Question on JFETs

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.

I don't think they would be all that good for laser diode drives.

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

George H.

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

Cheers

Phil Hobbs

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.

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

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.

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

Just a few >:-} ...Jim Thompson

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

I'm here to be helpful.

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

OK, I may not be

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.

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

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.