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Also called a Precision Rectifier. This circuit is like a half-wave rectifier, except the opamp uses closed-loop negative feedback to cancel the diode forward voltage drop. The opamp is configured like a voltage follower, but instead of the feedback path monitoring the opamp's output voltage, it is monitoring the diode's output voltage. In the positive (diode forward voltage) direction, the opamp attempts to make V- = V+. The diode requires VF to turn on, so the opemp output continues to rise above V+ until it reaches VF + V+. Then, V- = V+. Note that in the negative direction, the opamp output shoots to the negative rail, attempting to force the diode's otuput to the required negative input voltage at V+. But, the diode does not conduct in the negative direction, so the output does not go below 0 V.
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