Noise performance of small package mosfets

[Bruce Varley]

Hi, I've started using small mosfets, such as the 2N7000, for discrete
linear design. It's almost like designing with 12AX7s again. Can anyone
advise what their noise performance is like, how would one go as a mike
preamp input stage, for example?

 

[Ken Smith]

Hi, I've started using small mosfets, such as the 2N7000, for discrete
linear design. It's almost like designing with 12AX7s again. Can anyone
advise what their noise performance is like, how would one go as a mike
preamp input stage, for example?

It is likely to be too noisy for microphones. For low noise, high
impedance applications, a JFET will be a better option. Look at the
LSK170.

A big part of the problem with using a 2N7000 for a low noise device is
that there is no noise spec. on them. This means you can't reject a batch
just for being noisy.

I have wondered for a long time what the noise of a large power MOSFET
would be like. I've never gotten motivated to measure it though.

 

[Winfield Hill]

It is likely to be too noisy for microphones. For low noise, high
impedance applications, a JFET will be a better option. Look at the
LSK170.

A big part of the problem with using a 2N7000 for a low noise device
is that there is no noise spec. on them. This means you can't reject
a batch just for being noisy.

Below 30mA, this is in the subthreshold region for these MOSFETs.

I measured two small TO-92 MOSFETs with my hp4470A: a 2n7000 and
its older brother, a VN01. Both MOSFETs were very noisy. The
VN01 was 50 to 100x more noisy than a quiet JFET like a 2sk170,
and the 2n7000 was 2 to 5x worse yet, over 1uV/rt-Hz. This was
at 100kHz - the noise was much higher at lower frequencies and
offscale below 3kHz. Also, this was at 10mA - the noise was
much higher at lower currents, like 1mA or 300uA.

I have wondered for a long time what the noise of a large power MOSFET
would be like. I've never gotten motivated to measure it though.

Me either. Very high gate capacitance, ugh.

 

[John Larkin]

Below 30mA, this is in the subthreshold region for these MOSFETs.

I measured two small TO-92 MOSFETs with my hp4470A: a 2n7000 and
its older brother, a VN01. Both MOSFETs were very noisy. The
VN01 was 50 to 100x more noisy than a quiet JFET like a 2sk170,
and the 2n7000 was 2 to 5x worse yet, over 1uV/rt-Hz. This was
at 100kHz - the noise was much higher at lower frequencies and
offscale below 3kHz. Also, this was at 10mA - the noise was
much higher at lower currents, like 1mA or 300uA.

Nice data. It wouldn't shock me if you found popcorn noise, too.

John

 

[Graham Holloway]

Nice data. It wouldn't shock me if you found popcorn noise, too.

John

A board we use for measurement purposes uses 8 channels of current
measurement amplifiers. Each one has a 2N7000 as a source follower running
at 3mA. It has always been a problem to get it through production because of
excessive noise. There is no money to redesign it. But I found that
Siliconix/Vishay versions were about 10 times less noisier than any others,
eg. ST. Guess what - they're now discontinued. The boss will have to get
them redesigned now.

Graham Holloway

 

[Rich Grise]

Below 30mA, this is in the subthreshold region for these MOSFETs.

I measured two small TO-92 MOSFETs with my hp4470A: a 2n7000 and
its older brother, a VN01. Both MOSFETs were very noisy. The
VN01 was 50 to 100x more noisy than a quiet JFET like a 2sk170,
and the 2n7000 was 2 to 5x worse yet, over 1uV/rt-Hz. This was
at 100kHz - the noise was much higher at lower frequencies and
offscale below 3kHz. Also, this was at 10mA - the noise was
much higher at lower currents, like 1mA or 300uA.


Me either. Very high gate capacitance, ugh.

Well, when you're PWMing a 25 HP motor, I don't think a few millivolts
of noise is going to be noticeable.

And you certainly wouldn't use one for a mic preamp! ;-)

FWIW, I just put "mosfet noise" WQ* into google, and got "about 603" hits;
my impression from reading the comprehensible ones ( ;-) ) seems to
reinforce something I seem to have picked up sometime in the mists of the
past - mosfets are noisy.

Cheers!
Rich
*WQ = With Quotes