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Low side clipping for LM324, works well for TLC274

Mobile Wave
Prodigy 130 points
Other Parts Discussed in Thread: LM324, TLC274

Hello,

I had a question about clipping of a 60 Hz AC signal when it is passing through an LM324. The LM324 is biased with 12V and DC and the input has a virtual ground at 1.65V. The gain of the op-amp is such that the output  signal will swing between 0V and 3.3V. In real life, this signal is swinging between 640mV and 3.3V.

However, when I replace the LM324 with TLC274 the output swings very cleanly from 0V to 3.3V. This setup is on a breadboard, and no other change is made except pulling out the LM324 and putting in the TLC274.

The signal is from a function generator with the intention of figuring out that a barebones op-amp like the LM324 would work for my application. I do understand that the LM324 has higher offset voltage and poorer CMRR, but I am not sure how that would cause this issue.

Thanks

 

  • 0
    TI__Guru*** 140136 points

    Mobile,

    The answer is in the device schematic seen in data sheet.
    While souring current or sinking low current (<30uA), VOL will be near ground. The ~50uA current sink has a low VOL.
    For higher sink current the PNP emitter output will provide the current. However the emitter (output) must rise by 1 VBE to turn the PNP transistor on.
    This is why the output is limited to 630mV in the application.

    1VBE = one diode forward voltage