[Laser] DC couple the K3PGP amp?

[F1AVYopto at aol.com]

James, 
The  discussion was not about boosting the sensitivity at the lowest  
frequencies but to find the frequency domain (FFTDSP spectral area in frequency  
domain) where the PGP RX gives the best S/N. 
It  is not for voice com experiments but to use FFTDSP programs to modulate  
and to extract under noise coded signals. 
The  PGP RX gives a very strong gain enhancement at ULFs. 
The  S/N is also improved near the ULFs. 
It  is not the inter stages capacitors effect but the RC time constant effect 
 from the internal photodiode capacitance and its internal resistor 
associated  with the FET impedance. 
If  we calculate the Fc = 1/2 pi RC even with very fast PIN photodiodes, the 
-3  dB cut off frequency is always lower than 100 Hz. 
If  you are interested in working with voice modulation you are fully  right 
and you need to reject VLF noise by, for example, reducing the  inter stages 
capacitors. 
Unfortunately  the DC coupling is not useful because most of PC sound card 
used for  FFTDSP have a 15 Hz LF cut off frequency because their internal input  
capacitors. 
73 Yves