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An improvement on the standard half-wave rectifier consisting of just a diode and a resistor.
For signals wilt peak voltages less than the diode’s forward voltage or just slightly above it (around 0.7V), it becomes difficult to capture the positive swings without losing most of the signal.
With the opamp, by adding the diode to it’s negative feedback loop, we allow the opamp’s gain to source and sink current through the diode.
The setup involves a unity buffer opamp configuration (when diode is ON) with a resistor connected at the end. When the diode is off, the negative loop is broken so the opamp functions as a comparator.
On the signal’s positive swing, the opamp produces a positive voltage at it’s output, turning the diode ON and completing the feedback loop. At this point, tracking occurs and the opamp re-balances itself such that the positive swing appears on the resistor (unity buffer).
On the negative swing, the opamp sinks current. That is current from the ground through the resistor tries to enter the opamp through the diode. However, this direction of flow can’t occur as diode is facing the opposite direction.
This puts diode in reverse bias => diode is OFF => no feedback loop. Hence, circuit functions as a comparator ensuring diode is OFF and output at end of resistor is 0.
NB:
-> Output at opamp pin compensates for diode
forward voltage as it is within the feedback
loop.
-> As a comparator, the opamp produces a
saturated output on negative swings. And
depending on the opamp’s slew rate, it could
be slow in transitioning from negative
saturation to 0V or input signal level.
This could lead to distortions when applying
this circuit to high frequency signals that
switch between positive and negative swings
at a rate higher than the opamp’s slew rate.
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