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Experimenting with Op Amps: active full-wave rectifier

Jan Cumps

The Art Of Electronics features a set of practical op-amp circuits. They have several full-wave rectifiers. I'm trying the simplest one. You don't need fancy devices. A pair of 741s will do.

The circuit:

image
image source: my copy of the book. fair use.

Looking at the node where the arrow points to:

  • for positive values, the first op-amp is an inverting buffer.
  • for negative values, the op-amp output has a  0.6V dc bias, the right side of  the feedback resistor will be at 0 V (ground). 
    hint: the 0.6 V output is generated by the op-amp to drive the inverting input to the same value as the + input (ground), effectively clamping the output to 1 diode drop above ground.

image

Looking at the second op-amp:

  • it is an op-amp with a summing node. Two signals at the inverting input are summed and inverted. The amplification factor is defined by the input resistor and the feedback resistor.
  • for the input signal, the amplification = -1 * R/R = -1 (the input resistance equals the feedback resistance, resulting in the -1 amplification factor, doubling and inverting the signal)
  • for the signal coming from op-amp 1, the amplification = -1 * R / (R/2) = -2 (the input resistance is half the feedback resistance, resulting in the -2 amplification factor)
  • it is an inverting amplifier for the input. Both positive and negative signals are amplified by -1.
  • for the signal coming from the first op-amp, it acts as a -2 amplifier.
  • for positive inputs, this results in  Vin * -1 + Vin * -1 * -2 = -Vin + 2Vin = Vin ->  result: Vin, positive Vin comes out as positive Vin (see comment)
  • for negative inputs, this results in Vin * -1 (making Vin positive)  + Vin * 0 * -1 -> result -Vin: negative Vin comes out as positive Vin

image

Here is the result: The blue line is the output. Underneath it, you see the input. The output overlaps the input for the positive part. It inverses the negative part. No diode drop.

image

Parents

  • It's a useful circuit that I used a lot in the 70s and 80s. I was wondering why I use it  a lot less now - and the answer is that I am much more likely to feed signals straight into a fast ADC in a micro and do the rectification in maths.

    I first saw this circuit in the National Semiconductors Linear Applications book (1972). This was where I learned about Op Amps in my (distant) youth. It's a great collection of Op Amp circuits.

    Just for fun try scoping the output of A1 and high frequencies and with a low amplitude signal - you might not be so happy with the 741 !

    MK

  • in reply to michaelkellett
    michaelkellett said:
    Just for fun try scoping the output of A1 and high frequencies

    40 kHz:

    image

  • in reply to Jan Cumps

    The first op amp in this design has a hard time because it has to swing the diode drop and more in a small fraction of a cycle time. The smaller the signal the worse the effect.

    Some of the true RMS chips have a similar problem, good frequency response with large signals but much less good with small. I can remember struggling to get the required accuracy over amplitude and frequency in a meter design for BT in about 1978.

    MK

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  • in reply to Jan Cumps

    The first op amp in this design has a hard time because it has to swing the diode drop and more in a small fraction of a cycle time. The smaller the signal the worse the effect.

    Some of the true RMS chips have a similar problem, good frequency response with large signals but much less good with small. I can remember struggling to get the required accuracy over amplitude and frequency in a meter design for BT in about 1978.

    MK

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