A tunable filter circuit includes a first differential pair biased by a first
current,
a second differential pair biased by a second current, a first capacitor and a
second capacitor. The tunable filter circuit of the present invention can be configured
as a bandpass filter or a bandstop filter by connecting the input voltage signal
to different input nodes of the tunable filter circuit. The tunable filter circuit
can be tuned by adjusting the values of the first current and the second current.
In an alternate embodiment, frequency tuning is achieved either by switching capacitive
loads or changing resistive impedances introduced at the emitter of the differential
pairs, which also extends the input voltage range of the filter. This emitter resistance
is implemented using MOS switches whose on-resistance can be controlled for a precise
tuning within a large frequency range.
