Purpose and function of MFB circuit:
MFB single operational amplifier active filter is a good topology technology of filter circuit. Compared with VCVS single operational amplifier active filter, its temperature stability is excellent, but three basic circuit parameters, namely characteristic frequency (including cutoff frequency fc, peak frequency fm or central frequency fo), Q value and gain, are controlled by two resistors, which are not independent, which brings inconvenience to design and adjustment. Bruce Carter and Ron ManCini put forward an improved Deliyannis circuit, which uses a pair of resistors Rmi=Ro/(2Q- 1) and Rma=Ro*(2Q- 1) to separate Q from the characteristic frequency fm, that is, the value of Q can be adjusted without changing fm. This idea is very clever and intriguing. But there are still some shortcomings: firstly, two resistors with different resistance values are needed; Secondly, the resistance of one of these two resistors is often relatively large, especially when the characteristic frequency is relatively low, which is often above Megaohms, so it is difficult to buy a high-precision resistor above 2 Megaohms in the market; Thirdly, it is difficult to prevent the thermal noise of a large resistor from being relatively large, which may be generated in the case of a wide frequency band.