The invention relates to a circuit forming an active RC filter to be used as a band-stop filter in the high and very-high frequency domains, having a second order transfer function, comprising a circuit that includes a building block for performing a filtering and an amplifying function, and a building block for performing the summation V.sub.S of the circuit output signal and the signal V.sub.E applied to the input of the circuit.
The invention is applied to the realisation of integrated circuits forming filters, in the high and very-high frequency domains, that can be used, for example in one embodiment, as band-stop filters in frequency doublers for rejecting the unwanted signal at the fundamental frequency or, in another embodiment, as a band-stop filter having differential inputs and outputs; or, in yet another embodiment, be particularly suitable for a fine adjustment of the rejection.
An active all-pass filter is already known from the prior art from the publication by J. TOW, in "IEEE Spectrum, December 1969, pp. 103-107", entitled "A Step-by-Step Active-filter Design".
This publication describes, among other things, an all-pass filter realisation having a second order transfer function, which filter is shown in FIG. 8 of the said document. This circuit comprises three operational amplifiers arranged in series and is looped back. A fourth operational amplifier performs the summation of the output signal of the first operational amplifier and the input signal of the filter circuit
The operational amplifiers that are used for constituting this circuit have a gain which is most certainly infinite at low frequencies, but which becomes very low at high and very-high frequencies. Therefore, the circuit known from the said document has the disadvantage of not being suitable for use in these frequency domains.
Furthermore, the looped back circuit has the disadvantage to cause the circuit to oscillate in certain conditions.
FIG. 6 of the said document also shows an active band-pass filter that has similar characteristic features and thus exactly the same disadvantages.
The circuits known from the said document further have the following disadvantages: on the one hand, they are formed by a large number of transistors, which:
causes manufacturing to be costly in many cases
requires a large surface and is disadvantageous for use in integrated circuits,
entails high consumption and is also disadvantageous for the use as mentioned above.
On the other hand, their characteristic frequency is not adjustable.
Finally, the capacitors used in this circuit have considerable dimensions and render this circuit not integrable
Therefore, the present invention has for its object to provide an active filter circuit that allows to get rid of these disadvantages and that specifically:
can operate at high or very-high frequencies;
and in the case of the use at very-high frequencies is easy to integrate, requires a small surface and has little consumption;
exhibits characteristic features so that the rejection frequency and the rejection are adjustable and easy to control;
can admit a differential input signal and a differential output signal.