[Laser] Re : Preamp design, etc.

[Garnier Yves]

Hi all
An other way to avoid all the link capacitors and to remove the day light effects on the bias points is to use a dynamic load in the FET drain.
This load auto adjust its resistor to the good middle drain voltage and give directly a great dynamic load to the high speed current fluctuations.
The price to pay is the second amplifying stage that must to have a great input impedance.
You can design the diagram from the example at :
http://f1avyopto.wifeo.com/images/photometric-occular2.JPG
73
Yves


 
Yves F1AVY
http://f1avyopto.wifeo.com


________________________________
 De : steve kavanagh <skavanagh73 at yahoo.ca>
À : laser at mailman.qth.net 
Envoyé le : Samedi 17 Décembre 2011 2h44
Objet : [Laser] Preamp design, etc.
 
Thanks for the clarifications, Stuart !

> The 33 ohm resistor is to save the charge in the electrolytic capacitor eroding the relay contacts 

Good point !

> You mention the size of the gate coupling capacitor, I use 0.001uF 

Yes, I saw that, and if my thinking is right about the ratio of this capacitor to the FET/LED capacitance it sounds about right....I imagine the LED must be around 100 pF.  By the way I measure my little LED's capacitance today....5 pF at 1.4 V !

> Several who have built this design in the UK have reported this necessary and have told of the sensitivity increase at the correct operating point.

OK.  Not sure I understand why...further thought needed !

>my circuits were used at both ends of the current record two-way contact distance in the UK, 117.6km. 

Good work, Stuart !  My ambition at the moment is more modest....to get a second receiver so 2-way contacts are possible !  It's tough at the moment trying to recruit anyone to build a second rig.  My initial aim is to arrange something that is easy operationally (and relatively simple)....that means broad beam transmitters that can be seen as soon as they are turned on....which means shorter distances ! For now, anyway.

73,
Steve VE3SMA
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