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I am implementing the circuit published in AN3025 Transimpedance Amplifier Design as shown below. enter image description here

When I am simulating this circuit, a few questions come to mind:

Q1. Why do we need a differential amplifier in stage 2? I think this stage is to remove the offset voltage of the first stage. Am I right? But when I am simulating the above circuit using a single power supply, I am getting offset of 29mV. So what is the need of the difference amplifier? enter image description here

Q2. When I used a dual power supply of 5V in the second stage, the offset voltage was completely removed. However, this power supply of 5V is not recommended in the datasheet. But why is this happening? enter image description here

Q3. When I have used a dual power supply of 2.5V then I got an offset of -2.47V. Why is this happening?

enter image description here

Q4. How do we completely remove the offset voltage?

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  • \$\begingroup\$ Take a look at 4.1. Design Goals of that document. It explains that this is an example that uses a single voltage rail, has no offset and does not invert the signal. If you have multiple voltage rails and don't mind the signal being inverted, you could use a single opamp if you wanted. \$\endgroup\$ Commented May 21, 2020 at 19:21
  • \$\begingroup\$ Positive input of stage 1 has 1k||(1+10)k. So the reference is not vcc/2. Why is that? \$\endgroup\$
    – Sadat Rafi
    Commented May 21, 2020 at 19:58
  • \$\begingroup\$ In the datasheet I have seen that pin 7 is base. And why the emitter and collector is unused? \$\endgroup\$
    – Sadat Rafi
    Commented May 21, 2020 at 20:02
  • \$\begingroup\$ HI User 1850479, you are right that there is no offset in 4.1, but when I simulated this circuit as above I obtained an offset of 28.77mV. Why is that? can you please explain? \$\endgroup\$
    – Parveen
    Commented May 23, 2020 at 19:56

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With a single supply, the op amp's output can't quite reach the negative rail.

Part of data sheet

You could try reducing the load on the output by making R4=100k and R3=10k. I suspect you'd reduce that output voltage a bit.

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