually 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.

You'll almost certainly need a seperate supply to drive the things off or the
audio may simply break through by modulating the gate-source or gate-drain
voltage. Beware, pinch-off voltage can vary widely too.

Graham

 

[Eeyore]

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.

The shunt / series configuration is very good but if you happen to have a higher
voltage handy for gate drive, series only will normally give you adequate isolation.

Graham

 

[Rich Grise]

I mean before I place the order.

If it's not fully spec'd, don't buy it.

Good Luck!
Rich

 

[tempus fugit]

So what's the matter with an MPF-102? Not available?

MOSFETs will work, too.

CMOS switches will probably provide soft switching if you ramp the
'enable' input -- the simpler ones are just a pair of MOSFETs with
none-too-spectacular Rds-on figures.

--

Actually 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.

Thanks