Current mirrors driving FETs

[tiziano]

Don't kill me if my assertion is a bull **** or looks alike that Simply it's a simple consideration due the fact that the MOSFET gate is insulated practically with glass ...

 

[tiziano]

I've made this morning the two fets circuit, and it works perfectly ... I'll make in the next day the others an I'll see. I bought also two MOSFETs to try ...

 

[tiziano]

Anyway I would like to thnak really very much Lineup for his help.
I'll take care about his suggestions ...

Thank you


Tiziano

 

[lineup]

Great, I thank you very much ...

Now a question: if I use MOS-FETs in place of FETs at the input stage ( the differential I mean ) to obtain very high impedance of input ( I have to use it with Piezo transducers ) do you think that will be possible without problem ? Or do you think thats is equal to use MOSFETs as well as FETs ?

I ask you that 'cause MOSFET has insulated gate by metal-oxyde layer, instead of FET has a normal NP or PN junction ... what do you think about ?

In pre amplifiers and low power (TO92) almost NEVER is MOSFET used for input.
Can happen, sometimes with only one input transistor,
but JFET transistors
and bipolar, like BC550C / 2SC2240 / 2N3904 / 2N5551
are most often much better: Low noise and high gain.


When differential input almost never mosfet!
I can not remember one pre amplifier with this
It is a bit difficult to find and match 2 JFET to get a good prefect pair,
and good balance in input differential.
If you buy 20 JFET transistors, maybe only find 1 Good Pair ....
very expensive!
With MOSFET also same problem. Vgs is different for every exemplar.

Not same problem with bipolar, BJT, transistors.
Easy find 2 exemplars with same, for example: 0.653 - 0.653 Vbe.

But JFET are good, high input impedance, and you can find 1 good pair
that MATCH ALMOST perfect.
==========================================


Long time past, I remember one member here say:
He think MOSFET input differential can be good to try.
Why not!
I would never say NO, if somebody want to try IDEA.
... maybe if we search forum, we can find about that old topic
==========================================


Also I know Nelson Pass have used some time small mosfet in his amplifiers
Even if i think now there are some better small MOSFETs,
Mr Pass used these small in IC package. This IC type is called HEXDIP.
IRFD110 MOS N HEXDIP, IRFD120 MOS N HEXDIP
IRFD9110 MOS P HEXDIP, IRFD9120 MOS P HEXDIP
http://www.irf.com/product-info/datasheets/data/irfd110.pdf
http://www.irf.com/product-info/datasheets/data/irfd120.pdf
http://www.irf.com/product-info/datasheets/data/irfd9110.pdf
http://www.irf.com/product-info/datasheets/data/irfd9120.pdf

But I think it was NOT pre amplifier he used them, it was for some function in a Power Amplifier.

But maybe such small mosfet could be used for output stage in pre amplifier.
========================================


I would go for JFET in input differential.
For example 2SK170 or like this. They have good price and are easy to find in all country.
Low noise and good for audio.
Nelson Pass uses 2SK170 in his tutorial examples here:
http://www.passdiy.com/pdf/diyopamp.pdf

2SK389 is probably the best JFET for audio input.
It is 2 N-JFET in same Capsule = same temperature in both
and they are very good matched!
They cost very much money, but they are 99% perfect for hifi quality amplifiers!

http://www.datasheets.org.uk/search.php?q=2SK389


----------------------------------------------------
In my audio pre amplifiers Myself use:
BC550C BC560C (max 45V) or 2SC2240 2SA970 (max 120V)
for low noise at little current and lower voltage.
And these are very nice for higher voltage: 2N5551 2N5401 (max 160V)
They are all bipolar transistors.


lineup
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