Tuneable circuit for canceling third order modulation
A CMOS transconductor for cancelling third-order intermodulation is provided. The transconductor includes a transconductance circuit and a tuneable distortion circuit. The transconductance circuit takes an input voltage and generates an output current having a transconductance element and an IM3 element. The distortion circuit takes the same input voltage and generates a current having an IM3 element of equal amplitude and opposite phase to the IM3 element of the transconductance circuit. A controller circuit tunes the distortion circuit to adjust its IM3 element to substantially equal the amplitude of the IM3 of the transconductance circuit. The distortion and transconductance circuits are arranged to sum their output currents thereby effectively cancelling the IM3 elements, leaving the transconductance relatively unmodified.
1. A CMOS transconductor for cancelling third-order intermodulation of an output current corresponding to an input voltage comprising:
a transconductance circuit for generating a first current as a function of said input voltage, said first current having a transconductance gain element and a first third-order intermodulation element, said transconductance circuit comprising a first differential cell having a first and second transistor;
a tuneable distortion circuit coupled to said transconductance circuit for generating a second current having a second third-order intermodulation element of opposite phase to said first third-order intermodulation element, said distortion circuit including a second differential cell and a current source; and
a controller circuit for tuning said current source of said distortion circuit to adjust said second third-order intermodulation element to substantially equal said first third-order intermodulation element, said distortion circuit and transconductance circuit arranged to sum said first and second currents generating said output current substantially having said transconductance gain element and substantially cancelling said first and second third-order intermodulation elements thereof.
2. The CMOS transconductor of claim 1 , wherein said second distortion circuit comprises a third and fourth transistor.
3. The CMOS transconductor of claim 1 , wherein said distortion circuit further comprises a resistance tuneable by said controller circuit.
4. The CMOS transconductor of claim 1 , wherein said controller circuit comprises digital circuitry and analog circuitry for changing the value of the said current source or said resistance.
5. The CMOS transconductor of claim 1 , wherein said first and second transistors are biased by decoupling capacitors.
6. The CMOS transconductor of claim 5 , wherein a first resistive load biases input of said transconductance circuit and a second resistive load biases inputs of said distortion circuit.
7. A CMOS mixer comprising:
a transconductance circuit for generating a first current as a function of said input voltage, said first current having a transconductance gain element and a first third-order intermodulation element in the third-order, said transconductance circuit comprising a first differential cell having a first and second transistor;
a tuneable distortion circuit coupled to said transconductance circuit for generating a second current having a second third-order intermodulation element of opposite phase to said first third-order intermodulation element, said distortion circuit including a second differential cell and a current source;
a controller circuit for tuning said current source of said distortion circuit to adjust said second third-order intermodulation element to substantially equal said first third-order intermodulation element in amplitude, said distortion circuit and transconductance circuit arranged to sum said first and second currents generating said output current substantially having said transconductance gain element and substantially cancelling said first and second third-order intermodulation elements thereof; and
a switching circuit coupled to outputs of said transconductance and distortion circuits, for switching said output current, said switched output current converted to an output voltage by an active resistive load coupled to said switching circuit.
8. The CMOS mixer of claim 7 , wherein said second differential cell comprises a third and fourth transistor.
9. The CMOS mixer of claim 8 , wherein said distortion circuit further comprises a resistance load tuneable by said controller circuit.
10. The CMOS mixer of claim 8 , wherein said controller circuit comprises digital circuitry and analog circuitry for changing the value of the said current source or said resistance.
11. A CMOS amplifier comprising:
a transconductance circuit for generating a first current as a function of said input voltage, said first current having a transconductance gain element and a first third-order intermodulation element in the third-order, said transconductance circuit comprising a first differential cell having a first and second transistor;
a tuneable distortion circuit coupled to said transconductance circuit for generating a second current having a second third-order intermodulation element of opposite phase to said first third-order intermodulation element, said distortion circuit including a second differential cell and a current source;
a controller circuit for tuning said current source of said distortion circuit to adjust said second third-order intermodulation element to substantially equal said first third-order intermodulation element in amplitude, said distortion circuit and transconductance circuit arranged to sum said first and second currents generating said output current substantially having said transconductance gain element and substantially cancelling said first and second third-order intermodulation elements thereof; and
active resistive loads coupled to outputs of said transconductance and distortion circuits, said active resistive loads for converting said output current to an output voltage.
12. The CMOS amplifier of claim 11 , wherein said distortion circuit further comprises a current source tuneable by said controller circuit.
13. The CMOS transconductor of claim 11 , wherein said controller circuit comprises digital circuitry and analog circuitry for changing the value of the said current source or said resistance.
14. A CMOS transconductor for cancelling third-order intermodulation of an output current corresponding to an input voltage comprising:
a transconductance circuit for generating a first current as a function of said input voltage, said first current following a first current path and having a transconductance gain element and a first third-order intermodulation element, said transconductance circuit comprising a first differential cell having a first and second transistor;
a tuneable distortion circuit coupled to said transconductance circuit for generating a second current having a second third-order intermodulation element of opposite phase to said first third-order intermodulation element, the second current following a second current path decoupled from the first current path, said distortion circuit including a second differential cell and a current source; and
a controller circuit for tuning said current source of said distortion circuit to adjust said second third-order intermodulation element to substantially equal said first third-order intermodulation element, said distortion circuit and transconductance circuit arranged to sum said first and second currents generating said output current substantially having said transconductance gain element and substantially cancelling said first and second third-order intermodulation elements thereof.
15. A CMOS amplifier comprising:
a transconductance circuit for generating a first current as a function of said input voltage, said first current having a transconductance gain element and a first third-order intermodulation element in the third-order, said transconductance circuit comprising a first differential cell having a first and second transistor;
a tuneable distortion circuit coupled to said transconductance circuit for generating a second current having a second third-order intermodulation element of opposite phase to said first third-order intermodulation element, said distortion circuit including a second differential cell and a resistance load;
a controller circuit for tuning said resistance load of said distortion circuit to adjust said second third-order intermodulation element to substantially equal said first third-order intermodulation element in amplitude, said distortion circuit and transconductance circuit arranged to sum said first and second currents generating said output current substantially having said transconductance gain element and substantially cancelling said first and second third-order intermodulation elements thereof; and
active resistive loads coupled to outputs of said transconductance and distortion circuits, said active resistive loads for converting said output current to an output voltage.