I can only understand a small part of a bandpass filter circuit. I would like to ask the experts to give me some advice and pointers.

wlwywlt

I can only understand a small part of a bandpass filter circuit. I would like to ask the experts to give me some advice and pointers. [Copy link]

 

Question: 1. I understand the first part a little bit, but I have no idea what the second part is for, and I have no idea what the third part is for. 2. The function that this circuit needs to achieve is a bandpass filter from tens to hundreds of Hz. 3. I have simulated it with Multisim, and the result is a bandpass filter of about 5Hz-70Hz. 4. Since you already know the result, why are you asking? Because I don’t understand the principle of this, and I want to modify the filter characteristics. How should I change it? 5. What books do you recommend about active filtering? Thank you all!

maychang

"I have no idea what the second part is for, and I have no idea what the third part is for either." We can only analyze the function of the electrical schematic, and as for what the original author "wanted to do," we can only guess.

maychang

"I have no idea what to do with Part 2" There are two diodes between the output and the inverting input of Part 2. First note: the non-inverting input is connected to Vref = 1V. When the input is in the positive half cycle, the output is negative, because the two input terminals are "virtually shorted", the inverting input terminal is also 1V, and the output terminal is 1V minus the tube voltage drop of the upper diode. When the input is in the negative half cycle, the output is positive, the lower diode is turned on, the two input terminals are "virtually shorted", the inverting input terminal is 1V, and the output terminal is the input voltage (negative value) multiplied by -2.4 times plus the tube voltage drop of the lower diode. If the positive and negative half cycles of the input signal are symmetrical, the output of Part 2 will become asymmetrical in addition to being inverted.

maychang

"I don't know what the third part is for." In the third part, the upper op amp is a voltage follower. Its output is half after the voltage is divided by R62R63, and then output by the voltage follower formed by the lower op amp. In other words, the AC potential at the lower end of the two capacitors C26C27 is always half of the AC potential of the 3rd pin of U11A. This is positive feedback (two voltage followers, both in phase), but because the feedback amount is only half, it will not produce oscillation, nor will it be "locked". It will only make the capacitance of the capacitor smaller when viewed from the upper end of C27 (the AC current flowing through C27 is half of that when the lower end of C27 is connected to Vref). As for "what it is for", we can only guess. In my opinion, it is a randomly built circuit.

maychang

"2. The function of this circuit is to achieve a bandpass filter from tens to hundreds of Hz." To achieve bandpass filtering, four op amps can form a very good high-order bandpass active filter, so there is no need to go through so much trouble.

maychang

"What books do you recommend about active filtering?" I recommend the Analog Filter and Circuit Design Handbook. Look for it in the materials uploaded by Laotyw. This book is very comprehensive, but you may find that it is too long (490 pages) and you cannot quickly find the circuit you want.

wlwywlt

maychang posted on 2018-7-26 12:10 "What books do you recommend about active filtering?" I recommend "Analog Filter and Circuit Design Handbook". Go to the materials uploaded by Laotyw...

Thank you for your guidance. I still need to think more about what you said. The original intention of this circuit is to extract breathing and snoring signals. We "borrowed" the board from our friends. I don't know if it is correct. When I took it and saw this part of the circuit, I was really confused. However, I have simulated this circuit with Multisim. From the Bode diagram, it can be seen that it is a bandpass. When I analyzed the first level, I felt very awkward. And even more so later. It is undeniable that I am really an amateur in active filtering. In addition, I am also a newcomer to this forum, so I would like to ask who is the old tyw? What is the specific ID? I want to find this book as soon as possible to study it. Thank you very much!

maychang

wlwywlt posted on 2018-7-26 14:13 Thank you for your guidance. I still need to think more about what you said. The original intention of this circuit is to extract breathing and snoring signals. It is our "reference" to friends...

模拟滤波器与电路设计手册 490_ 74.9M 高清书签版.part1.rar (30 MB, downloads: 469)

2018-7-26 14:39 上传

点击文件名下载附件

模拟滤波器与电路设计手册 490_ 74.9M 高清书签版.part2.rar (20.92 MB, downloads: 454)

2018-7-26 14:40 上传

点击文件名下载附件

maychang

wlwywlt posted on 2018-7-26 14:13 Thank you for your guidance. I still need to think more about what you said. The original intention of this circuit is to extract the breathing and snoring signals. It is our "reference" to friends...

May I ask who is the old tyw? That is tyw.

wlwywlt

maychang posted on 2018-7-26 14:42 May I ask who is the old tyw? It is tyw.

3Q

maychang

wlwywlt posted on 2018-7-26 14:55 3Q

The compressed file I posted was downloaded from old tyw, and there is tyw's watermark in it. Thanks to old tyw.

maychang

wlwywlt posted on 2018-7-26 14:55 3Q

In addition, you can search for "gmchen" in this section. Teacher gmchen has posted seven posts on active filters in the simulation section. You should take a closer look.

wlwywlt

Maychang posted on 2018-7-26 15:13 In addition, you can search for "gmchen" in this forum. Teacher gmchen has posted seven posts on active filters in the simulation section. You should take a closer look...

Well, thank you maychang for leading me into the world of filtering, and thank you tyw, gmchen.

sky9218

Downloaded it to learn. Thank you.

gmchen

I'll try to analyze it. In my opinion, the main function of this circuit is not bandpass filtering. Let's analyze it one by one.

The first part is indeed a bandpass filter.

C23 and R54 form a first-order high-pass filter with a cutoff frequency of 16Hz.

C24 and R56 form a first-order low-pass filter with a cut-off frequency of 80Hz.

R55 is connected in series with a large capacitor, so it has little effect on the frequency response, but that capacitor ensures that the DC gain of the amplifier is 1.

gmchen

The second part is a precision half-wave rectifier circuit.

Those two diodes are Schottky diodes. You can look up information about the principles of precision rectifier circuits yourself.

Note that C25 and R57 form a first-order high-pass filter with a cutoff frequency of 72Hz. Combined with the low-pass filter cutoff frequency of 80Hz in the first part, the signal that can actually enter the rectification of this circuit is only between 72Hz and 80Hz.

gmchen

The third part is a second-order low-pass filter.

Its voltage transfer function is 1/[(1+sRC)(1+sRC/2)], where RC=1M*68n, so the cutoff frequency is about 2Hz.

Therefore, the function of this circuit is to extract the DC component of the signal rectified by the second part of the circuit.

gmchen

To sum up, the function of this circuit should be:

Detect the DC component of the input signal at about 70~80Hz.

gmchen

I just realized after I finished writing this that this was a post from a few years ago