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.
-- -- kensmith@rahul.net forging knowledge
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.
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
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
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