Topband: Slightly OT - amplifier noise

[Richard (Rick) Karlquist]

On 3/14/2020 10:20 AM, GEORGE WALLNER wrote:
> Steve,
> Sorry, I did not directly answer you question. Look at the specs of the 
> IF3602. (But also look at the price!)
> George
> AA7JV
> 
>
JFET's have a channel resistance that has thermal
noise like any other resistor.  That sets the
noise voltage for the device.  It is trivial to
simply produce a larger JFET or alternately
parallel a bunch of small ones, and you can
get as low a noise voltage as you want.  JFET's
also inherently have very low noise current.

The catch is that scaling up devices also scales
up capacitance.  No vendor AFAIK has made any
breakthru in terms of reducing capacitance vs IDSS.
That is the true figure of merit.  Without a
detailed analysis of the MFJ circuit, it is
hard to know if it could tolerate more capacitance.
Also, there may be other noise sources in the
MFJ as well.

Rick N6RK