[Laser] DC couple the K3PGP amp?

[TWOSIG at aol.com]

I have been trying to digest the discussion about low  frequency signals.  My 
first thought is that for the signals I am  interested in working with 
(voice), the problem that should be attacked is  eliminating the sensitivity to low 
frequency.  To me that is noise.
 
The discussion on the list seems to be about boosting the sensitivity of  the 
lower frequencies.  Or perhaps it is better said that lower frequencies  are 
better and that the circuits being used are limiting performance.
 
I was looking at the K3PGP circuit, with the intent of "improving it" for  my 
use.  Then it hit me that the reason that the circuit limits low  frequency 
performance is that it is capacitively coupled.  Whatever is  done to the basic 
circuit is only going to change the frequency where it limits  performance.   
Then I remembered an old text on transistor circuits  had DC coupled 
amplifiers.
 
I don't remember that much of what the textbook said, but I remember it  
talked about complimentary stages (and the Darlington circuit, but I don't see  
how that would help here).
 
I began to wonder if there was a way to DC couple the K3PGP circuit.   The 
task is way over my head, but maybe I can get you guys thinking about  it.  I 
realize that it might ruin the low noise nature of the design.  
 
Maybe it would help if you replace the resistor and capacitor at the lower  
end of the FET.  ( Ok.  Don't make fun of me because I can't remember  if it is 
the source or the drain.  I was trained on vacuum tubes so it  looks like a 
cathode to me.)  In its place put a string of diodes to set  the correct 
voltage drop.  A zener might set the right voltage, but my  "gut" tells me it would 
add noise we don't want.
 
If you then remove the capacitor from the FET to the base of the  transistor, 
you would need to pull the resistor from its collector to  base.  Then you 
need to raise the emitter off ground potential without  sacrificing  to much 
gain or adding noise.  Any suggestions from  you?
 
Maybe the way to go is to replace the single NPN transistor with a DC  
coupled PNP to NPN complementary amplifier circuit.  It may not have the  low noise 
figure of the original, but it should have good gain charactoristics  all the 
way down to DC.
 
I hope this gives you some ideas that you can use.  If not, maybe you  can 
think of a way to improve the voice band performance while you are trying to  
improve the sub-audible frequency performance.
 
 
73
 
James
N5GUI