Low-noise amplifier (LNA) transformer notch

1. A circuit for signal processing, comprising:

a first transformer having a first inductive element magnetically coupled with a second inductive element;

a second transformer having a third inductive element magnetically coupled with a fourth inductive element, wherein the first inductive element is coupled in series with the third inductive element; and

a capacitive element coupled in parallel with the fourth inductive element, the capacitive element and the fourth inductive element forming a notch circuit, the notch circuit coupled in series with the second inductive element.

2. The circuit of claim 1 , wherein:

the second transformer is effectively shorted with respect to a first frequency; and

the first transformer and the second transformer are operational with respect to a second frequency, the first frequency being higher than the second frequency.

3. The circuit of claim 1 , wherein an equivalent impedance of the capacitive element in parallel with the fourth inductive element has higher impedance with respect to a first frequency as compared to the equivalent impedance with respect to a second frequency that is lower than the first frequency.

4. The circuit of claim 1 , wherein:

the first inductive element is a primary winding of the first transformer;

the second inductive element is a secondary winding of the first transformer;

the third inductive element is a primary winding of the second transformer; and

the fourth inductive element is a secondary winding of the second transformer.

5. The circuit of claim 1 , wherein the capacitive element has a variable capacitance.

6. The circuit of claim 5 , wherein the capacitive element comprises a plurality of capacitive elements, each coupled in series with a switch.

7. The circuit of claim 1 , further comprising a mixer coupled to the second inductive element and the notch circuit.

8. The circuit of claim 7 , wherein the mixer is configured to receive a local oscillator (LO) signal, wherein a resonant frequency of the notch circuit comprises three times a frequency of the LO signal.

9. The circuit of claim 1 , further comprising a low-noise amplifier (LNA) having an output coupled to the first inductive element.

10. The circuit of claim 1 , wherein a turn ratio of the first transformer is same as a turn ratio of the second transformer.

11. A wireless device comprising the circuit of claim 1 , the wireless device comprising:

at least one antenna; and

a receiver coupled to the at least one antenna, the receiver comprising the circuit for signal processing.

12. A circuit for signal processing, comprising:

a first inductive element magnetically coupled to a second inductive element, wherein the first inductive element is a primary winding of a first transformer, wherein the second inductive element is a secondary winding of the first transformer;

a third inductive element magnetically coupled to a fourth inductive element, wherein the third inductive element is a primary winding of a second transformer, wherein the fourth inductive element is a secondary winding of the second transformer;

wherein the first inductive element is coupled in series with the third inductive element;

a first terminal of the first inductive element is coupled to a first terminal of the third inductive element, a second terminal of the first inductive element is coupled to an input node of the circuit, and a second terminal of the third inductive element is coupled to a reference potential node;

a first terminal of the second inductive element is coupled to a first terminal of the fourth inductive element, a second terminal of the second inductive element is coupled to a first output node of the circuit, and a second terminal of the fourth inductive element is coupled to a second output node of the circuit; and

a capacitive element coupled between the first terminal and the second terminal of the fourth inductive element.

13. The circuit of claim 12 , wherein:

the second transformer is effectively shorted with respect to a first frequency, the first transformer and the second transformer being operational with respect to a second frequency, and the first frequency being higher than the second frequency.

14. The circuit of claim 12 , wherein the capacitive element and the fourth inductive element form a notch circuit, wherein an equivalent impedance of the notch circuit has higher impedance with respect to a first frequency as compared to the equivalent impedance with respect to a second frequency that is lower than the first frequency.

15. The circuit of claim 12 , wherein a turn ratio of the first transformer is same as a turn ratio of the second transformer.

16. The circuit of claim 12 , wherein the capacitive element has a variable capacitance.

17. The circuit of claim 16 , wherein the capacitive element comprises a plurality of capacitive elements, each coupled in series with a switch.

18. The circuit of claim 12 , further comprising a mixer coupled to the first output node and the second output node.

19. The circuit of claim 18 , wherein the mixer is configured to receive a local oscillator (LO) signal, wherein the fourth inductive element and the capacitive element form a notch circuit having a resonant frequency of three times a frequency of the LO signal.

20. The circuit of claim 12 , further comprising a low-noise amplifier (LNA) having an output coupled to the input node of the circuit.

21. A wireless device comprising the circuit of claim 12 , the wireless device comprising:

at least one antenna; and

a receiver coupled to the at least one antenna, the receiver comprising the circuit for signal processing.

22. A method for signal processing, comprising:

receiving an input current flowing in a first signal path, wherein the first signal path comprises a first inductive element coupled in series with a second inductive element; and

generating an output current flowing in a second signal path, wherein the second signal path comprises a third inductive element coupled to a fourth inductive element, the first inductive element being magnetically coupled to the third inductive element, and the second inductive element being magnetically coupled to the fourth inductive element, wherein:

the second signal path has a high impedance with respect to a first frequency as compared to an impedance of the second signal path with respect to a second frequency, the second frequency being lower than the first frequency.

23. The method of claim 22 , wherein the second signal path comprises a capacitive element coupled in parallel with the fourth inductive element.

24. The method of claim 23 , further comprising setting the first frequency by setting a capacitance of the capacitive element coupled in parallel with the fourth inductive element.

25. The method of claim 22 , further comprising:

mixing the output current with a local oscillator (LO) signal, wherein the first frequency comprises three times a frequency of the LO signal.

26. A circuit for signal processing, comprising:

a low-noise amplifier (LNA) having an output coupled to a first inductive element;

a first transformer including the first inductive element magnetically coupled with a second inductive element;

a second transformer including a third inductive element magnetically coupled with a fourth inductive element, wherein the first inductive element is coupled in series with the third inductive element;

a capacitive element coupled in parallel with the fourth inductive element, the capacitive element and the fourth inductive element forming a notch circuit, the notch circuit coupled in series with the second inductive element; and

a mixer coupled to the second inductive element and the notch circuit.

27. The circuit of claim 26 , wherein the mixer is configured to receive a local oscillator (LO) signal, wherein a resonant frequency of the notch circuit comprises three times a frequency of the LO signal.