Topband: pHEMT and MOSFET preamps and 1/f noise on 160 meters

[Richard (Rick) Karlquist]

On 9/22/2015 11:40 AM, Art Snapper wrote:

> The KD9SV pHEMT FET preamp is a nice addition to the shack. I was able to
> do an in/out test this morning, and it made a big improvement. This was the
> first time since installation during the summer, that the atmospheric noise
> was low enough to give it a decent test.
>
> de Art NK8X
> S.W. Michigan
>
Just an FYI:

After seeing this posting, I looked at various KD9SV products.
The pHEMT preamp is advertised as "Featuring components with rated noise 
figures as low as 0.4 dB".  Yes, but pHEMT's are not specified
at 1.8 MHz, and for good reason.  They have a boatload of 1/f noise.
The NF is many dB higher at 1.8 MHz (how many dB?  well it's not
specified so you have to test each one.)  But it is bad.  Other
KD9SV use MOSFET's like BF981.  These also have high 1/f noise
at 1.8 MHz.  These 1/f issues are well known in the industry,
although maybe not to the designer.  It isn't clear that the
NF was actually measured.

I spent a lot of time characterizing pHEMT's and MOSFET's in
hopes of making a low noise preamp for 160 meters. They are
non-starters, IMHO.  I specifically tested the BF981.
You could do much better with bipolars like 2N5109 or JFET's.

Rick N6RK