Stabilization circuit and method for second order tunable active filters

An amplifier for use in an active filter having a predetermined cutoff frequency, comprising:

• a first transistor having a collector, a base and an emitter, said collector of said first transistor being coupled to a first supply voltage terminal, said base of said first transistor being coupled to receive an input voltage;
• a second transistor having a collector, a base and an emitter, said collector of said second transistor being coupled to said first supply voltage terminal, said base of said second transistor being coupled to receive a first reference voltage;
• a third transistor having a collector, a base and an emitter, said base of said third transistor being coupled to said collector of said third transistor;
• a fourth transistor having a collector, a base and an emitter, said base and collector of said fourth transistor being coupled to said emitter of said second transistor, said emitter of said fourth transistor being coupled to said emitter of said third transistor;
• a fifth transistor having a collector, a base and an emitter, said collector of said fifth transistor being coupled to an output node of the amplifier to provide an output current, said base of said fifth transistor being coupled to said collector of said third transistor;
• a sixth transistor having a collector, a base and an emitter, said collector of said sixth transistor being coupled to said first supply voltage terminal, said base of said sixth transistor being coupled to said collector of said fourth transistor, said emitter of said sixth transistor being coupled to said emitter of said fifth transistor;
• a resistor being coupled between said emitter of said first transistor and said collector of said third transistor;
• a first current mirror circuit having an input and an output, said output of said first current mirror circuit being coupled to said emitter of said third transistor to provide a first current;
• a second current mirror circuit having an input and an output, said output of said second current mirror circuit being coupled to said emitter of said fifth transistor to provide a second current;
• and
• circuit means coupled to said inputs of said first and second current mirror circuits for respectively providing third and fourth currents thereat, said third and fourth currents controlling a ratio of said first and second currents such that the cutoff frequency of the active filter is substantially independent of temperature, said third and fourth currents both being a function of a second reference voltage.