In a precision rectifier circuit, when Vin is negative, what happens to the diode and the output?

In a precision rectifier circuit, when Vin is negative, what happens to the diode and the output?

• A. The diode conducts and the output follows Vin.
• B. The diode is off and the output saturates near 0.
• C. The output becomes a negative voltage equal to Vin.
• D. The op-amp saturates high.

Answer: (B)

In a precision rectifier, the diode acts as a switch that controls whether the op-amp’s feedback is closed. When the input is negative, the op-amp would try to drive its output negative to pull the feedback node toward ground, but that makes the diode reverse-biased. With the diode off, the feedback loop is opened, so the output node is no longer forced by the op-amp. As a result, the output remains at or very near zero (ground), while the op-amp output may swing toward the negative rail behind the scenes. This is why the diode is off and the output sits at zero for negative input.

In a precision rectifier, the diode acts as a switch that controls whether the op-amp’s feedback is closed. When the input is negative, the op-amp would try to drive its output negative to pull the feedback node toward ground, but that makes the diode reverse-biased. With the diode off, the feedback loop is opened, so the output node is no longer forced by the op-amp. As a result, the output remains at or very near zero (ground), while the op-amp output may swing toward the negative rail behind the scenes. This is why the diode is off and the output sits at zero for negative input.