Bs170-bs250

[lumanauw]

These are small mosfets. Are they any good for front end differential or VAS?

How do they compare with Jfet of similiar size (like 2SK170-J74) or other small mosfets (ZVP-ZVN3310)?

[jacco vermeulen]

David,

BS170 is a retirement home transistor.
Good for switching or a cheapo buffer, but far too noisy compared to an SK170.

[lumanauw]

Is it more noisy than IRF610 or not?
What graph / data to look for the noise characteristic in Mosfet datasheet?

[jacco vermeulen]

David,

a BS170 is a MOSFET and a 2SK170 is a JFET, a JFET has a much lower noise level at LF than a MOSFET.
Thermal noise is the most important noise contributor, even for switching devices, for MOSFETs it's much higher due to impurities.
MOSFETs behave like resistors for thermal noise.
The noise level has a correlation with transconductance, compare the one of the BS170 with that of an IRF610.

[lumanauw]

Thanks Jacco
So more transconductance equals noisier device?

I just wonder why I never see BS170-BS250 used as front end transistors in audio amps.

[jacco vermeulen]

Eeeh David,

in my relic recollection, it hasn't been that long when i never saw an IRF device on the input position in a schematic.
Other way around, an IRF610 has a very high transconductance number.
Other issue, with some MOSFETs the noise goes down at a higher drain current level. The IRF and BS are also of different MOSFET technology type.

Btw: run into a golden oldy title in dustcovered amp design classes paperwork:
Cherry, amplifiying devices and low pass amplifier design.
Maybe a reference has already been posted earlier, couldn't find it with a search though.
For heavy Boltzmann equation stuff on thermal noise you need someone else, only paperwork i have is written in Cheese.