As shown in the figure, this is a kind of precision full-wave rectifier circuit of op amp on the Internet, 4 diode type.
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Among them, the functions of D1 and D3 are not very clear to me. I think it is still feasible after removing them. Can you give me some ideas?

Rebeka 게시 날짜 August 26, 2020
Generally used for input protection and limiting.

Rina 게시 날짜 August 26, 2020
Thank you, is that understanding?
For example: when ui>0, R1 and R2 form positive feedback on A2, and D1 clamps the voltage on the right side of R1 to 0?

Lainie 게시 날짜 August 26, 2020
Simply put, it is clamping to remove the diode's tube voltage drop effect.