Patent ID: 7683703

Claim:
An active filter comprising: a first stage ( 10 ) provided with: a first operational amplifier ( 11 ) having an inverting input ( 11 a ), a noninverting input ( 11 b ) and an output ( 11 c ); a resistor ( 12 ) having a first end ( 12 a ) connected with the inverting input ( 11 a ) of said first operational amplifier ( 11 ) and a second end ( 12 b ) set to receive an input signal (Vs); a second resistor ( 13 ) having a first end ( 13 a ) connected to the inverting input ( 11 a ) of said first operational amplifier ( 11 ) and a second end ( 13 b ) connected with the output ( 11 c ) of said first operational amplifier ( 11 ); a second stage ( 20 ) provided with: a second operational amplifier ( 21 ), having an inverting input ( 21 a ), a noninverting input ( 21 b ) and an output ( 21 c ); a resistor ( 22 ) having a first end ( 22 a ) connected with the inverting input ( 21 a ) of said second operational amplifier ( 21 ) and a second end ( 22 b ) connected to the output ( 11 c ) of said first operational amplifier ( 11 ); feedback means ( 23 ) having a first end ( 23 a ) connected to the inverting input ( 21 a ) of said second operational amplifier ( 21 ) and a second end ( 23 b ) connected to the output ( 21 c ) of said second operational amplifier ( 21 ); a third stage ( 30 ) provided with: a third operational amplifier ( 31 ) having an inverting input ( 31 a ), a noninverting input ( 31 b ) and an output ( 31 c ); a resistor ( 32 ) having a first end ( 32 a ) connected to the inverting input ( 31 a ) of said third operational amplifier ( 31 ) and a second end ( 32 b ) connected to the output ( 21 c ) of said second operational amplifier ( 21 ); feedback means ( 33 ) having a first end ( 33 a ) connected to the inverting input ( 31 a ) of said third operational amplifier ( 31 ), and a second end ( 33 b ) connected to the output ( 31 c ) of said third operational amplifier ( 31 ); a main feedback branch ( 50 ) defined by a resistor ( 51 ) and having a first end ( 50 a ) connected to the output ( 310 ) of said third operational amplifier ( 31 ), and a second end ( 50 b ) connected to the inverting input ( 11 a ) of said first operational amplifier ( 11 ), further comprising a fourth stage ( 40 ) provided with: a fourth operational amplifier ( 41 ) having an inverting input ( 41 a ), a noninverting input ( 41 b ) and an output ( 41 c ), said inverting input ( 41 a ) being connected to the noninverting input ( 11 b , 21 b or 31 b ) of one of said first, second and third operational amplifiers ( 11 , 21 , 31 ), either directly or through a resistor; a first resistor ( 42 ) having a first end ( 42 a ) connected to the inverting input ( 41 a ) of said fourth operational amplifier ( 41 ) and a second end ( 42 b ); a second resistor ( 43 ) having a first end ( 43 a ) connected to the inverting input ( 41 a ) of said fourth operational amplifier ( 41 ) and a second end ( 43 b ) connected to the output ( 41 c ) of said fourth operational amplifier ( 41 ).