Electronic – How to decrease capacitance of PIN photodiode

amplifieranalogcapacitoroperational-amplifierphotodiode

I am using PIN photodiode BPW34 (1).

I am then using an opamp (LM324) (2) to amplify and get an output voltage from the current source generated through the photodiode.

Following is the circuit I am using to realise this.

enter image description here

Following is the output I am getting at particular frequencies. I am actually giving square wave input light signal to photodiode.Following image is the output we got from the digital oscilloscope(Left-400kHz,Right-2kHz).

I also tried changing parameters for Cf in the diagram and also putting direct square wave voltage source(Left image) instead of photodiode with equivalent resistance.

Following is the output I got.

enter image description here

After all the experimentations, I feel that frequency response is sluggish(Right image) due to RC limitations of the circuits and that too due to capacitance of photodiode.

Someone please suggest me something for the same. I have read a lot of documents related to the same but couldn't find anything that is concrete.

Best Answer

I have used bootstrapping on photodiode amplifiers like this: -

enter image description here

The idea behind it is that any AC the capacitance of the photodiode is largely muted by applying the same voltage at its cathode thus you can use a much smaller feedback capacitor Cf.

The problem comes down to reducing the noise gain of the op-amp when used as TIA (trans impedance amplifier) - because the JFET reduces the capacitance seen at the inverting input, high frequency noise (due to the op-amp) is not amplified as much therefore Cf can be lower and bandwidth can be significantly bigger.

I am then using an opamp (LM324)

The voltage noise of that op-amp is terrible for applications like this. Choose a device with much better performance and much improved lower bias currents.

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