Variable-gain low noise amplifier

1. A variable-gain low noise amplifier, comprising:

a plurality of amplifier modules, having a plurality of input nodes capacitively coupled for receiving a RF signal, and outputting currents for output; and

a current steering module, coupled to the plurality of amplifier modules for selectively directing the currents for output according to a plurality of control signals;

wherein, a power gain of the low noise amplifier is a linear combination of power gains of the plurality of amplifier modules;

wherein, when operating in a first mode, the current steering module allows the plurality of amplifier modules to couple in parallel, such that the low noise amplifier has a transconductance proportional to a sum of transconductance of the amplifier modules;

wherein, when operating in a second mode, the current steering module allows the plurality of amplifier modules to cross couple, such that the low noise amplifier has a transconductance proportional to a subtraction of the transconductance of the amplifier modules;

wherein, when operating in a third mode, the current steering module terminates at least one of the amplifier module such that the power gain of the low noise amplifier is independent from one of the amplifier modules.

2. The variable-gain low noise amplifier of claim 1 , wherein the current steering module comprises a plurality of transistors capable of switching current paths to allow cross coupling, parallel coupling, and termination of the plurality of amplifier modules.

3. The variable-gain low noise amplifier of claim 1 , wherein the plurality of amplifier modules have the same amplifier configuration and operate at different biasing conditions.

4. The variable-gain low noise amplifier of claim 1 , wherein one of the amplifier modules is a Class A amplifier and another is a Class A or AB amplifier.

5. The variable-gain low noise amplifier of claim 1 , wherein the amplifier modules are common-source amplifiers with a common source coupled to a ground voltage via an inductor.

6. The variable-gain low noise amplifier of claim 1 , further comprising a capacitor coupled between the input nodes of the amplifier modules.

7. A low noise amplifier, comprising:

a plurality of main and auxiliary amplifier modules, having input nodes capacitively coupled for receiving a RF signal, and respectively having main and auxiliary transconductances for the inbound RF signal; and

a gain control module, coupled to receive output currents from the main and auxiliary amplifier modules;

a second controllable device that is capable of decoupling the auxiliary amplifier modules from the main amplifier modules;

wherein, the gain control module is configured in a way that when the low noise amplifier operates in a high-gain mode, the low noise amplifier has a first transconductance substantially equaled to the main transconductance plus a first portion of the auxiliary transconductance; and

when the low noise amplifier operates in a moderate-gain mode, the low noise amplifier has a second transconductance substantially equaled to the main transconductance.

8. The low noise amplifier of claim 7 , wherein the gain control module is configured in a way that when the low noise amplifier operates in a low-gain mode, the low noise amplifier has a third transconductance substantially equaled to a subtraction of the main transconductance and a second portion of the auxiliary transconductance.

9. The low noise amplifier of claim 7 , wherein the gain control module comprises a first controllable device that is capable of parallel coupling the main and the auxiliary amplifier modules.

10. The low noise amplifier of claim 9 , wherein the gain control module further comprises a third controllable device capable of cross coupling the main and the auxiliary amplifier modules.

11. The low noise amplifier of claim 7 , wherein the main and the auxiliary amplifier modules share a common inductor connected to a ground voltage.

12. The low noise amplifier of claim 7 , wherein the main and auxiliary amplifier modules have a same amplifier configuration and operate at different biasing conditions.

13. A method of processing a RF signal, comprising:

providing a low noise amplifier with a plurality of main and auxiliary amplifier modules;

coupling the main and auxiliary amplifier modules in parallel when a selection indicates the low noise amplifier operating in a high-gain mode, such that the low noise amplifier has a transconductance proportional to a sum of transconductances of the amplifier modules;

cross coupling the main and auxiliary amplifier modules when the selection indicates the low noise amplifier operating in a low-gain mode, such that the low noise amplifier has a transconductance proportional to a subtraction of the transconductance of the amplifier modules;

decoupling the auxiliary amplifier modules from the main amplifier modules when the selection indicates the low noise amplifier operating in a moderate-gain mode, such that a power gain of the low noise amplifier is independent from the auxiliary amplifier modules; and

terminating the auxiliary amplifier modules.

14. The method of claim 13 , further comprising:

capacitively coupling an input of the main amplifier module to an input of the auxiliary amplifier module; and

operating the main and auxiliary amplifier modules in different biasing conditions.

15. The method of claim 13 , wherein the main and auxiliary amplifier modules have a same amplifier configuration.

16. The method of claim 13 , further comprising: measuring the signal power currently output by the low noise amplifier; and making the selection according to the signal power.