Modular low voltage filter with common mode feedback

A filter comprising:

• a first transconductor including a first input, a second input, a first output, and a second output, said first and second inputs receiving an input differential voltage for filtering;
• a first amplifier including a first input coupled to said first input of said first transconductor, a second input coupled to said second input of said first transconductor, a first output, and a second output;
• a first capacitor coupled between said first output of said first transconductor and said first output of said first amplifier;
• a second capacitor coupled between said second output of said first transconductor and said second output of said first amplifier;
• a first common mode amplifier including a first input coupled to said first output of said first transconductor, a second input coupled to said second output of said first transconductor, a third input for receiving a common mode reference voltage, and an output;
• a third capacitor coupled between said first output of said first transconductor and said output of said first common mode amplifier;
• a fourth capacitor coupled between said second output of said first transconductor and said output of said first common mode amplifier;
• a second transconductor including a first input coupled to said second output of said first transconductor, a second input coupled to said first output of said first transconductor, a first output coupled to said first output of said first transconductor, and a second output coupled to said second output of first transconductor;
• and
• a first load circuit including a first load terminal coupled to said first output of said first transconductor, a second load terminal coupled to said second output of said first transconductor, and a control input coupled to said output of said first common mode amplifier.

A filter comprising:

• a first transconductor including a first input, a second input, a first output, and a second output, said first and second inputs receiving an input differential voltage for filtering;
• a first amplifier including a first input coupled to said first input of said first transconductor, a second input coupled to said second input of said first transconductor, a first output, and a second output;
• a first capacitor coupled between said first output of said first transconductor and said first output of said first amplifier;
• a second capacitor coupled between said second output of said first transconductor and said second output of said first amplifier;
• a second transconductor including a first input, a second input, a first output coupled to said first output of said first transconductor, and a second output coupled to said second output of first transconductor;
• a first buffer for providing increased drive capability, said first buffer having a first input coupled to said first output of said first transconductor, a second input coupled to said second output of said first transconductor, a first output, and a second output;
• a first common mode amplifier including a first input coupled to said first output of said first buffer, a second input coupled to said second output of said first buffer, a third input for receiving a common mode reference voltage, and an output;
• a third capacitor coupled between said first output of said first transconductor and said output of said first common mode amplifier;
• a fourth capacitor coupled between said second output of said first transconductor and said output of said first common mode amplifier;
• a first load circuit including a first load terminal coupled to said first output of said first transconductor, a second load terminal coupled to said second output of said first transconductor, and a control input coupled to said output of said first common mode amplifier;
• a third transconductor including a first input coupled to said first output of said first buffer, a second input coupled to said second output of said first buffer, a first output, and a second output wherein said first and second inputs of said second transconductor respectively couples to said second and first outputs of said third transconductor;
• a second amplifier including a first input coupled to said first input of said first transconductor, a second input coupled to said second input of said first transconductor, a first output, and a second output;
• a fifth capacitor coupled between said first output of said second amplifier and said first output of said third transconductor;
• a sixth capacitor coupled between said second output of said second amplifier and said second output of said third transconductor;
• a second common mode amplifier including a first input coupled to said first output of said third transconductor, a second input coupled to said second output of said third transconductor, a third input receiving said common mode reference voltage, and an output;
• a seventh capacitor coupled between said first output of said third transconductor and said output of said second common mode amplifier;
• an eighth capacitor coupled between said second output of said third transconductor and said output of said second common mode amplifier;
• a fourth transconductor including a first input coupled to said second output of said third transconductor, a second input coupled to said first output of said third transconductor, a first output coupled to said first output of said third transconductor, and a second output coupled to said second output of said third transconductor;
• and
• a second load circuit including a first load terminal coupled to said first output of said third transconductor, a second load terminal coupled to said second output of said third transconductor, and a control input coupled to said output of said second common mode amplifier.

A biquadratic filter utilizing common mode feedback, the biquadratic filter filtering an applied input differential signal and providing a filtered output differential signal, the biquadratic filter comprising:

• a first load circuit having a first terminal and a second terminal for summing signals applied thereto, loading at said first and second terminals of said first load circuit being responsive to a first control signal;
• a first transconductor of a first transconductance responsive to the input differential signal and providing a differential signal to said first and second terminals of said first load circuit;
• a first amplifier of a first gain being capacitively coupled to said first and second terminals of said first load circuit, said first amplifier being responsive to the input differential signal and providing a differential signal to said first and second terminals of said first load circuit;
• a first common mode amplifier capacitively coupled to first and second terminals of said first load circuit for providing common mode feedback, said first common mode amplifier being responsive to a voltage across said first and second terminals of said first load circuit and a common mode reference voltage, said first common mode amplifier providing said first control signal for varying loading at said first and second terminals of said first load circuit;
• a second transconductor of a second transconductance responsive to said voltage across said first and second terminals of said first load circuit, said second transconductor providing a differential signal to said first and second terminals of said first load circuit;
• a second load circuit having a first terminal and a second terminal for summing signals applied thereto, loading at said first and second terminals of said second load circuit being responsive to a second control signal, said first and second terminals of said second load circuit providing the filtered output differential signal;
• a third transconductor of a third transconductance responsive to said voltage across first and second terminals of said first load circuit and providing a differential signal to said first and second terminals of said second load circuit;
• a second amplifier of a second gain being capacitively coupled to said first and second terminals of said second load circuit, said second amplifier being responsive to the input differential signal and providing a differential signal to said first and second terminals of said second load circuit;
• a second common mode amplifier capacitively coupled to said first and second terminals of said second load circuit for providing common mode feedback, said second common mode amplifier being responsive to the filtered output differential signal and said common mode reference voltage, said second common mode amplifier for providing said second control signal to said second load circuit for varying loading at said first and second terminals of said second load circuit;
• and a fourth transconductor of a fourth transconductance responsive to the filtered output differential signal, said fourth transconductor providing a differential signal to said first and second terminals of said second load circuit.