What are FETs Used for?

[Nelson Win]

I'm attending an amplifier course at a community college. The course
has finished with Op Amps and BJTs and is moving onto FETs. I'm
thinking of dropping out of class as nearly all the circuits I've seen
use Op Amps and NPN transistors. I'm interested in sequencing,
flashing and fading LEDs. Grateful for any advice whether FETs could
be of any use to me later on. Don't want to regret it.

Nelson

 

[Tim Kettring]

A FET is a high impedance input that does not load-down the circuit feeding
it like a NPN or PNP typically would .

I have an old voltmeter with a FET input , one of the best in those days .

FETs basically replace tubes in modern electronics .

tim

 

[Tim Kettring]

FETs have a very high impedance input , they are great where you dont want
to load the circuit feeding it down .

tim

 

[Anthony Fremont]

I'm attending an amplifier course at a community college. The course
has finished with Op Amps and BJTs and is moving onto FETs. I'm
thinking of dropping out of class as nearly all the circuits I've seen
use Op Amps and NPN transistors. I'm interested in sequencing,
flashing and fading LEDs. Grateful for any advice whether FETs could
be of any use to me later on. Don't want to regret it.

I think you should stick with it. As Tim already told us (a couple of
times ;-), FETs have high input impedance and can be quite useful.
There are different types and some FETs have interesting characteristics
like being able to switch an AC circuit or pass current in either
direction. I believe you'll also find that some of them have extremely
low DC resistance when saturated. This is quite useful when you wish to
switch large currents and don't want a huge heatsink. If you think any
of this might be useful in the future, you should check them out.

michael

 

[Dr. Anton Squeegee]

I'm attending an amplifier course at a community college. The course
has finished with Op Amps and BJTs and is moving onto FETs. I'm
thinking of dropping out of class as nearly all the circuits I've seen

I have one word of advice.

Don't.

To expand on that: Stay with the class! FETs are incredibly useful
as high-impedance amplifiers, and certain types of FETs (GaAsFETs) are
even more useful in low-noise high-gain RF amplifiers.

I made the mistake of dropping out of my early electronics classes
when I was well on my way to getting my A.A.S. degree. That was in 1978.
It was not until 1997 that I started back up again, and I regret letting
it go that long.

Stay with the class. You won't regret it in the long run.

 

[Nelson Win]

Stay with the class! FETs are incredibly useful

as high-impedance amplifiers, and certain types of FETs (GaAsFETs) are
even more useful in low-noise high-gain RF amplifiers.
You won't regret it in the long run.

Thanks, guys. I will stay with the class. The instructor will be
suprised to see me next week as I told him I'm withdrawing...he's a
good instructor, knows his stuff inside out and has a nice smile, come
to think of it, I enjoy his class, don't understand everything. But
who does?

Cheers,
Nelson

 

[Fritz Schlunder]

I'm attending an amplifier course at a community college. The course
has finished with Op Amps and BJTs and is moving onto FETs. I'm
thinking of dropping out of class as nearly all the circuits I've seen
use Op Amps and NPN transistors. I'm interested in sequencing,
flashing and fading LEDs. Grateful for any advice whether FETs could
be of any use to me later on. Don't want to regret it.

Nelson

FETs are great, but in particular the MOSFET (metal oxide semiconductor
field effect transistor) is probably the single most important electrical
component of modern electronics. Their popularity is well justified for
they really do have some extremely valuable characterisitics. If you have
any interest in electronics at all, I'm sure you won't regret knowing what a
MOSFET is and how to use it.

What are FETs in general used for? Anything and everything basically.
Digital integrated circuits are typically almost exclusively built up from
logic gates composed of a few MOSFETs each. The millions of transistors
composing the CPU of the computer you posted with are not BJTs, but rather
MOSFETs. In the analog world FETs continually have been gaining ground and
popularity against the traditional BJT. Most of the latest high performance
Op Amps coming out these days have rail to rail I/O made possible with FETs.

In the discrete electronics world power MOSFETs offer truely impressive
capability. For example the IRLMS2002:

http://www.irf.com/product-info/datasheets/data/irlms2002.pdf

Allows one to realistically switch currents in excess of 4Amps in a little
tiny SOT23-6 package with very low loss.

Or on the bigger side of things the IRF2804:

http://www.irf.com/product-info/datasheets/data/irf2804.pdf

Can switch seriously wicked current levels in a TO-220 package with little
or no heatsinking. With no heatsink it can easily switch loads in excess of
17 Amps. With a modest heatsink (of say 20W or less dissipation) the device
can switch 75 Amps of juice with no problems. Try to compare that against
any BJT offering and you will find there is no comparison.

 

[Watson A.Name - Watt Sun, Dark Remover]

Or on the bigger side of things the IRF2804:

http://www.irf.com/product-info/datasheets/data/irf2804.pdf

Can switch seriously wicked current levels in a TO-220 package with little
or no heatsinking. With no heatsink it can easily switch loads in excess of
17 Amps. With a modest heatsink (of say 20W or less dissipation) the device
can switch 75 Amps of juice with no problems. Try to compare that against
any BJT offering and you will find there is no comparison.

That'z exactly right. FETs can't compare with IGBTs.

http://www.elec.gla.ac.uk/groups/dev_mod/papers/igbt/igbt.html
"The operation of the IGBT can therefore be considered like a wide-
base pnp transistor whose base drive current is supplied by the MOSFET
current through the channel." (see pictures)

IGBTs can switch thousands of volts at hundreds of amps.
http://www.dynexsemi.com/products/igbt/


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[Fritz Schlunder]

IGBTs can switch thousands of volts at hundreds of amps.
http://www.dynexsemi.com/products/igbt/

Oh yeah baby. That company produces some pretty sweet devices. I wouldn't
mind having a few of those 3300V 1200A IGBTs in my junk parts bin.

http://www.dynexsemi.com/assets/DNX_DIM1200ESM33-F000.pdf

3300V @ 1200A works out to just under 4MW of maximum theoretical power
handling capability. Of course a real implementation wouldn't likely
achieve quite this much continuous power, but still. You could
theoretically juice up a whole town with just one of those beasts. I wonder
how much they cost each? Anyone have any ideas? Anyone know of
substantially larger IGBTs?