J
Jon Kirwan
I've not used JFETs before and was thinking about them first
as high impedance current sources to get started learning
more about them.
(I already gather that their potentially very high input
impedance and low Ciss is a good thing and I'm broadly aware
that FET and BJT input opamps divide their world into low
bias current vs low offset voltage camps.)
Looking over a datasheet on one, I see a chart picturing Id
vs Vgs (for one specific Idss) and illustrating several
curves for different temperatures. I note in this case that
they all cross at Id=10mA (and on this part, that happens in
the selected world of this datasheet at Vgs=-0.4V.) That
suggests that setting things up for Id=10mA would seem to
mitigate temperature variation in a simple current source
based on this device. (Similar on the real part, but not
necessarily exactly there, of course.)
However, there is a complication. I'd like that current
source to also be relatively _flat_ over the Vds compliance
range as well as temperature. Looking over the Id vs Vds
curves, it gets pretty nice and flat but close to the
pinch-off Vgs(off) and not close to Id=10mA, where the
d(Vds)/d(Id) slope is closer to about 3k ohms, compared to
about 26k ohms at Id=1mA. So it seems to get the temperature
stability tighter I have to loosen up on stability vs Vd, in
this part.
Is that generally the case for simple JFET current sources?
Am I reading this wrong? (If right, are there JFETs that get
both of these benefits relatively well optimized at close to
the same point of operation?)
Jon
as high impedance current sources to get started learning
more about them.
(I already gather that their potentially very high input
impedance and low Ciss is a good thing and I'm broadly aware
that FET and BJT input opamps divide their world into low
bias current vs low offset voltage camps.)
Looking over a datasheet on one, I see a chart picturing Id
vs Vgs (for one specific Idss) and illustrating several
curves for different temperatures. I note in this case that
they all cross at Id=10mA (and on this part, that happens in
the selected world of this datasheet at Vgs=-0.4V.) That
suggests that setting things up for Id=10mA would seem to
mitigate temperature variation in a simple current source
based on this device. (Similar on the real part, but not
necessarily exactly there, of course.)
However, there is a complication. I'd like that current
source to also be relatively _flat_ over the Vds compliance
range as well as temperature. Looking over the Id vs Vds
curves, it gets pretty nice and flat but close to the
pinch-off Vgs(off) and not close to Id=10mA, where the
d(Vds)/d(Id) slope is closer to about 3k ohms, compared to
about 26k ohms at Id=1mA. So it seems to get the temperature
stability tighter I have to loosen up on stability vs Vd, in
this part.
Is that generally the case for simple JFET current sources?
Am I reading this wrong? (If right, are there JFETs that get
both of these benefits relatively well optimized at close to
the same point of operation?)
Jon