MFB Bandpass Filter

戒骄戒躁

MFB Bandpass Filter [Copy link]

When using the MFB topology to make a bandpass filter, it is found that when the input signal is very small, the bandpass filter has a large DC bias at the output. The second figure is an input of 3mv, the filter output is normal, and the third figure is an input of 0.4MV, the filter output has a large DC bias

maychang

[It was found that when the input signal was very small, the bandpass filter had a large DC bias at the output]

It is probably caused by the offset voltage of the op amp.

Note that the Y-axis of oscilloscope A channel in the previous picture and the next picture differs by 10 times.

戒骄戒躁

maychang posted on 2023-11-6 19:29 [It is found that when the input signal is very small, the output of the bandpass filter has a large DC offset] It is estimated that it is caused by the offset voltage of the op amp. Note the previous...

Because the amplitude of the two input signals differs by ten times, if the Y-axis coordinates are the same, it may not be clear.

maychang

Jie Jiao Jie Zao published on 2023-11-6 20:13 Because the amplitude of the two input signals differs by ten times, if the Y-axis coordinates are the same, it may not be clear

The Y axis of the previous picture is 50mV/div, and the Y axis of the next picture is 5mV/div. The output offset of the next picture is about one and a half grids, that is, 7.5mV. 7.5mV is 0.15 grids in the previous picture. Look carefully, is there a 0.15 grid offset in the red waveform in the previous picture?

maychang

Jie Jiao Jie Zao published on 2023-11-6 20:13 Because the amplitude of the two input signals differs by ten times, if the Y-axis coordinates are the same, it may not be clear

The offset of the red waveform in the first and second pictures is roughly the same. So I guess it is caused by the op amp offset voltage. And the op amp offset voltage is about 7.5mV. The voltage drop of the bias current on R5 is not considered here.

戒骄戒躁

maychang posted on 2023-11-6 20:29 The offset of the red waveform in the previous picture and the next picture is roughly the same. So I guess it is caused by the offset voltage of the op amp. And the offset voltage of the op amp is large...

Thank you, brother. Both pictures do have offsets, but when the input amplitude is large, the offset is not easy to see. When the input is small, it is very clear. I have learned something new.

maychang

Jie Jiao Jie Zao posted on 2023-11-6 21:48 Thank you, brother, these two pictures do have offsets, but when the input amplitude is relatively large, the offset is not easy to see, and when the input is relatively small...

[When the input amplitude is large, the offset is not easy to see, but when the input is small, it is very clear]

By analogy, it should be easiest to see when the input amplitude is zero. Since this is a simulation, it is easy to input zero, so why not give it a try.

戒骄戒躁

maychang posted on 2023-11-7 07:23 [When the input amplitude is large, the offset is not easy to see, and when the input is small, it is very clear] By analogy, the input amplitude...

Yes, when the input is zero, the output does have a bias of about 7.5mV, which is caused by the op amp itself. You are awesome.