[Laser] New optical front ends

[F1AVYopto at aol.com]

Steve

>Can you explain the purpose of the 2.2 Meg  resistor,
>the 0.47/0.1 uF capacitors and the 1k resistor  (BF
>circuit) ?

The simplest explanation is this one:
The FET with the photodiode  in gate is called the input FET.
THe FET (FETs) with the 2,2M resistor in gate is (are) called the load  FET. 
If you fix the source gate voltage of a FET it becomes a nearly  perfect 
current source.
A current source has infinite impedance.
The 0,47  MF capacitor fixes the dynamic gate voltage of the load FET so the 
real drain  dynamic load of the input FET becomes 2,2M at is Idss current and 
at its max  transimpedance.
The gain becomes very high! 
With such a load the output  stage must have a very high impedance to keep 
the 2,2 M load value and so to  keep the gain.
The 1 K resistor gives a very high impedance "source  follower" with the load 
FET without to change the gain.
The VLF  schematic uses two FETs as a load because they must provide all the 
Idss current  of the input FET.
A single FET could have an Idss current lower the input FET  one.  
In the BF schematic, the input FET source resistor reduces the  current so a 
single FET can be used as load.

>What does "BF"  mean ? Basses frequences ?

Sorry, yes, BF means "basses fréquences"  because this front end bandwidth is 
the same a K3PGP RX with a small photodiode  capacitance.

>What modulation frequencies are you using with  each
circuit ?

Theses circuits are for light amplitude  modulation for audio frequencies 
(BF) and for VLF (10-300 Hz for optical NLOS  and FFTDSP).

73 Yves F1AVY.