Mr. Tang's lecture on operational amplifier (Lecture 7) -- Application of operational amplifier

Two 、 Active power filter

3、 ... and 、 passive filter

Four 、 First order low-pass active filter

5、 ... and 、 Second order low pass filter

It drops faster than the first-order stopband

Application, for example, （ Be careful ： Twice at the filter RC It doesn't have to be equal ）

Amplitude frequency characteristics of second-order low-pass filter ：

solve Sallen-Key Three methods of high frequency feed through of low pass filter ：

Sallen-Key Low pass filter high frequency feed through It refers to the phenomenon that high-frequency signals can pass through the second-order low-pass filter ： For example, the cut-off frequency of the designed low-pass filter is 28Hz, The result can pass 28KHz Noise interference , There are several solutions ：

Another method is shown in the figure below , Add an op amp as a buffer .

Actual design case

6、 ... and 、 Second order high pass filter

The second-order low-pass RC Exchange positions to get a second-order high pass .

Calculation of amplification factor and cut-off frequency of second-order low high pass filter

7、 ... and 、 Bandpass filter

The circuit in the upper right of the figure below is suitable for bandpass filters with narrow passband , If the passband is large , High pass before low pass design is required

The following figure shows the design of bandpass filter with low pass first and then high pass , The design is suitable for the case of large passband .

The other is high Q Value of the bandpass filter DAPF

Bandpass filter scheme

The ECG signal goes through AD620 After zooming in , First, go through a high pass , The cut-off frequency is 0.05Hz, And then go through a low-pass , The cut-off frequency is 100Hz

8、 ... and 、 Band stop filter ( Notch filter )

General design 50Hz perhaps 60Hz Band stop filter , In this way, the power frequency interference can be removed .

Design example of band stop filter
When do I need to filter 50Hz( China ) perhaps 60Hz( Abroad ) interfere , There are two conditions ：
One is 50Hz perhaps 60Hz The signal is in the useful frequency band
Second, the system uses the mains power for power supply , If it is powered by dry battery , There is no need to filter .

Nine 、 All pass filter

Ten 、 Selection of passive components in active power filter

The principle of frequency adjustment ： Fix the capacitor first , Then a potentiometer is used to replace a certain resistance , Adjust the center frequency by rotating the potentiometer , When the frequency value reaches the set value , Fix the potentiometer with glue .
Capacitors are recommended NPO capacitance ：NPO The capacitance between them is good , But it's big

Active filter GBW And swing rate selection ：