Tow-Thomas biquad filter

[liletian]

Hi all

Does anyone know why Tow-Thomas biquad casode two op-am and make R as a feedback? What is the point there?

For example, why can not make it two seperate first order filter cascade together to get a second order filter?

Does any have explanation here?

Thanks a lot

 

[Audioguru]

The feedback from the resistor allows the output of the first integrator to be a bandpass and the value of feedback resistor allows adjustment of the Q.

 

[liletian]

The feedback from the resistor allows the output of the first integrator to be a bandpass and the value of feedback resistor allows adjustment of the Q.

Yes, it makes sense, since for DC due to the integrator, it will be stable in the DC.

How to calibrate the stability of the filter? There will have two op-am which has to be stable. How about the big loop stability?

Are there other benefits? Is there a good tutorial here?

Thank you.

 

[LvW]

The Tow-Thomas-Biquad is one of the popular state-variable filter topologies.
The benefit of these structures is the availability of more than one filter functions at the same time.
The Tow-Thomas filter contains three blocks: (1) A damped integrator, (2) an undamped integrator and (3) an inverter.
It provides three ouputs: At (1) a bandpass function, at (2) a lowpass function and at (3) an inverted lowpass function.

The basic function is as follows: Without the damping feedback resistor at (1) the loop of these three blocks will constitute an oscillator (two inverting intergators with a phase shift of -180deg at the desired oscillation frequency). That means: It is the purpose of the damping resistor to inhibit oscillations and to reduce the Q-factor from infinity (oscillation) to the desired lower value.

This filter topology is extensively described in all relevant textbooks and in many filter tutorials which are available in the Internet. Try a search.