FILTER HAVING MINIMIZED CUT-OFF FREQUENCY VARIATIONS

摘要

A filter (10) having a semiconductor chip (12) and a low pass filter section (14). The low pass filter section (14) includes a filter transconductor (18, 20) and a filter capacitor (Cf). The low pass filter section (14) has a cut-off frequency related to the transconductance, (gmf), of the filter transconductor (18, 20) and the capacitance of the filter capacitor (Cf). The gm control circuit (22) includes a first oscillator (26) for producing a reference frequency. The first oscillator (26) has a portion thereof on the chip and an off-chip capacitor (Cext) and off-chip resistor (Rext). A second oscillator (28) has an on-chip capacitor (Cint). A control circuit transconductor (24) is fed by the control circuit transconductor control signal to vary the transconductance (gmc) of the control circuit transconductor in accordance with the control circuit transconductor control signal, correspondingly changing the transconductance of the filter transconductor (18, 20) and hence the cut-off frequency of the low pass filter section (14).