[AMPS] High isolation RF amplifers.

[Jon Ogden]

>
>Does anyone know how one goes about designing a wide-band RF amplifer
>(small signal, say +10 dBm max output) that has high isolation between
>output and input (ie |S12| is small) ??

What is small |S12|?  As you say, a good narrow band design with minimal 
collector to base feedback should easily have 20 to 25 dB or better of 
isolation.  The problem is you generally have to add some feedback in 
order to get wideband.  It is a trick of the trade.  And since all my 
designs have been narrow band, I don't know.

>
>Whilst I could easily to this for a narrow band amp, I'm intrigued how
>companies like Minicircuits can do it for wide band amps. Any ideas ????

Well, I may sell the stuff, but I can't let you in on our trade secrets! 
:-)  Fact is, I have no idea really.  You raise a good question.  I would 
assume there is a fair amount of attenutation involved.  Which you say 
below.
>
>The only way I can think of would be to amplify, then attenuate, then
>amplify again. I'm just intrigued if there is a better way.

Thinking about it some more: Depending on how wide a bandwidth you want, 
there are things that you can do with wide band transformer matches.  The 
guys at Mini-Circuits are the kings of wide band transformer design.  So 
if you make a wideband transformer that steps from 50 Ohms down to what 
ever the input impedance of your transistors are and then back up from 
the output to 50 Ohms, I bet you would have a fairly wide band amp with 
minimal |S12|.  I think that's how the Mini-Circuits folks probably do it.

73,

Jon
KE9NA
 

-------------------------------------
Jon Ogden
KE9NA

http://www.qsl.net/ke9na


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


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