# Instantaneous "on" FET

S

#### Stuart Garet

Jan 1, 1970
0
I want to use an FET as the first stage in an op amp darlington
follower. The ones I have on hand require a threshold signal voltage
to begin conducting. Hence, I am loosing swing at the output. Can
anyone recommend an FET that turns on from 0V upward, or as close as
possible?

Stu Garet

M

#### MooseFET

Jan 1, 1970
0
I want to use an FET as the first stage in an op amp darlington
follower. The ones I have on hand require a threshold signal voltage
to begin conducting. Hence, I am loosing swing at the output. Can
anyone recommend an FET that turns on from 0V upward, or as close as
possible?

Stu Garet

www.aldinc.com makes some.

C

#### Charles

Jan 1, 1970
0
Stuart Garet said:
I want to use an FET as the first stage in an op amp darlington
follower. The ones I have on hand require a threshold signal voltage
to begin conducting. Hence, I am loosing swing at the output. Can
anyone recommend an FET that turns on from 0V upward, or as close as
possible?

Have you investigated JFETs and depletion mode MOSFETs?

D

#### D from BC

Jan 1, 1970
0
I want to use an FET as the first stage in an op amp darlington
follower. The ones I have on hand require a threshold signal voltage
to begin conducting. Hence, I am loosing swing at the output. Can
anyone recommend an FET that turns on from 0V upward, or as close as
possible?

Stu Garet

Some way ..somehow there's has to be a ~3V difference between the gate
and the source for a mosfet to begin to conduct.
Example...Vs is ~-3V and Vg= 0V..... or Vs=0V and Vg=3V
Otherwise...it's not a mosfet..
D from BC

D

#### D from BC

Jan 1, 1970
0

Damn new shit coming out all the time!
So I checked it out...
First glance ...the PITA alarms went off..ding ding ding..
I'm guessing it's an expensive min quantity PITA part..
I'll be surprised if the OP will be jumping for joy...

However..it's been an education...so hopefully I'll remember the IC
for some quirky design someday..
"Yeah..I think it was some 0 Vgs chip that ran off 5V..."
D from BC

J

#### John Larkin

Jan 1, 1970
0
Some way ..somehow there's has to be a ~3V difference between the gate
and the source for a mosfet to begin to conduct.
Example...Vs is ~-3V and Vg= 0V..... or Vs=0V and Vg=3V
Otherwise...it's not a mosfet..
D from BC

Supertex makes depletion-mode mosfets, which can be very handy. They
make nice current limiters.

John

S

#### Spehro Pefhany

Jan 1, 1970
0
Some way ..somehow there's has to be a ~3V difference between the gate
and the source for a mosfet to begin to conduct.
Example...Vs is ~-3V and Vg= 0V..... or Vs=0V and Vg=3V
Otherwise...it's not a mosfet..
D from BC

http://www.aldinc.com/pdf/ALD110800.pdf

Best regards,
Spehro Pefhany

D

#### D from BC

Jan 1, 1970
0
Supertex makes depletion-mode mosfets, which can be very handy. They
make nice current limiters.

John

The OP wrote "turns on from 0V and upward"..
I interpreted that as an enhancement mode mosfet.
D from BC

C

#### colin

Jan 1, 1970
0
D from BC said:
Some way ..somehow there's has to be a ~3V difference between the gate
and the source for a mosfet to begin to conduct.
Example...Vs is ~-3V and Vg= 0V..... or Vs=0V and Vg=3V
Otherwise...it's not a mosfet..
D from BC

well its not an enhancement mode mosfet anyway,
but you Can get depletion mode mosfets,
theyr just not as common,
at least not in power types anyway,
theyve been available in small signal devices for some time,
usualy combination of enhancement/depletion,
particularly the dual gate devices I like to use.

some low voltage logic level power mosfets are getting with lower and lower
vgth.

the vgth spread of course will still be an issue.

the jfet type obviously cant be enhancement mode at all.
so jfet could be used to compensate for the folowing stages voltage drop,

Colin =^.^=

S

#### Spehro Pefhany

Jan 1, 1970
0
Damn new shit coming out all the time!
So I checked it out...
First glance ...the PITA alarms went off..ding ding ding..
I'm guessing it's an expensive min quantity PITA part..

bitch, bitch, bitch ;-)

Small quantities in stock at Mouser, they show reasonable lead times
1.50 and 2.00 in qty 100 for the quad array. Not 3 cents, of course.
I'll be surprised if the OP will be jumping for joy...

However..it's been an education...so hopefully I'll remember the IC
for some quirky design someday..
"Yeah..I think it was some 0 Vgs chip that ran off 5V..."
D from BC

Often the special the part has 'de'parted by the time you need it.

Best regards,
Spehro Pefhany

W

#### Winfield

Jan 1, 1970
0
Stuart said:
I want to use an FET as the first stage in an op amp darlington
follower. The ones I have on hand require a threshold signal voltage
to begin conducting. Hence, I am loosing swing at the output. Can
anyone recommend an FET that turns on from 0V upward, or as close
as possible?

Stu, we're confused, what do you mean by "op amp darlington follower"
-- an op amp voltage follower doesn't need or involve a Darlington,
which is a bipolar transistor configuration. As for FET input stages,
generally we use JFETs. These are depletion type devices, which can
be operated at their full drain current, called Idss, where Vgs = zero
volts (it's enhancement-mode MOSFETs that need a volt or two to get
going). So your question is unclear on that account as well.

Finally, you can get pretty good JFET opamps, which in all but a few
special cases are fine. Some special cases would be where you need
noise levels below 2.5nV/rt-Hz, where you need to work from low supply
voltages, or where the $8 to$15 cost of low-noise JFET opamps is

J

#### Joerg

Jan 1, 1970
0
Spehro said:
bitch, bitch, bitch ;-)

Small quantities in stock at Mouser, they show reasonable lead times
1.50 and 2.00 in qty 100 for the quad array. Not 3 cents, of course.

Often the special the part has 'de'parted by the time you need it.

Like ye olde SD5400. Hard to obtain these days and for most of my
designs now prohibitively expensive. And it's always been so good to me,
it is (was?) a true analog engineer's delight. <sigh>

J

#### Joerg

Jan 1, 1970
0
Winfield said:
Stu, we're confused, what do you mean by "op amp darlington follower"
-- an op amp voltage follower doesn't need or involve a Darlington,
which is a bipolar transistor configuration. As for FET input stages,
generally we use JFETs. These are depletion type devices, which can
be operated at their full drain current, called Idss, where Vgs = zero
volts (it's enhancement-mode MOSFETs that need a volt or two to get
going). So your question is unclear on that account as well.

Finally, you can get pretty good JFET opamps, which in all but a few
special cases are fine. Some special cases would be where you need
noise levels below 2.5nV/rt-Hz, where you need to work from low supply
voltages, or where the $8 to$15 cost of low-noise JFET opamps is

$8-$15? Ouch! Which one would that be? At that cost I'd go discrete.

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