Negative aspects of LM324 and LM358 (by Brindsley Archer)

Component: LM324

Category: Opamps

These opamps are produced by many manufacturers and have become industry building blocks. There are some traits that will preclude them from some uses.

-The inputs are specified to go to the negative rail and they do, BUT input impedances fall as the internal PNP transistors run out of gain. This means that feedback resistors have to be orders of magnitude lower than expected if accuracy is to be preserved.

-The output behaviour if the inputs are swung negative depends on the chip manufacturer. The LT version appears best in this respect.

-The output stage can move from class A into class B, the transition point depends on output voltage and load resistance. HF oscillations accompany the transition point explaining why the chip sounds "harsh" when used for audio. Best to not use for audio but if you must, keep load resistance high or pull down output to negative rail with resistor of less than the load resistance. This will keep the output stage in class A.

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