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LM224: LM224D op-amp misbehaving at saturation

Part Number: LM224
Other Parts Discussed in Thread: TLV9004

Dear Sir,

I am using op-amp LM224A of TI make and LM224D of ON-semi make in one of our ciruit. I have observed one weired behaviour which I am not able to understand.

Voltage Gain : 3.7

Voltage Input : Rectified DC inverted sine wave

With ON semi make Op-amp I observed after saturation op-amp peak start to clip off at 2.2V and on increasing the voltage at the input, the behaviour of the output used to reach towards a square wave. If voltage at input is increased further then output behaviour gets inverted and distorted and it comes out of saturation mode. Please refer the image for the same.

However with TI make the observation is : no matter of whatever is the input voltage once the reaches to saturation it gives perfectly square wave at extreme saturation.

kindly guide , which parameter is causing the output of op-amp in extreme saturation to get inverted and distorted.

  • Hello Ankit,

    From your description, I drew your circuit. Is this correct?

    The input signal is -13.4V peak and the gain is -3.7 therefore the theoretical output peak would be +50V.
    The output maximum is limited by the VOH of the op amp. You already know this.

    Once the output gets to VOH level. The voltage at the two inputs will no longer be the same. In other words the "virtual ground" is no more and the op amp is in "overload" which means there is a small time delay coming out of this mode, overload recovery time. Not an issue in this application.

    The main problem is that the inverting input goes below ground. It would be, in theory, -10V with input at -13.4V and output at +2.1V. However, both op amps have parasitic diodes than limit the inverting voltage to around roughly -0.7V and will have input current that must be limited (-1mA max suggested) for device's safety. This parasitic current flow will affect the  operation of the amplifier in unexpected ways. For the competitor's LM224D, I see inversion that brings output low; even lower than ground. Most (and likely all) LM224A  produced today and in the past will force output high (the desired effect) as this is what the parasitic current does in LM224A.  

    The risks are:
    1) That all samples don't do this (unlikely today) 
    2) Distant future LM224A product could be changed and the output might not go high in this application.

    For 3.3V supply, I suggest the TLV9004 which will go up to 3.3V on output and won't have inversion as the parasitic diode paths are isolated from the amplifier core function, "inversion free".