Why is the DC characteristic of this filter not a straight line?

飞鸿浩劫

Why is the DC characteristic of this filter not a straight line? [Copy link]

I used an online design tool to generate a low-pass filter, and tried it under simulation software. The AC characteristics were fine, but why is the DC characteristic a broken line with two bends? Why is there an inflection point?

飞鸿浩劫

Could it be that the LM358 is not rail-to-rail?

chunyang

It is normal that there is a voltage drop between the lowest output and the highest output of a non-rail-to-rail amplifier. However, if a rail-to-rail op amp is used, the output will be a diagonal line.

maychang

It is obvious that the upper and lower (or positive and negative) sides of the op amp are saturated. When the output of the op amp is saturated, the output does not change with the input, which is shown as a horizontal straight line in the figure.

飞鸿浩劫

maychang posted on 2018-9-13 09:04 It is obvious that the upper and lower (or positive and negative) directions of the op amp are saturated. After the output of the op amp is saturated, the output does not change with the input, which is shown as a horizontal straight line in the figure.

5V power supply is removed and 3V is lost

飞鸿浩劫

chunyang posted on 2018-9-12 23:06 This is normal. There is a voltage drop between the lowest and highest outputs of non-rail-to-rail amplifiers. If a rail-to-rail op amp is used, the output will be a slash.

Try changing the chip

maychang

Feihong Haojie published on 2018-9-13 10:55 5V power supply loses 3V

This is what the old op amp looks like. Your circuit uses two-stage inverting amplification. The first stage forward output loses 1.5V, and the second stage also loses 1.5V forward output, totaling 3V.

maychang

Feihong Haojie posted on 2018-9-13 10:56 Try changing the chip

By changing to a rail-to-rail op amp, the output swing can be close to the power rail.