[Amps] low Q output matching

[TexasRF at aol.com]

Hi Carel, I would have expected the two inductors to require an increase in 
 value. I have not modeled the problem so could be wrong.
 
73,
Gerald K5GW
 
 
 
 
In a message dated 11/11/2012 1:46:22 A.M. Central Standard Time,  
pc5m at xs4all.nl writes:

Hi, I  was wondering what else, besides bad harmonic suppression, will 
happen
when  I design my (single band) output matching network (PI-L)  with low  Q
(lowest Q needed to get the match done) ?

When using low Q design  I will get
- much smaller (variable) capacitors, especially for the bigger  load C, so
easier to get components 
- broader no-tune range
-smaller  RF stress on the components

As an example for a 160mtrs amp, with Rload  of 1700Ohm and intermediate
impedance around 330Ohm:
With Q of  10:
CLoad  =1200pF
L-l=10uH
Ctune=293pF
Lpi=33uH



With lowest  Q:

CLoad=400pF
L-l=10uH
Ctune=132pF
Lpi=33uH



When  you plot both matching networks  in a Smith diagram you see  immediate
the difference. Low Q is much more straightforward.
To get the  needed harmonic suppression a lowpass filter with fixed
components is easy  to construct (first harmonic at 3,8MHz needs 25dB and
second one 20dB to  get the same values as higher Q design). But maybe I 
have
overlooked  something . Thanks for your ideas
73's Carel,  pc5m





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