Re: Noise performance of small package mosfets
- From: "Winfield Hill" <hill@xxxxxxxxxxx>
- Date: 28 Feb 2007 12:06:55 -0800
Ken Smith wrote:
Bruce Varley wrote:
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.
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.
.
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