Jim,
I believe you are measuring the IMD of the K3 or at least seeing results
that are influenced by the K3. I have made IMD measurements of the K3
and they are not that great and were the best at the 30-40W level. I
built a simple fixture for IMD testing that used 2 separate transmitters
and had IMD3 of better than 60dB. See
http://www.w0qe.com/Technical_Topics/imd_testing_of_amplifiers.html
NXP has an app note describing how much better the source IMD needs to
be in order to measure the output IMD to a certain level of
uncertainty. Do a search for "NXP RF transmitting transistor and power
amplifier fundamentals" and it is table 3.1. This paper is better than
any other doc I have seen for transistor high power amps. It discusses
many other transmitter topics such as bias level etc. vs IMD. This
paper does NOT have the usual NXP ECOxxxxx prefix and has a 1998 date.
Other testing I have done shows that to have really good IMD you need to
significantly under drive the amp. Some 300W Freescale LDMOS parts were
great at the 100W level and OK at 150W. Beyond 150W the IMD began a
constant rise. Unfortunately backing down the output level kills the
efficiency.
I thought the same as you about increasing negative feedback. However
there is just not enough excess gain to make much of a difference if the
goal is increasing IMD3 from 30dB to say 40dB. Simulations in LTSpice
unfortunately confirmed the measurements. A big downside of negative
feedback is the reduction in input impedance. To really drive the IMD
lower you would need another gain stage within the negative feedback
loop. Source degeneration is a good idea and it increases input
impedance while reducing gain.
These are my opinions and are at least backed up by measurements and
simulations but I am not a high power amp expert.
73,
Larry, W0QE
On 8/7/2013 7:46 AM, Jim Thomson wrote:
A buddy is building the 250w SS HF amp you see in the last few arrl books.
Its almost finished. It only requires aprx 7-10 watts of drive to the pair of
VRF-151G finals.
He also ran a series of tests on his K3....and found that min IMD occurs with
30 w pep out. It increases above or below 30 w pep. Its very clean
at the 30 watt level. The original plan was to use a 5-6db pad between the K3
and the hb 250w SS amp. Upon close examination of the 250w ss amp,
it occurred to me that if the NFB was increased by 5-6 db, the 5-6db pad
between k3 and SS amp could be eliminated.
I think the base resistor values on the SS amp can be increased to do this.
Resulting IMD from the 250w SS amp...on paper, should drop a bunch.
The 250w SS amp has pretty good IMD as is, at 250w pep out...and a lot better
at 225w..and better still at 200w. The bias on the SS amp could also be
tweaked a bit to improve it some more.
OK, for every 1 db increase in NFB on the SS amp, how many db will the
IMD3-5-7-9-11-13 drop ?? I have no info on this anywhere.
Is it going to be a 1 to 1 deal, or does the IMD drop several db for each 1 db
increase in NFB ? What about a tube amp, does it follow
the same pattern as a SS amp re: NFB vs IMD. ?
On paper it appears this scheme should work. What we are after is a squeaky
clean 180-250 watts pep out. The K3 only puts out
100w pep.....and the IMD at 100w pep out is nothing to brag about....its lousy.
The thought here was we could kill about 3 birds at once. Increasing idle
current on the K3 final stage plus its mating driver is not appealing
due to heat issues, plus minimal IMD improvement. Ditto with the 250 W SS
amp. Now whether the 250w SS amp will handle the 30w of drive,
and no 5-6 db pad....and just relying on a 5-6 db increase in NFB is another
issue. Any thoughts would be much appreciated.
later........ Jim VE7RF
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