WHAT'S SPECIAL ABOUT "FETS"?

[ghamilton]

The AU-919 talks about 100% FET inputs. I guess we're not talking about "MOS"-FETS here.

 

[kevzep]

No they are Junction FETS.
You don't need decoupling caps to use Junction fets, you can direct couple them and then have lower distortion and better frequency response..

 

[Hyperion]

I guess we're not talking about "MOS"-FETS here.

Correct - just plain old JFET's

Oops - posts crossed.

 

[ghamilton]

So fets and Jfets are the same thing?

 

[Hyperion]

'Field Effect Transistor' and 'Junction Field Effect Transistor' - different acronyms for the same thing IIRC. Back in the day they were playing tunes with FET fabrication methods and a couple of other acronym combinations were used.

You also have 'Enhancement' or 'Depletion' types and 'P channel' and 'N channel' types - this last being the equivalent of PNP and NPN for bipolar transistors.

I suggest you 'Google' to fill in the gaps - I forget most of what I ever knew about FET's

 

[kevzep]

Yep but basically its all about frequency response, phase, and distortion, you can achieve the best results using JFETS....

 

[kevzep]

Thats what the whole "Direct Coupled" thing is about "DC Pure Power" etc etc...

 

[Hyperion]

Back then someone said that 'FET's were the semiconductor equivalent of a Valve' (Tube) - being a 'voltage controlled' device rather than 'current controlled' (as are bipolar transistors) - and suddenly everyone had to have them in their designs.

 

[Sansuiman]

^ Hence the way tubes (ok, valves for you Brits; but as a plumber I don't think of 2A3's, 6L6's and 12AX7's when speaking of valves) are sometimes referred to colloquially as glassfets. Naturally, I imagine some purists would take issue with that term. And with good reason.

 

[kevzep]

Back then someone said that 'FET's were the semiconductor equivalent of a Valve' (Tube) - being a 'voltage controlled' device rather than 'current controlled' (as are bipolar transistors) - and suddenly everyone had to have them in their designs.

Except you need decoupling with valves/tubes.....
But I have heard this said before....

 

[avionic]

VFET -- verticle field effect transistor
MOSFET-- metal oxide semiconductor field effect transistor

 

[dlucy]

Back then someone said that 'FET's were the semiconductor equivalent of a Valve' (Tube) - being a 'voltage controlled' device rather than 'current controlled' (as are bipolar transistors) - and suddenly everyone had to have them in their designs.

Two of the things I go to when trying to explain why tube gear sounds different than solid state:

1. Tubes, driven to their optimal performance range with high voltage, don't produce perfectly linear amplification like solid state does.

2. When they distort, tubes produce even-order distortion and solid state produces odd-order distortion.

So, I understand I can have the above two statements wrong. If so, please say so.

My questions for this thread are:

1. Is FET operating characteristic curves mostly linear like solid state? Or are they curved?

2. When pushed beyond their operating curves, do FETs produce even-order or odd-order distortion?

 

[BLAH BLAH]

Two of the things I go to when trying to explain why tube gear sounds different than solid state:

1. Tubes, driven to their optimal performance range with high voltage, don't produce perfectly linear amplification like solid state does.

2. When they distort, tubes produce even-order distortion and solid state produces odd-order distortion.

So, I understand I can have the above two statements wrong. If so, please say so.

My questions for this thread are:

1. Is FET operating characteristic curves mostly linear like solid state? Or are they curved?

2. When pushed beyond their operating curves, do FETs produce even-order or odd-order distortion?

So for slimplicty sake...
When tubes do distort...their even distortion is preferable/pleasanter to a SS uneven distortion...!?
Uneven distortion is sharper and more revealing like Jimi Hendrix on guitar!?!
While even distortion is mellower...

 

[ghazzer]

Maybe I got it wrong also, but I thought that FETs were basically used as a switches. They are "ON" if no controlling voltage is applied to the gate, and they are turned "OFF" as voltage to the gate is increased.

 

[Pio1980]

Maybe I got it wrong also, but I thought that FETs were basically used as a switches. They are "ON" if no controlling voltage is applied to the gate, and they are turned "OFF" as voltage to the gate is increased.

For input switching and for switchmode class 'D' operation, yes.

 

[ghazzer]

Wikipedia lists 23 different kinds of FETs, so very little would surprise me.

 

[dlucy]

Reading the curves and description, im guessing the value of fet is the "ohmic" region where it operates as a voltage controlled resistor.