[Amps] Designing the Cleanest Linear with RF Negative Feedback

[Tom Cathey]

Hi Marv!

Now THAT'S  the kind of info I've been looking for, OM.

OK on using the 'J' tube versions and the TRW  class A chips. I imagine the 
J's are harder to find used.

I went to a site that has a CA5106  TRW RF Broadband stick... is this the 
right one?

Here's the link - they have lots of interesting low level RF chips:
http://www.sphere.bc.ca/test/rf-semis.html

Your website schemtic of the amplifier with negative feedback is quite 
helpful. I can see where they neutralize each stage, grid driven, and see 
the simple neg feedback from the final to the driver using a 20pf cap from 
plate to the driver cathode.  Though, I didn't see any feedback for the pre 
driver, the 6146. I guess this amplifier was is just for an example since 
the 6146 w/o feedback is not that clean, I understand.  I don't think this 
will be too hard to pull off using other tubes and these basic techniques.

I just did some reading about adaptive feed forward systems that you 
mentioned can approach -80db 3rd order IMD.  They talk about cancellors. I'm 
not sure if I should persue that - might be too complex for the job. Then, 
as you said, the FT-1000D even at the low level 10mW tap, would not be a 
good driver for this super clean system..  The homebrew ssb generator is 
intriguing. I do have experience constructing SS stuff, but wonder how much 
of the FT-1000D I would retain as a unit with this outboard stuff.  I think 
the 10mW FT-1000D output with simple negative feedback in the linear might 
be all I need to grab another -15 to-20db IMD over the 3-500Z amps.  That's 
what I'm looking for.

Thanks again, Marv.

73,
Tom, K1JJ





Hi Tom,
  Well, you have a certainly started a string here.

  A couple of suggestions for you.

  Use the 4CX350FJ.  May as well start with the lowest distortion.

  Don't try to employ RF feedback across more than two stages.

  Use one of the old TRW - Motorola - M/ACom class A bricks for the first 
stage.

  Since you are operating narrowband, consider the feed forward 
configuration.  If you start with a -50dB distortion amp, you should be able 
to slice another 30dB off that figure for a -80dB system.

  I doubt if your exciter, at any level, is clean enough to drive the above 
FF system.

  Fortunately, a clean 80M SSB generator is easy to construct compared to 
the balance of this undertaking.

73 & Good afternoon,
  Marv WC6W