Low noise amplifier and method of input impedance control for terrestrial and cable modes

1. A low-noise amplifier (LNA) for a receiver circuit, the LNA comprises:

amplifier circuitry including a plurality of input nodes and an output;

an adjustable impedance network including an input for receiving a radio frequency signal and a plurality of control inputs, the adjustable impedance network including:

a plurality of transistors, each of the plurality of transistors including a drain and a source coupled between the input of the adjustable impedance network and a respective one of the plurality of input nodes of the amplifier circuitry, and including a gate coupled to one of the plurality of control inputs; and

a controller coupled to the plurality of control inputs and configured to control an impedance of the adjustable impedance network to provide a first impedance in a terrestrial mode and to provide a second impedance in a cable mode.

2. The LNA of claim 1 , wherein the first impedance is configured to substantially match an impedance of a cable providing the radio frequency signal.

3. The LNA of claim 1 , wherein the adjustable impedance network further comprises:

a plurality of resistors, each of the plurality of resistors coupled between one of the plurality of transistors and the input of the adjustable impedance network.

4. The LNA of claim 1 , wherein the controller activates the plurality of transistors in the terrestrial mode.

5. The LNA of claim 1 , wherein the controller deactivates some of the plurality of transistors in the cable mode.

6. The LNA of claim 1 , wherein the plurality of transistors have substantially equal width-to-length ratios.

7. The LNA of claim 1 , wherein the plurality of transistors have substantially equal resistances.

8. A method comprising:

determining a receiving mode of a receiver, the receiving mode including one of a terrestrial mode and a cable mode; and

activating a plurality of transistors of an adjustable impedance network in response to determining the receiving mode is a terrestrial mode; and

activating some, but not all, of the plurality of transistors of the adjustable impedance network in response to determining the receiving mode is a cable mode;

wherein:

in the terrestrial mode, the plurality of activated transistors provide a first impedance; and

in the cable mode, the plurality of activated transistors provide a second impedance.

9. The method of claim 8 , wherein activating the plurality of transistors comprises reducing an input impedance of the receiver.

10. The method of claim 8 , wherein activating some, but not all, of the transistors comprises increasing an input impedance of the receiver.

11. The method of claim 8 , wherein before activating, the method further comprises:

providing a control signal to an impedance adjustment circuit; and

decoding the control signal within the impedance adjustment circuit to produce driver signals for activation of selected transistors according to determining the receiving mode.

12. The method of claim 8 , wherein each transistor of the plurality of transistors that is activated has an impedance that is approximately equal to impedances of other transistors of the plurality of transistors.

13. The method or claim 8 , wherein

the second impedance is greater than the first impedance.

14. The method of claim 8 , wherein activating some, but not all, of the transistors of the adjustable impedance network comprises activating approximately half of the plurality of transistors of the adjustable impedance network.

15. A receiver comprising:

a low noise amplifier including an adjustable impedance network and an amplifier circuit including a plurality of input nodes, the low noise amplifier including an input for receiving a radio frequency signal, a plurality of control inputs, and an output, the adjustable impedance network including a plurality of transistors, each of the plurality of transistors including a drain, a source, and a gate, one of the drain and the source coupled to one of the plurality of input nodes of the amplifier circuit, and the gate coupled to one of the plurality of control inputs; and

a controller configured to control the adjustable impedance network to provide a first input impedance in a terrestrial mode and to provide a second input impedance in a cable mode.

16. The receiver of claim 15 , wherein, in the cable mode, the input impedance substantially matches an impedance of a transmission line providing the radio frequency signal.

17. The receiver of claim 15 , wherein the adjustable impedance network further comprises:

a plurality of resistors, each of the plurality of resistors including a first terminal coupled between one of the plurality of transistors and the input of the adjustable impedance network.

18. The receiver of claim 17 , wherein:

in the terrestrial mode, the controller controls the adjustable impedance network to provide a first impedance; and

in the cable mode, the controller controls the adjustable impedance network to provide a second impedance.

19. The receiver of claim 18 , wherein the second impedance is greater than the first impedance.

20. The receiver of claim 17 , wherein the plurality of transistors and the plurality of resistors are substantially matched.

21. The receiver of claim 17 , wherein:

in the terrestrial mode, the controller activates each of the plurality of transistors and each of the plurality or transistors conducts a first current that is substantially equal to currents conducted by others of the plurality of transistors; and

in the cable mode, the controller activates some, but not all, of the plurality of transistors and each of the plurality of transistors conducts a second current that is substantially equal to currents conducted by other active ones of the plurality of transistors; and

wherein the second current is greater than the first current.

22. The receiver of claim 15 , wherein the controller comprises a micro-controller unit.