M
M. Hamed
Guest
I was doing some measurements and came up with a value of 150K of drain source resistance for a FET in saturation. Does this sound like a reasonable value?
P
Phil Allison
Guest
M. Hamed wrote:
> I was doing some measurements and came up with a value of 150K of drain source resistance for a FET in saturation. Does this sound like a reasonable value?
** No.
The "on resistance" of different FETs varies from a few hundred ohms to a few milliohms.
You are way beyond clueless.
..... Phil
> I was doing some measurements and came up with a value of 150K of drain source resistance for a FET in saturation. Does this sound like a reasonable value?
** No.
The "on resistance" of different FETs varies from a few hundred ohms to a few milliohms.
You are way beyond clueless.
..... Phil
J
John Larkin
Guest
On Sat, 17 Jan 2015 19:36:40 -0800 (PST), "M. Hamed"
<mhdpublic@gmail.com> wrote:
>I was doing some measurements and came up with a value of 150K of drain source resistance for a FET in saturation. Does this sound like a reasonable value?
Some people call the "saturation region" of a fet to be the high
voltage, constant-current region
https://en.wikipedia.org/wiki/Field-effect_transistor#Operation
where you could well see 150K for a small jfet.
Most people think that the low voltage, ohmic region is the
"saturation" region, like a bipolar transistor.
What fet did you use, and what were the bias voltages?
--
John Larkin Highland Technology, Inc
picosecond timing laser drivers and controllers
jlarkin att highlandtechnology dott com
http://www.highlandtechnology.com
<mhdpublic@gmail.com> wrote:
>I was doing some measurements and came up with a value of 150K of drain source resistance for a FET in saturation. Does this sound like a reasonable value?
Some people call the "saturation region" of a fet to be the high
voltage, constant-current region
https://en.wikipedia.org/wiki/Field-effect_transistor#Operation
where you could well see 150K for a small jfet.
Most people think that the low voltage, ohmic region is the
"saturation" region, like a bipolar transistor.
What fet did you use, and what were the bias voltages?
--
John Larkin Highland Technology, Inc
picosecond timing laser drivers and controllers
jlarkin att highlandtechnology dott com
http://www.highlandtechnology.com
W
whit3rd
Guest
On Saturday, January 17, 2015 at 7:36:43 PM UTC-8, M. Hamed wrote:
> I was doing some measurements and came up with a value of 150K of drain source resistance for a FET in saturation. Does this sound like a reasonable value?
In a discrete device, no. That value is scale-dependent, though; a power MOSFET will
be lower than a small-signal MOSFET, and p-JFET values for a current-setting element
in an op amp (or even for the input transistors (which need thick depletion layers
for voltage standoff) might be that high. In some applications, if it didn't matter
what the Rds is (like a follower) an IC designer might be able to 'optimize' that
value up into that range.
One wouldn't usually write up datasheets and sell 'em as discrete devices, though.
CD4007 gets up into the 1000 ohm range on its 'discrete' NMOS elements, that's as high
a value as I'd ever expect to see in print.
> I was doing some measurements and came up with a value of 150K of drain source resistance for a FET in saturation. Does this sound like a reasonable value?
In a discrete device, no. That value is scale-dependent, though; a power MOSFET will
be lower than a small-signal MOSFET, and p-JFET values for a current-setting element
in an op amp (or even for the input transistors (which need thick depletion layers
for voltage standoff) might be that high. In some applications, if it didn't matter
what the Rds is (like a follower) an IC designer might be able to 'optimize' that
value up into that range.
One wouldn't usually write up datasheets and sell 'em as discrete devices, though.
CD4007 gets up into the 1000 ohm range on its 'discrete' NMOS elements, that's as high
a value as I'd ever expect to see in print.
M
M. Hamed
Guest
On Sunday, January 18, 2015 at 4:26:20 PM UTC-7, John Larkin wrote:
I was measuring an electret mic. So Vgs is pretty much fixed. Current is almost constant at around 0.3 mA and it was biased to about 2V. I remember being taught that this is called the saturation region, and the current is Idsat.
I was measuring an electret mic. So Vgs is pretty much fixed. Current is almost constant at around 0.3 mA and it was biased to about 2V. I remember being taught that this is called the saturation region, and the current is Idsat.
T
Tim Wescott
Guest
On Sat, 17 Jan 2015 19:57:52 -0800, Phil Allison wrote:
Speaking of who's clueless, "saturation" in a FET means current
saturation in the level part of the I vs. V curve, as opposed to
"saturation" in a BJT, which means voltage saturation in the near-
vertical part.
--
www.wescottdesign.com
Speaking of who's clueless, "saturation" in a FET means current
saturation in the level part of the I vs. V curve, as opposed to
"saturation" in a BJT, which means voltage saturation in the near-
vertical part.
--
www.wescottdesign.com
T
Tim Wescott
Guest
On Sat, 17 Jan 2015 19:36:40 -0800, M. Hamed wrote:
In spite of my zing on Phil "I know more than God" Allison, your question
is unclear.
Do you mean that you are measuring the slope of the I/V curve at high
voltage, nearly-constant-current? Or are you measuring the slope of the
I/V curve at a gate drive well above threshold and a drain to source
voltage around zero? Or are you doing something else?
--
www.wescottdesign.com
In spite of my zing on Phil "I know more than God" Allison, your question
is unclear.
Do you mean that you are measuring the slope of the I/V curve at high
voltage, nearly-constant-current? Or are you measuring the slope of the
I/V curve at a gate drive well above threshold and a drain to source
voltage around zero? Or are you doing something else?
--
www.wescottdesign.com
J
Jim Thompson
Guest
On Sun, 18 Jan 2015 15:50:13 -0600, Tim Wescott <tim@seemywebsite.com>
wrote:
A definitional difference which has always annoyed the hell out of me,
having been brought up on bipolar devices first.
Slope in "saturation", for a MOSFET, is the channel-length modulation
term... the imperfection of the ID current versus VDS.
RDS is in the "linear" region where the FET is acting like a switch.
...Jim Thompson
--
| James E.Thompson | mens |
| Analog Innovations | et |
| Analog/Mixed-Signal ASIC's and Discrete Systems | manus |
| San Tan Valley, AZ 85142 Skype: skypeanalog | |
| Voice
480)460-2350 Fax: Available upon request | Brass Rat |
| E-mail Icon at http://www.analog-innovations.com | 1962 |
I love to cook with wine. Sometimes I even put it in the food.
wrote:
A definitional difference which has always annoyed the hell out of me,
having been brought up on bipolar devices first.
Slope in "saturation", for a MOSFET, is the channel-length modulation
term... the imperfection of the ID current versus VDS.
RDS is in the "linear" region where the FET is acting like a switch.
...Jim Thompson
--
| James E.Thompson | mens |
| Analog Innovations | et |
| Analog/Mixed-Signal ASIC's and Discrete Systems | manus |
| San Tan Valley, AZ 85142 Skype: skypeanalog | |
| Voice
480)460-2350 Fax: Available upon request | Brass Rat || E-mail Icon at http://www.analog-innovations.com | 1962 |
I love to cook with wine. Sometimes I even put it in the food.
J
John Larkin
Guest
On Sun, 18 Jan 2015 15:09:17 -0700, Jim Thompson
<To-Email-Use-The-Envelope-Icon@On-My-Web-Site.com> wrote:
That's all backwards. We call the low-voltage ohmic region
"saturation" for any transistor, bipolar or fet. We call the other
thing the "constant current region."
--
John Larkin Highland Technology, Inc
picosecond timing laser drivers and controllers
jlarkin att highlandtechnology dott com
http://www.highlandtechnology.com
<To-Email-Use-The-Envelope-Icon@On-My-Web-Site.com> wrote:
That's all backwards. We call the low-voltage ohmic region
"saturation" for any transistor, bipolar or fet. We call the other
thing the "constant current region."
--
John Larkin Highland Technology, Inc
picosecond timing laser drivers and controllers
jlarkin att highlandtechnology dott com
http://www.highlandtechnology.com
T
Tim Wescott
Guest
On Sun, 18 Jan 2015 18:28:01 -0800, M. Hamed wrote:
So you're not talking about Rds_on -- you're talking h_od or r_o or some
such, depending on what model you want to pick.
Yes, 150k seems high, but I think JT pointed out that this can be
controlled somewhat by jiggering the device geometry, and if it's a
really small part it'll be higher, and it'll definitely be higher if
someone's put in a source degeneration resistor.
--
www.wescottdesign.com
So you're not talking about Rds_on -- you're talking h_od or r_o or some
such, depending on what model you want to pick.
Yes, 150k seems high, but I think JT pointed out that this can be
controlled somewhat by jiggering the device geometry, and if it's a
really small part it'll be higher, and it'll definitely be higher if
someone's put in a source degeneration resistor.
--
www.wescottdesign.com
J
John Larkin
Guest
On Tue, 20 Jan 2015 15:07:27 -0600, Tim Wescott <tim@seemywebsite.com>
wrote:
Cheap electret mikes are the electret element (essentially a film
capacitor, source to gate) and a jfet... nothing else. The jfet runs
at or very near Idss.
--
John Larkin Highland Technology, Inc
picosecond timing laser drivers and controllers
jlarkin att highlandtechnology dott com
http://www.highlandtechnology.com
wrote:
Cheap electret mikes are the electret element (essentially a film
capacitor, source to gate) and a jfet... nothing else. The jfet runs
at or very near Idss.
--
John Larkin Highland Technology, Inc
picosecond timing laser drivers and controllers
jlarkin att highlandtechnology dott com
http://www.highlandtechnology.com