Question on JFETs

[George Herold]

On Thursday, November 10, 2016 at 12:43:37 PM UTC-5, Jim Thompson wrote:

On Thu, 10 Nov 2016 09:02:34 -0800 (PST), George Herold
gherold@teachspin.com> wrote:

On Thursday, November 10, 2016 at 9:57:29 AM UTC-5, Jim Thompson wrote:
On Thu, 10 Nov 2016 06:38:32 -0800 (PST), George Herold
gherold@teachspin.com> wrote:

[snip]

See figure 3.8 in AoE.
I haven't used a lot of jfet's so I'm not much of an authority.

I thought H&H did a good job of making clear what could be a confusing subject.

(I haven't used depletion mode fet's either... except playing around
with mkaing 'em into current sources.)

George H.

I love it when microchip foundry processes include a depletion mode
MOSFET... they make fabulous kick-starters for bandgap references,
then disconnect from rail-induced current variations.
Hmm.. not sure I fully follow. Hey! You could write a book!
Phil's working on his second, you don't want a
snooty PhD leaving more of a legacy. :^)

George H.


I'm more into trade secrets that pass on to the offspring >:-}
I understand... just my shameless attempt to pick your brain.

I also assume I don't have to apologize to Phil for calling him
snooty... (Tongue in cheek .:^)

George H.

E.G. My most recent OpAmp model is a cryptic (but not encrypted)
equation whose coefficients are derived from unpublished calculations.

...Jim Thompson
--
| James E.Thompson | mens |
| Analog Innovations | et |
| Analog/Mixed-Signal ASIC's and Discrete Systems | manus |
| STV, Queen Creek, AZ 85142 Skype: skypeanalog | |
| Voice480)460-2350 Fax: Available upon request | Brass Rat |
| E-mail Icon at http://www.analog-innovations.com | 1962 |

I'm looking for work... see my website.

 

[George Herold]

On Thursday, November 10, 2016 at 6:33:26 PM UTC-5, Gunther Heiko Hagen wrote:

On Thu, 10 Nov 2016 06:38:32 -0800, George Herold wrote:

(I haven't used depletion mode fet's either... except playing around
with mkaing 'em into current sources.)

In that case, would they make good drivers for laser diodes, do you think?

I don't think they would be all that good for laser diode drives.
1.) I don't think they can get much current... but I don't know.
2.) Temperature stability may not be all that great.

I make laser diode drives with voltage reference - opamp-
fet and resistor to measure current. Yeah that's a lot more parts.

George H.

 

[Chris]

On Thu, 10 Nov 2016 18:41:35 -0800, John Larkin wrote:

> AoE is brilliantly clear.

I agree! Normally it is. But the discourse on FETs could have been
written so much more clearly if the two types were treated separately.

They behave very much alike: wiggling the gate voltage modulates the
conductivity of the source-drain channel.

Indeed. "A voltage controlled current source" I believe it's called.

You seem to not understand fundamentals, like voltage and current. You
might consider backing up and getting the basics right; that will make
things like semiconductors easier to understand.

That's a bit patronising. I'm really not *that* stoopid! OK, I may not be
in your venerable, stratospheric league but I have a *pretty* good
overall grasp of the science. There are just a few gaps here and there,
though; hence the original question.

 

[Gunther Heiko Hagen]

On Fri, 11 Nov 2016 09:15:35 -0500, Phil Hobbs wrote:

Normally I_DSS and V_P aren't well enough controlled for that. For a
simple diode laser bias supply, you're much better off with a BJT, a
voltage reference, and a resistor. Or a 3-terminal positive voltage
regulator (not LDO) connected as a current source.

What is this "LDO" please? (English is not my first language).

 

[Phil Hobbs]

On 11/10/2016 06:32 PM, Gunther Heiko Hagen wrote:

On Thu, 10 Nov 2016 06:38:32 -0800, George Herold wrote:

(I haven't used depletion mode fet's either... except playing around
with mkaing 'em into current sources.)

In that case, would they make good drivers for laser diodes, do you think?

Normally I_DSS and V_P aren't well enough controlled for that. For a
simple diode laser bias supply, you're much better off with a BJT, a
voltage reference, and a resistor. Or a 3-terminal positive voltage
regulator (not LDO) connected as a current source.

Cheers

Phil Hobbs

--
Dr Philip C D Hobbs
Principal Consultant
ElectroOptical Innovations LLC
Optics, Electro-optics, Photonics, Analog Electronics

160 North State Road #203
Briarcliff Manor NY 10510

hobbs at electrooptical dot net
http://electrooptical.net

 

[Phil Hobbs]

On 11/11/2016 10:55 AM, George Herold wrote:

On Thursday, November 10, 2016 at 12:43:37 PM UTC-5, Jim Thompson wrote:
On Thu, 10 Nov 2016 09:02:34 -0800 (PST), George Herold
gherold@teachspin.com> wrote:

On Thursday, November 10, 2016 at 9:57:29 AM UTC-5, Jim Thompson wrote:
On Thu, 10 Nov 2016 06:38:32 -0800 (PST), George Herold
gherold@teachspin.com> wrote:

[snip]

See figure 3.8 in AoE.
I haven't used a lot of jfet's so I'm not much of an authority.

I thought H&H did a good job of making clear what could be a confusing subject.

(I haven't used depletion mode fet's either... except playing around
with mkaing 'em into current sources.)

George H.

I love it when microchip foundry processes include a depletion mode
MOSFET... they make fabulous kick-starters for bandgap references,
then disconnect from rail-induced current variations.
Hmm.. not sure I fully follow. Hey! You could write a book!
Phil's working on his second, you don't want a
snooty PhD leaving more of a legacy. :^)

George H.


I'm more into trade secrets that pass on to the offspring >:-}
I understand... just my shameless attempt to pick your brain.

I also assume I don't have to apologize to Phil for calling him
snooty... (Tongue in cheek .:^)

George H.

Nah, Jim still has all those MC1488/9s running the internet backbone.

Cheers

Phil Hobbs


--
Dr Philip C D Hobbs
Principal Consultant
ElectroOptical Innovations LLC
Optics, Electro-optics, Photonics, Analog Electronics

160 North State Road #203
Briarcliff Manor NY 10510

hobbs at electrooptical dot net
http://electrooptical.net

 

[Phil Hobbs]

On 11/11/2016 01:43 PM, Gunther Heiko Hagen wrote:

On Fri, 11 Nov 2016 09:15:35 -0500, Phil Hobbs wrote:

Normally I_DSS and V_P aren't well enough controlled for that. For a
simple diode laser bias supply, you're much better off with a BJT, a
voltage reference, and a resistor. Or a 3-terminal positive voltage
regulator (not LDO) connected as a current source.

What is this "LDO" please? (English is not my first language).

Low dropout regulator. You want something like an LM317, because its
adjust pin current is low and stable. LDOs have quiescent currents that
vary a lot with voltage drop, which can make the bias current less accurate.

Cheers

Phil Hobbs

--
Dr Philip C D Hobbs
Principal Consultant
ElectroOptical Innovations LLC
Optics, Electro-optics, Photonics, Analog Electronics

160 North State Road #203
Briarcliff Manor NY 10510

hobbs at electrooptical dot net
http://electrooptical.net

 

[Jim Thompson]

On Fri, 11 Nov 2016 18:37:12 -0000 (UTC), Chris <cbx@noreply.com>
wrote:

On Thu, 10 Nov 2016 18:41:35 -0800, John Larkin wrote:

AoE is brilliantly clear.

I agree! Normally it is. But the discourse on FETs could have been
written so much more clearly if the two types were treated separately.


They behave very much alike: wiggling the gate voltage modulates the
conductivity of the source-drain channel.

Indeed. "A voltage controlled current source" I believe it's called.

You seem to not understand fundamentals, like voltage and current. You
might consider backing up and getting the basics right; that will make
things like semiconductors easier to understand.

That's a bit patronising. I'm really not *that* stoopid! OK, I may not be
in your venerable, stratospheric league but I have a *pretty* good
overall grasp of the science. There are just a few gaps here and there,
though; hence the original question.

Just a few >:-}

...Jim Thompson
--
| James E.Thompson | mens |
| Analog Innovations | et |
| Analog/Mixed-Signal ASIC's and Discrete Systems | manus |
| STV, Queen Creek, AZ 85142 Skype: skypeanalog | |
| Voice480)460-2350 Fax: Available upon request | Brass Rat |
| E-mail Icon at http://www.analog-innovations.com | 1962 |

I'm looking for work... see my website.

 

[John Larkin]

On Fri, 11 Nov 2016 18:37:12 -0000 (UTC), Chris <cbx@noreply.com>
wrote:

On Thu, 10 Nov 2016 18:41:35 -0800, John Larkin wrote:

AoE is brilliantly clear.

I agree! Normally it is. But the discourse on FETs could have been
written so much more clearly if the two types were treated separately.


They behave very much alike: wiggling the gate voltage modulates the
conductivity of the source-drain channel.

Indeed. "A voltage controlled current source" I believe it's called.

A fet can be approximated as a VCCS under some conditions. Sometimes a
voltage controlled resistor is a better model.

You seem to not understand fundamentals, like voltage and current. You
might consider backing up and getting the basics right; that will make
things like semiconductors easier to understand.

That's a bit patronising.

I'm here to be helpful.

> I'm really not *that* stoopid!

It's not a matter of brute intelligence. It is a matter of acquiring
electronic instincts.

OK, I may not be

in your venerable, stratospheric league but I have a *pretty* good
overall grasp of the science. There are just a few gaps here and there,
though; hence the original question.

There are lots of people who sort of start in the middle, with
transistors and fets and opamps, without first getting good with
resistors and capacitors and inductors... the basic math.

Jfets and mosfets are very similar in their behavior, in the
constant-current and in the ohmic regions. Their drain curves look a
lot alike. They share the same key parameters, like transconductance
and Rds-on. They are after all using about the same physics.

The guys at Bell who invented the bipolar transistor were actually
trying to make a jfet, and their point-contact bipolar was an
experimental accident.





--

John Larkin Highland Technology, Inc

lunatic fringe electronics

 

[Winfield Hill]

Jim Thompson wrote...

I love it when microchip foundry processes include a
depletion mode MOSFET... they make fabulous kick-starters
for bandgap references, then disconnect from rail-induced
current variations.

Why do they have to characterize a depletion-mode
MOSFET, can't you roll-your-own from scratch?
They don't need to be precise for your applications.


--
Thanks,
- Win

 

[Jim Thompson]

On 5 Jun 2017 08:26:14 -0700, Winfield Hill <hill@rowland.harvard.edu>
wrote:

Jim Thompson wrote...

I love it when microchip foundry processes include a
depletion mode MOSFET... they make fabulous kick-starters
for bandgap references, then disconnect from rail-induced
current variations.

Why do they have to characterize a depletion-mode
MOSFET, can't you roll-your-own from scratch?
They don't need to be precise for your applications.

Unfortunately most foundries have fixed process steps and don't allow
you to meddle. In the "good-old-days" we could experiment, cut a mask
set, and have the results back over a weekend.

...Jim Thompson
--
| James E.Thompson | mens |
| Analog Innovations | et |
| Analog/Mixed-Signal ASIC's and Discrete Systems | manus |
| STV, Queen Creek, AZ 85142 Skype: skypeanalog | |
| Voice480)460-2350 Fax: Available upon request | Brass Rat |
| E-mail Icon at http://www.analog-innovations.com | 1962 |

Thinking outside the box... producing elegant solutions.

"It is not in doing what you like, but in liking what you do that
is the secret of happiness." -James Barrie

 

[rickman]

Gunther Heiko Hagen wrote on 11/11/2016 1:43 PM:

On Fri, 11 Nov 2016 09:15:35 -0500, Phil Hobbs wrote:

Normally I_DSS and V_P aren't well enough controlled for that. For a
simple diode laser bias supply, you're much better off with a BJT, a
voltage reference, and a resistor. Or a 3-terminal positive voltage
regulator (not LDO) connected as a current source.

What is this "LDO" please? (English is not my first language).

While LDO stands for Low Drop Out regulator, it is typically used for any
linear regulator as compared to a switching regulator. Low Drop Out implies
that it indeed has a low drop out voltage, but since most linear regulators
are built that way, it has become nearly synonymous with linear regulators.

--

Rick C