OPA846: Question About Input Impedance and Phase Margin in TIA Circuit with OPA846 - Amplifiers forum - Amplifiers

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?? ?

Prodigy 40 points

Part Number: OPA846

Tool/software:

Hello, I recently started learning about analog circuits, and I am currently designing a simple photodiode amplifier circuit.

The photodiode has a parasitic capacitance of approximately 20pF, and I estimated the op-amp's parasitic capacitance to be around 10pF. I selected a feedback resistor of 50kΩ and a feedback capacitor of 0.3pF, allowing me to design a TIA circuit with a bandwidth of approximately 13MHz.

The OPA846 datasheet states that the differential mode input impedance is approximately 6.6kΩ, but my feedback resistor is 50kΩ, which is nearly 10 times larger. Would this cause signal distortion?

If I apply the voltage divider effect, I estimate that the signal could be attenuated by around -20dB. If my calculation is incorrect, please let me know.

To investigate the phase margin, I added an inductor and a capacitor to the circuit and performed a simulation.

This is my first time analyzing the phase margin. I am wondering if I have properly marked the probe to investigate the phase margin. I would appreciate any insights or corrections to my understanding. Thank you!

7 months ago

+1 Aditya Gosavi 7 months ago

TI__Intellectual 2065 points

Hi,

Thank you for providing all the details. Few points I would like to add from my side:

For OPA846 the inverting input capacitance is around 3.8pF which is mentioned in the datasheet under "input impedance" on page 3. With this value you can accurately design your TIA circuit.
You are correct that the OPA846 datasheet states that the differential mode input impedance is approximately 6.6kΩ, but in this TIA circuit, you should be concerned only about input common mode impedance. Common mode input impedance refers to the impedance seen at each input terminal with respect to ground. So the common mode impedance is 4.7MΩ, hence it would barely attenuate any input signal.
For designing such TIA circuits you can use our new TIA calculator tool: Analog circuit design studio where you can provide all the required circuit parameters like bandwidth requirement, output voltage range, photodiode current + capacitance, etc. The tool will also suggest devices/ TIA as per the user input and the tool also has comprehensive noise analysis. I used the tool to design your circuit, and it looks okay to me.
(please ignore the supply warning)
An issue I saw was that the error voltage due to OpAmp's input bias current is 0.95V (50k * 19uA). I am not sure if you want such high DC output at your TIA.

Please try the tool and let me know what do you think.

Regards
Aditya

0 ?? ? 7 months ago in reply to Aditya Gosavi

Prodigy 40 points

Thank you for your response.
I have a few more questions.

If I modify the circuit by adding a compensation resistor for the input bias at the non-inverting terminal, can I eliminate the output error voltage (offset)?

Also, I am currently using PSpice for TI to simulate the circuit. I am doing this to become more familiar with circuit simulation.
Is it possible to simulate a circuit like this using PSpice for TI?

I appreciate your advice.

+1 Aditya Gosavi 7 months ago in reply to ?? ?

TI__Intellectual 2065 points

Hi,

Yes you can reduce the effect of input bias current by adding a series compensation resistor at the non-inverting terminal, but cannot guarantee its elimination (because amplifier might have some non-zero input offset current).

If you want to know more how the compensation scheme works, please refer similar thread: https://e2e.ti.com/support/amplifiers-group/amplifiers/f/amplifiers-forum/1422066/opa2626-impedance-match-for-input-bias-current/5450476?tisearch=e2e-sitesearch&keymatch=%2520user%253A595890#5450476

Yes its possible to simulate a circuit like this using PSpice for TI.

Some helpful resources that you can use to learn about PSpice for TI:

Regards
Aditya

0 Aditya Gosavi 6 months ago in reply to Aditya Gosavi

TI__Intellectual 2065 points

Hi,

We would love to hear your feedback on the tool mentioned above: its usability, accuracy and any improvements you'd suggest. Please let us know your thoughts!

Thanks

Aditya Gosavi

0 ?? ? 6 months ago in reply to Aditya Gosavi

Prodigy 40 points

Thank you very much for your response.

I am planning to redesign the circuit by adding a compensation resistor to the non-inverting terminal.

I have been referring to videos and documents for guidance.
All my questions have been answered. Thank you.

It seems that simulating a photodiode using PSpice for TI is not very straightforward.