finding op-amps without phase inversion/latch up

Discussion in 'General Electronics Chat' started by sravet, Oct 9, 2010.

  1. sravet

    Thread Starter New Member

    Oct 8, 2010
    In another thread I found a link to a precision full rectifier using single supply opamps with out needing any diodes.

    As noted in the article, the circuit relies on having an op-amp that doesn't latch up when the input goes negative, dropping below the ground rail.

    Those op-amps are $3-$4 in single quantities, are there any other op-amps to choose from that are protected from phase inversion?

    I am using this circuit to rectify a line level audio signal, with the rectified signal being sampled by an ADC to eventually result in a VU meter type display.

    thanks for any suggestions,
  2. Audioguru

    AAC Fanatic!

    Dec 20, 2007
    Something is wrong with that circuit.
    The input goes to plus 5V and -5V then the two 1k resistors in series cuts the voltage to the input of the opamp in half.
    The max allowed negative input voltage of the Cmos opamp used is only -0.3V which is the same as an ordinary old LM324 quad or LM358 dual opamp but the circuit is giving the first opamp peaks that go to -2.5V which might destroy it.
  3. sravet

    Thread Starter New Member

    Oct 8, 2010
    In the datasheet for the opamp it says as long as the input current is limited to 5ma then the device is protected and phase inversion won't occur. Figure 2 shows a +/- 7 volt input applied with a supply voltage of +3V

  4. DonQ

    Active Member

    May 6, 2009
    Rather than depending on the internal protection diodes of the amplifier (and other bad design decisions), it makes more sense to just use a proper circuit where you include your own diodes. If properly designed, the voltage drop of the diode is not applied to the signal. This makes amplifier selection a trivial problem.


    (The link seems to not take everyone to page 10, figure 25, so go there manually)

    Don't use the amps they suggest. This was first published in the 1969! and it's been working since then in a wide variety of incarnations. Any reasonable amp will work, and you can DC couple it if you need.
    Last edited: Oct 10, 2010
  5. marshallf3

    Well-Known Member

    Jul 26, 2010
    Good advice, internal protection diodes are not really meant to be used on a constant basis, they're basically there for accidental situations.