Precision Rectifier

  1. I'm having trouble understanding the following sentence, regarding an improved precision rectifier circuit:

    This circuit has the benefit that the op-amp never goes into saturation, but its output must change by two diode voltage drops (about 1.2 V) each time the input signal crosses zero.

    (1) What is meant by "change by"? If for example, the input is 5 volt, would the output change by 5 - 1.2 = 3.8 Volt?

    (2) Is it still necessary in this circuit, to overcome the forward voltage?

    http://en.wikipedia.org/wiki/Precision_rectifier
     
  2. jcsd
  3. AlephZero

    AlephZero 7,298
    Science Advisor
    Homework Helper

    One of the diodes D1 and D2 is always conducting and the other one is not.

    When the input goes through 0V, the other diode starts to conduct. The output of the op-amp has to flip between about +0.6V and -0.6V to provide the voltage drop across the correct diode.

    If the input changes between say +1mV and -1mV, The output of the whole circuit only changes by about 1mV, even though the output of the op amp changes by about 1.2V.
     
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