Practical application of operational amplifiers. Part two. Encyclopedia of radio electronics and electrical engineering

Encyclopedia of radio electronics and electrical engineering / Beginner radio amateur

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So, in the first part, we considered the circuits for switching on the op-amp as amplifiers, in this part we will consider the inclusion of the op-amp as filters.

High Pass Filter (HPF, High-Pass - whatever)

It is required to cut off a signal whose frequency is below a certain threshold, which, by the way, is called the cutoff frequency.

The simplest HPF looks like this:

The first circuit with a non-inverting inclusion of the op-amp, the second - with an inverting one.

This is a first-order filter with attenuation of an unnecessary signal - a steepness - 6dB per octave. You can determine the cutoff frequency by calculating the reactance of the capacitor. When it becomes equal to the resistance of the resistor connected in series with the capacitor, this will be the very thing.

The formula is:

Where F is the frequency in Hertz, C is the capacitance in Farads, Ec is the resistance in Ohms.

If the slope of the first order filter seems to be insufficient, you can fix the second order filter - with a slope of 12 dB per octave as shown in the figure.

This is the so-called Butterworth filter. Named so, apparently, in honor of the dude who invented it.

To calculate its cutoff frequency, you can use the following relationships:

R1=R2; С1=2С2;

When choosing resistors, it should be taken into account that their values ​​should be in the range of 10-100 kOhm, since the output resistance of the filter increases with the frequency, and if the values ​​of the resistors go beyond the above limits, this may affect the operation of the filter. Negatively, of course - otherwise why warn?

Low Pass Filter (LPF, Low-Pass - whatever)

The operation of this filter is directly opposite to the previous one - it cuts off the signal, the frequency of which is higher than the cutoff frequency. In principle, everything is the same as in the previous case, only the capacitor is not connected in series with the resistor, but in parallel with it.

The first circuit is a non-inverting inclusion, the second is an inverting one. The cutoff frequency is calculated in exactly the same way as in the case of the HPF.

Well, the second-order filter circuit is the same Butterworth citizen.

Again - everything is considered exactly the same as described above.

Bandpass Filter (Band pass)

A bandpass filter is used when it is necessary to isolate a certain frequency band from the entire spectrum. For example, in spectrum analyzers or something like that.

I will not give the calculation formulas here - they are hefty. To calculate the bandpass filter, I advise you to use the wonderful program - Filter Wiz Pro from Schematica Software. However, it can also be used to calculate any other filters.

Plug filter (Notch Filter)

If you need to attenuate (almost to zero) some selected frequency, then this filter is just for you.

The calculation formula is this:

where R=R3=R4, C=C1=C2;

When constructing this filter, the accuracy of the component ratings is very important - the degree of "killing" the selected frequency depends on this. So, when using resistors and capacitors with a tolerance of 1%, you can get a frequency attenuation of up to 45dB, although, theoretically, you can achieve 60dB. For example, if you want to bang the hated frequency of 50Hz, then we take the following values: R1=R2=10kOhm, R3=R4=68kOhm, C1=C2=47nF.

Filter plug with double T-bridge

Using this filter, you can not only attenuate the selected frequency, but also adjust the degree of its attenuation with a variable resistor R4. The formula for calculating the denominations is the same as in the previous case.

Publication: radiokot.ru

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