[Amps] Designing the Cleanest Linear with RF Negative Feedback

[Tom Cathey]

Thanks for the replies.

Let's address a few comments. I'm trying to get the right direction to 
follow, then do some serious, focused study in the right areas.

One comment:  Why bother to get better linearity?

I think for most bands, the standard GG open loop 3-500Z type amplifier is 
FB. You can hide in the 40M BC crap, the 20M loud crowd, 160M noise, etc.

But, notice my intended amp is a 75M mono-bander. The 75M DX window is a 
unique situation where extremely clean signals are required often - due to 
the low noise floor at grayline or late afternoon when weak DX signals begin 
coming in S3, etc.  The window is maybe 15kc wide. [3785-3800]   If a local 
is S9+50 over, even if he is 50db down 5kc away, that is still going to 
cover an S3 DX station. And, if the quality is worse, the window gets pretty 
tough to use.

So, I am looking for ways to improve upon a few exisiting GG 3-500Z's, an 
8877,  160-10M amplifiers as well as build up a new, dedicated 75M 
monobander that can give IMD numbers higher than what is achieved with open 
loop GG amps, etc to address this unique 75M window problem.

Let's see if we can sort out some NFB techniques here....  I don't want to 
pit one guy against the other, so I will not mention names from private 
emails or archives, just opinions... so we can discuss things without egos 
getting involved.

Richard, you mentioned here to try 100 ohms in the cathode of my GG amp. I 
believe it would put the amp into cutoff, since only 10 ohms drops the idle 
by 1/2. It was suggested it would require a choke across this resistor to 
idle the amp and eliminate the DC drop and bias variation during ssb 
eaks  - and keep the RF feedback working.  There is talk in the archives of 
using this choke, while others strongly disagree in it's use.

Here's an email I recieved from an RF engineer addressing the subject of 
using a cathode resistor in a GG amplifier. He is addressing my question the 
other day of using this cathode resistor for GG amps. [printed with his 
permission, but I opted to leave the name out - unless he comes into the 
discussion himself]

" His logic is WRONG! Adding a CATHODE RESISTOR to a GROUNDED GRID (Cathode
 Driven ) amp does NOT increase negative feedback!  The drive and the output 
are  IN PHASE. His logic applies to GRID-DRIVEN amps, where the CATHODE and 
SCREEN are at/very near R.F. ground. (Usually TETRODE or PENTODE amps)-With 
an  amp
 such as he described,(Grounded-Grid) he will need to add resistance IN 
SERIES with the R.F. Drive, NOT the cathode ground return."

So there you are.

Another NFB technique  - Some, as above, have suggested to put a resistor in 
the grid circuit. I see mention of it in the archives - BUT, some say it 
will destabilize things - an old Orr mistake. I would like to try that 
technique too and will add one in series with the driver coupling cap as 
suggested in a private email. I'll publish results..

These two techniques sound like a great way for the average guy to slightly 
improve the linearity of his GG linear. So it's worth talking further about 
them.

For my 75M mono-bander, I may go the Harris design route. I am leaning 
towards building something like a 6CL6 driving a 4CX-350, driving another 
4CX-350, driving a 4CX-3000 with heavy feedback back to the 6CL6.  100mW in 
and 1500+ QRO out as clean as I can make it. We'll see.

BTW,  [Tnx for the correction of  using"m" mW instead of "M', MW,  mega vs: 
milli... :-)

73,
Tom, K1JJ