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Why you need to match active filters to the opamp

The problem

You may be familiar with the following problem: You have carefully dimensioned an active filter - either using the appropriate formulas from a book or using a filter design program. You set up your circuit to try it out and find that the cutoff frequency and/or characteristics of your (high-pass or low-pass) filter deviate from your specifications. In the case of a band-pass filter, the center frequency is next to your target value and the quality is not correct. Now replace fixed resistors with trimmers and tune your circuit while making frequency response measurements over and over again.

You will reach your goal in this way - but the way to the goal can be very long.

Have you ever wondered if this is the only and best solution?

Think carefully about what you actually did. You had to change your precisely calculated component values on the circuit you built in order to get the desired frequency response. Why did you have to do this, you did the math, didn't you? Yes, you probably did the math correctly. Then what is the cause?

The cause

All formulas that you will find in textbooks and practice books for filter design are based on an ideal amplifier, an ideal opamp. For example, an ideal opamp has a consistently high gain at all frequencies. You will not achieve this property with a real opamp. A real opamp always has a gain that decreases at higher frequencies. For example, the well-known opamp uA741 has a gain of over 10,000 at a very low frequency of 1 Hz. At 100kHz, its gain is less than 10. At these high frequencies, the phase shift between the input and output of the op-amp also changes.

Also remember that this frequency dependent gain and phase component - the real opamp - is part of your filter circuit.

It stands to reason that the real opamp, with its frequency dependent gain and phase, will more or less change the properties of your filter. Textbooks usually say that the operational amplifier's frequency-dependent gain is linearized by the negative feedback, but negative feedback does not produce an ideal amplifier either. The phase shift that increases at high frequencies cannot be compensated for by negative feedback.

The solution

Since we can't buy or build an ideal amplifier, we need to solve the problem similar to what you did: by changing component values.

There is a unique computer program for this that saves you the tedious adjustment work and outputs a dimensioning that is perfectly matched to your opamp type in just a few seconds: AktivFilter 3.3 software, with which you can dimension active high, low and bandpass filters as well as bandstops and at the same time Match your design to one of more than 300 opamp types built into AktivFilter 3.3. After each calculation, AktivFilter 3.3 shows you the frequency response of your calculated filter together with the target frequency response. A SPICE call interface that allows you to quickly check your design is also integrated into AktivFilter 3.3.

This allows you to develop your filter circuits with real operational amplifiers the way you originally wanted: enter specifications in the program, have the filter calculated, set up the filter, and you're done! For all the people who need to balance their filters, all you have to do is smile...

The program also plots the frequency response that the filter would exhibit if it were sized without considering the real operational amplifier characteristics (traditional design). This allows you to see immediately how good or bad your design would be if you didn't use AktivFilter.

As you can see, there is also a short way to the goal...

AktivFilter 3.3 software

AktivFilter 3.3 is a mature 32-bit application software for Windows XP, Vista, 7, 8, 8.1, 10, and Windows 11. You can use it to create high-pass and low-pass filters up to the 30th order of the Bessel, Butterworth and Chebyshev characteristics or with a self-defined characteristic in Design Sallen-Key structure and multiple negative feedback (double negative feedback). You can specify any ripple up to a maximum of 20 dB for the Chebyshev characteristic. You can also design bandpass filters up to 30th order with multiple negative feedback and active double-T band rejection. According to our information, AktivFilter is the only program available worldwide that is able to tune filter designs to specific operational amplifier types.

How to buy

Each full version of AktivFilter 3.3 includes more than 300 operational amplifier models built into the program. For users who want to add their own opamp models to the program, we offer the AktivFilter 3.3 Professional Edition. We deliver this edition together with the program OpAmp-Modell, with which you can quickly and easily create your own opamp models. Here you can purchase AktivFilter 3.3.