Low power agile active filter with digitally controlled center-frequency

New voltage-mode bandpass shadow filter

“…Since its inception the concept of shadow filters has attracted the attention of many researchers [1]- [11]. This is attributed to its ability to modify the parameters of the filter; for example the center frequency, over a wide range by acting only from outside the filter and without disturbing the active and passive components of the filter itself.…”

“…The proposed shadow filter is built around the commercially available current-feedback operationalamplifier (CFOA) instead of the commercially unavailable CCCII, ZC-CITA, VDTA and CDTA active elements used in references [1]- [3], [5]- [11]. Moreover, despite working in the voltage-mode, rather than the current-mode as in [1]- [3], [5]- [11], no externallyconnected voltage-summing circuit is needed. The proposed voltage-mode shadow filter can realize bandpass response with variable center frequency, bandwidth and gain.…”

“…In [9] and [10] current-mode shadow filters using the current difference transconductance amplifier (CDTA) and the voltage difference transconductance amplifier (VDTA) of different classes are presented. Unfortunately the CCCII+, the CDTA, the VDTA and the ZC-CITA active elements used in [1]- [3] and [5]- [11] are not commercially available integrated circuits. The operational-amplifier (OA) based realization suggested in [4] can be practically implemented.…”

“…The circuit presented in [1] uses four CCCIIs and can realize a bandpass filter with variable center frequency and quality factor such that the bandwidth is kept constant. The CCCII was also used in [11] for realizing badpass shadow filters. In [2] the authors extended the idea of [1] for controlling the bandwidth and the quality factor without disturbing the center frequency.…”

Shadow filters based on DDCC

“…Although, the first shadow filters are presented as voltage-mode circuits [1], in the literature we can find that most of the realisations of shadow filters are in current-mode where the summing circuit is simply realised by a circuit's node [1][2][3][4][5][6][7][8][9][10][11]. Hence, unlike the voltage-mode shadow filters, the realisation of the current-mode ones is much easier since no extra summing circuit is required.…”