Ken said:
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.