Active filter circuit

An active filter circuit comprising:

• a voltage-current converting circuit having a variable gain and having a positive input end, a negative input end and an output end;
• a first input terminal connected to said positive input end;
• a buffer circuit having an input end connected to said output end of said voltage-current converting circuit, and an output end;
• a first capacitor having a first end connected to said input end of said buffer circuit and a second end, and having a first capacity;
• a second input terminal connected to said second end of said first capacitor;
• an output terminal connected in common to said output end of said buffer circuit and said negative input end of said voltage-current converting circuit;
• said voltage-current converting circuit including a first current source with two output ends leading first and second currents, a second current source producing a third current, and a pair of first internal resistors having a first resistance value;
• said first current source including a first compensating current source circuit and a second compensating current source circuit;
• said first compensating current source circuit including a second capacitor with a second capacity to produce a first compensating current in proportion to said second capacity;
• said second compensating current source circuit including a second internal resistor with a second resistance value to produce a second compensating current in inverse proportion to said second resistance value;
• said first current being in proportion to said first compensating current;
• said second current being in proportion to a difference between a current in proportion to said first compensating current and a current in proportion to said second compensating current;
• said second current source including a third internal resistor with a third resistance value to produce a third current which is in proportion to an inverse number of said third resistance value and in proportion to said ambient temperature converted into an absolute temperature;
• an active load;
• a differential amplifier connected to said active load;
• and
• first and second current drive transistors;
• said active load including first and second transistors and a third current source;
• said first transistor having a control electrode connected to a first one of said output ends to lead said first current in said first current source, a first current electrode connected to said first potential point and a second current electrode;
• said second transistor having a control electrode connect to a second one of said output ends to lead said second current in said first current source, a first current electrode connected through said fourth current source to said first potential point and connected to said output end of said voltage-current converting circuit, and a second current electrode;
• said third current source producing a dc component of the current lead by said second current electrode in said second transistor;
• said differential amplifier including third and fourth transistors;
• said third transistor having a control electrode connected to said positive input end, a first current electrode connected to said first potential point, and a second current electrode connected through a first one of said first internal resistors to a first end of said second current source;
• said fourth transistor having a control electrode connected to said negative input end, a first current electrode connected in common to said second current electrodes of said first and second transistors, and a second current electrode connected through a second one of said first internal resistors to said first end of said second current source;
• said first current drive transistor having a control electrode and a first current electrode connected with each other, and a second current electrode connected to said first output end of said first current source to lead said first current;
• said second current drive transistor having a control electrode and a first current electrode connected with each other, and a second current electrode connected to said second output end of said first current source to lead said first current;
• and
• said second current source having a second end connected to said second potential point.