Simple Frequency Response question

[Catalyst]

Hi folks,

Have a quick question.

I am trying to find the frequency response of a diode which i am representing by a simple R and C in series (organic diode).

R = 100 MegaOhms
C = 12 pico Farads.

How would i calculate the frequency response of this device? What other parameters do i need?

Cheers

Cat

 

[keith1200rs]

It depends on the load you connect it to. A very small load resistor and it will be a lot quicker than with a very large load resistor.

Keith.

 

[Catalyst]

Thanks for yuor kind reply Keith

The load resistance is 1 MegaOhm. Sorry forgot to add that before.

My main problem is understanding the theory behind the calculation of the frequency response.

I am looking at an organic diode which I am trying to calculate a rough estimate for its cut-off frequency / frequency response but i do not know where to start.

Any advice is much appreciated.

Kind regards

Cat

 

[keith1200rs]

Cat,

With that sort of load it will be very slow: f = 1/(2.pi().R.C) so 13kHz. The leakage R doesn't really matter. It will make a very slight difference, but the resistance is dominated by the load resistor.

If you need more bandwidth you need to look at a transimpedance amplifier. This can give you effective resistances into the mega ohms while presenting a low resistance to the diode.

Keith.

 

[Catalyst]

Keith,

So for your calculation you used the R for the load and not the R of the device? I'm not totally clear why the R of the device is not important in this case.

Is the cut off frequency the same as the devices frequency response?


Much appreciated

Cat

 

[keith1200rs]

Cat,

It would be useful to know what the device is, but in general, if you are using the device as a current source (rather than photovoltaic mode) then a simple model is as a current source with parallel resistance and capacitance. When you add a load resistor this is in parallel with the internal resistance for AC signals so the effective resistance is the two in parallel. In you case (and most cases) the load resistor is considerably smaller than the leakage resistance so that dominates.

The device frequency response could mean anything - you need to find the test conditions on the data sheet to know what it really means. For example, the SFH203 is quoted as 5ns rise and fall with a 50 ohms load. The S2386-18K is quoted as 400ns with 1k load- it is difficult to just compare the speed without knowing the load.

Keith.

 

[biff44]

Where do you get 100 Meg ohm from? The IV curves in this paper show 600K ohm dynamic resistance (fig b) for instance.

**broken link removed**