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Re: Want to Homebrew a Preamp

on 9/26/00 3:10 AM, bronson@eece.maine.edu at bronson@eece.maine.edu wrote:

> I have not used relays to isolate the pre amps yet but as soon as I get some
> good ones I will.  They can be pretty pricey.

This is true.  I did purchase some nice small, Italian made ones from Alan
Bond of Max Gain Systems.  Go to www.mgs4u.com and look in the President's
page.  He's got some nice relays for just about $4 each.

> Since LEO RF hits the ground pretty hard (compared to VHF, UHF
> DX and EME) and you need a beam antenna anyway, NF isnít a be all end all.
> Some may feel that my previous statement is a bit of a contradiction... LEO
> signals are strong but you need a beam... seems mutually exclusive.

Agreed.  I also agree with your other statements about NF as well.  If you
get get near 1.5 dB on 2m and 70cm, you will be fine.  It doesn't make much
sense to try to get to the 0.2 dB point on 2m.  Even with my UHF preamp kit
from Down East, I tuned the amp for max signal gain and didn't worry about
the fact that the noise figure might be a 1/4 or 1/2 dB higher than optimum.
I figured I could use more gain.

> The trick to designing pre amps is to choose enough NF for your application
> and then finding the transistor in the market place.  The major manufacturers
> (NEC, HP, Motorola, Zetex...) are pretty good at providing data on their
> transistors, especially the ones that are used in the cellular phone bands.
> They also provide good application notes on employing their transistors.  The
> problem is that they donít make enough for their vendors to stock them or the
> devices are in such demand that the vendors run out of stock in a hurry.  The
> data sheets are important because they tell you where to bias the device for
> best NF or gain.  You will never have both, it is always one or the other.  If
> you want gain you pay with NF.  If you want NF you pay with gain.

Being a person who sells electronic components in the current market, I can
agree that it is hard to get hold of devices!  The demand right now is so
enormously high.  The companies just do not have enough capacity.

> Also available for pre amp use are MMICs (Mini-Circuits, HP...) .  These in
> general are useful for the second stage of a pre amp and do not offer low
> enough NF for use as a first stage.  For the best performance a GaAs FET with
> a noise figure well below 1dB is used as a front end amp after a filter of
> some kind and then the MMIC.

Whoa!  Now it looks like you contradict yourself.  You said above that a NF
well below a dB isn't necessary in most ham preamps.  Yes, for the higher
bands it's necessary, but for 2m and 70cm, some of the newer MMIC amps might
do the job just fine.  Many of those are around 1.5 dB which is sufficient.

All in all, Bronson, an EXCELLENT post.  It was good reading.



------------------------------------- Jon Ogden NA9D (ex: KE9NA)



"A life lived in fear is a life half lived."

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