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LM358A: Output waveform in case of large input signal

suy
Expert 1045 points
Part Number: LM358A
Other Parts Discussed in Thread: TINA-TI, LM358, LM358LV

Hi team,

   I am using LM358A as a non-inverting amplifier to simulate the circuit. When the input signal is 10mV, the output waveform looks normal; when the input signal is 50mV, there will be a small steps in the negative half cycle of the output waveform. What is the reason for this phenomenon? ? Is it normal?

  • 0
    Guru 287370 points

    Neither the LMX358 nor LTspice are made by TI.

    What is the output voltage without AC coupling? In theory, 2 V should work.

  • 0 in reply to Clemens Ladisch
    Expert 1045 points

    The simulation model is downloaded from the TI official website.

    snom268c.zip

  • 0
    Guru 140200 points

    Hi Suy,

    I can confirm the issue in TINA-TI Relaxed

    You may want to add a pull-up:

    suy_lm358.TSC

    The LM358 is well known to profit from a pull-up at the output in such cases.

    Kai

  • 0 in reply to kai klaas69
    Expert 1045 points

    Hi Kai，

       I‘m appreciate your reply.

       I added the pull-up 10k resistance as you suggested, and the output waveform was normal. In addition, I found that the same problem occurred again when the input increased to 500mV, and the output was normal again after the pull-up 10k resistance was reduced to 1k. So I want to ask what the root cause is and why adding pull-up resistance can prevent this from happening.

  • 0 in reply to suy
    Guru 140200 points

    Hi Suy,

    Ron is an expert on this and can explain better than I could. You may want to read his appnote:

    3531.sloa277a.pdf

    Kai

  • +1 in reply to kai klaas69
    TI__Mastermind 29156 points

    Hey Suy,

    If you look at the current through the output without the pull-up resistor, this happens when the output current is close to 50uA. The LM358 has a 50uA constant current sink on the output, which is helpful for being able to drive very light loads close to the V- rail (think capacitive load or very large resistances). 

    This can yield nonlinear behavior when the output tries to source around 50uA of current and fights the constant current sink. Adding this pull up resistor forces the output to sink current. When you increase the input voltage, you then drive the output to a point where it will start to source current again and fight the output current sink. This is why adding a stronger pull-up fixes this effect. There is a sink current limit however, and eventually the LM358 will no longer be able to pull the voltage down from the pull-up resistor.

    Best,
    Jerry

  • 0 in reply to Gerasimos Madalvanos
    Expert 1045 points

    Hi Jreey，

       Thank you very much for explaining the specific reasons, and thank kai for providing the documents.

  • +1 in reply to suy
    Guru 140200 points

    Hi Suy,

    keep in mind that the LM358 is a dino. It was a sensational OPAmp at the time it came to the market many decades ago. But today we have better suited OPAmps. You may want to replace the LM358 by the LM358LV:

    Shown are seven different input signal amplitudes ranging from 10mV to 220mV.

    Kai

  • 0 in reply to kai klaas69
    Expert 1045 points

    Hi Kai,

       I got it.

       Thank you for your special reminder.