frilance
Member level 1
[For everyone: I want to say sorry in advance for my mistakes writting in english]
Hello Fellas,
I designed a TIA photoreceiver circuit in order to obtain an output voltage from a input current coming from a photodiode.
The schematic of the design and the simulation of the expected frequency response is showed in the attached file "schematicSimulation.PNG" (
). The only difference from the schematic and the actual circuit I'm testing is the photodiode (respresented in the schematic as a Input Current Source and a parallel 11pF capacitor).
I just want to have an ouput voltage from the first TIA stage up to 50 MHz (Rf = 5KOhms and Cf = 0.7pF can make it) and then divide the signal into 2 paths (with the same cut-off frequency, 10 KHz). I used an active filter for the AC path, using 2 capacitors in the second OpAmp of around 150nF. For the DC path, i used a simple Low Pass filter.
For the Transfer Function - Frequency Response of the circuit, I used an input Laser-Diode Beam focused on the photodiode, and modulated with the Network Analyzer signal (from 5 Hz to 500 MHz.).
The result obtained in the DC output is showed in picture "dcTransferFunction" (
), and it´s quite the expected one (Sorry for the quality of the pictures, I didn´t have any DAQ card for that Network Analyzer). The cut-off frequency is about 10 KHz.
The problem comes with the AC response. "acTransferFunction".
The high cut-off frequency is about 50 MHz, so that looks OK, but for the low cut-off frequency, first it has this ripple I didn´t expect (this is the worst problem), and then it looks like the response rises again after a short interval going down. I identified some possible sources of errors, but I would appreciate some help to understand what is going on here.
- First, the Laser Diode´s behaviour at low frequencies. It´s quite bad, so I don´t know if the strange behaviour of the ripple is related with that. I think that at least the rising in the response below 10 KHz is related with the Laser Diode.
- I used a piramide-shape series connection of two 1 pF capacitors to make the "Cf = 0.7 pF". The method wasn´t the best, but seeing that the high cut-off frequency is around the expected one (50 MHz) I would say it´s non-related with this capacitor.
- Maybe the use of the same cut-off frequency for both paths (10 Khz) makes the circuit have a resonance or oscilation. Phase margin problems and so on... I don´t know.
- The DC passive low-pass filter affecting the AC part....
Those are some ideas, but maybe somebody can understand better how this works and could help me with it. Maybe it´s just a Laser Diode problem and it´s working as expected....
Thank you in advance.
Kind Regards,
frilance
Hello Fellas,
I designed a TIA photoreceiver circuit in order to obtain an output voltage from a input current coming from a photodiode.
The schematic of the design and the simulation of the expected frequency response is showed in the attached file "schematicSimulation.PNG" (
). The only difference from the schematic and the actual circuit I'm testing is the photodiode (respresented in the schematic as a Input Current Source and a parallel 11pF capacitor). I just want to have an ouput voltage from the first TIA stage up to 50 MHz (Rf = 5KOhms and Cf = 0.7pF can make it) and then divide the signal into 2 paths (with the same cut-off frequency, 10 KHz). I used an active filter for the AC path, using 2 capacitors in the second OpAmp of around 150nF. For the DC path, i used a simple Low Pass filter.
For the Transfer Function - Frequency Response of the circuit, I used an input Laser-Diode Beam focused on the photodiode, and modulated with the Network Analyzer signal (from 5 Hz to 500 MHz.).
The result obtained in the DC output is showed in picture "dcTransferFunction" (
), and it´s quite the expected one (Sorry for the quality of the pictures, I didn´t have any DAQ card for that Network Analyzer). The cut-off frequency is about 10 KHz.The problem comes with the AC response. "acTransferFunction".
The high cut-off frequency is about 50 MHz, so that looks OK, but for the low cut-off frequency, first it has this ripple I didn´t expect (this is the worst problem), and then it looks like the response rises again after a short interval going down. I identified some possible sources of errors, but I would appreciate some help to understand what is going on here.
- First, the Laser Diode´s behaviour at low frequencies. It´s quite bad, so I don´t know if the strange behaviour of the ripple is related with that. I think that at least the rising in the response below 10 KHz is related with the Laser Diode.
- I used a piramide-shape series connection of two 1 pF capacitors to make the "Cf = 0.7 pF". The method wasn´t the best, but seeing that the high cut-off frequency is around the expected one (50 MHz) I would say it´s non-related with this capacitor.
- Maybe the use of the same cut-off frequency for both paths (10 Khz) makes the circuit have a resonance or oscilation. Phase margin problems and so on... I don´t know.
- The DC passive low-pass filter affecting the AC part....
Those are some ideas, but maybe somebody can understand better how this works and could help me with it. Maybe it´s just a Laser Diode problem and it´s working as expected....
Thank you in advance.
Kind Regards,
frilance
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