Precision half wave rectifier

An ordinary diode rectifier cannot rectify signals smaller than the diode's cut-in voltage (about 0.6 V), because the diode does not conduct below it. A precision half-wave rectifier (super diode) places the diode inside the feedback loop of an op-amp. The effective cut-in voltage is divided by the open-loop gain, so even millivolt signals are rectified accurately.

Circuit diagram

                +---------[ Rf ]----------+
                |                         |
                |     D1                  |
                +-----|>|------+          |
                |              |          |
 Vin o---[ R ]--+--(-)\        |    D2    |
                       >-------+---|>|----+-----o Vo
           GND ----(+)/                   |
                                        [RL]
                                          |
                                         GND

D1 has its anode at the inverting node and its cathode at the op-amp output. D2 has its anode at the op-amp output and its cathode at the output node Vo. Rf runs from Vo back to the inverting node.

Operation (inverting version)

Positive half-cycle of Vin: The op-amp output goes negative. D1 turns on (D2 is off), closing the feedback loop and keeping the inverting input at virtual ground. No current flows to the load: Vout = 0.

Negative half-cycle of Vin: The op-amp output goes positive. D2 turns on (D1 is off). Now the op-amp works as an inverting amplifier with the feedback resistor Rf:

Vout = −(Rf/R) Vin

So the output is a positive half-wave, scaled by Rf/R (unity when Rf = R).

 Vin     _         _
       /   \     /   \
 -----/-----\---/-----\---/---> t
             \_/       \_/

 Vout         _         _
             / \       / \
 -----------/---\-----/---\---> t

The output is zero during the positive half-cycles of Vin; the negative half-cycles appear inverted (and scaled by Rf/R).

The diode drop is divided by the open-loop gain A (≈ 10⁵), so the circuit works for inputs of a few millivolts. Applications: signal detection, AC voltmeters, AM demodulation and instrumentation.