[Laser] DC couple the K3PGP amp?

[KY1K]

John is correct-

If you want to do voice, you need to use a photodiode in PV mode with 
an op amp and appropriate feedback resistor. The feedback resistor 
provides very good control and quantification of the upper frequency 
limit. For every 2X the bandwidth is reduced, a 4X reduction in noise 
occurs-so it doesn't take a rocket scientist to understand that a big 
increase in performance can be had by minimizing the frequency 
response to the highest frequency you are interested in receiving.

Don't make a 10 Khz cutoff frequency if you're only interested in 
communications quality (300 to 3 Khz).

The traditional PD and op amp receiver does nothing to eliminate 
frequencies below 300 Hz, which aren't needed for communications 
grade fidelity however.

I know I've said this before, but Burr-Brown integrated photodide/op 
amp chips are inexpensive and offer a lot of performance and are easy 
to fabricate. OPT-101 and OPT-301's are available on ebay.

Regards,

Art


Don't At 01:51 AM 3/16/2006, you wrote:
>F1AVYopto at aol.com wrote:
>
>[snip] 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. [snip]
>
>If you want frequency response from a photo diode, back bias it to 
>widen the Space
>Charge Layer and run it into an opamp input (a virtual ground)  with 
>a FB resistor.
>It works well to > 1 MHz.
>
>-John
>
>
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