Abstract:
Low noise amplifiers and related control methods for multiple radio standards are disclosed. An exemplary low noise amplifier comprises input ports, an output port, amplifier stages, and a degeneration inductor. Each amplifier has a gain stage and a buffer stage connected in series. The buffer stage selectively channels an output of the gain stage to the output port or a power supply. The degeneration inductor is commonly connected to the gain stage in each of the amplifier stages.

Description:
BACKGROUND 
       [0001]    The present disclosure relates generally to the design and implementation of radio frequency (RF) receivers, and more specifically, to the design and implementation of low-noise amplifiers (LNAs) for multiple radio standards. 
         [0002]    Single integrated circuit chips required to support multiple radio standards tend to be an intuitive simple combination of multiple circuit modules in the IC, with each module accommodates only a single radio standard. Frequency bands allocated for Enhanced Data Rates for GSM (EDGE) and General Packet Radio Services (GPRS) are 850 MHz, 900 MHz, 1.8 GHz and 1.9 GHz, for example, and conventional chips for EDGE and GPRS have been designed using three or four independent Low-Noise Amplifiers (LNAs) cascading with the same number of mixers whose outputs are merged together to feed a single baseband circuit. This kind of approaches has several disadvantages. A costly semiconductor area is required, for instance, because each of LNAs and mixers needs at least one inductive device, which is huge in size. Furthermore, for merging outputs from different mixers, long-distance routings crossing over a large semiconductor area are required, but it is hard for such a routing to achieve low signal loss, low parasitic resistance, and low parasitic capacitance. 
         [0003]    In view of the foregoing, it is highly desirable and advantageous to providing a system and method that employs fewer LNAs and/or mixers on a single chip than that currently employed using known techniques while supporting multiple radio standards. 
       SUMMARY 
       [0004]    An exemplary low noise amplifier is disclosed in the specification, comprising a plurality of input ports, an output port, a plurality of amplifier stages, and a degeneration inductor. Each amplifier has a gain stage and a buffer stage connected in series between one of the input ports and the output port. The buffer stage selectively channels an output of the gain stage to the output port or a power supply. The degeneration inductor is commonly connected to the gain stage in each of the amplifier stages. 
         [0005]    An exemplary method for operating a low-noise amplifier is also disclosed. The low-noise amplifier comprises a plurality of input ports, an output port, and a plurality of amplifier stages, each comprising a gain stage coupled between one of the input ports and the output port. A first amplifier stage among the amplifier stages is disabled by biasing the gain stage of the first amplifier stage to an off state and channeling an output current from the gain stage of the first amplifier stage to a power supply. A second amplifier stage among the amplifier stages is enabled by biasing the gain stage of the second amplifier stage to an on state and channeling an output current from the gain stage of the second amplifier stage to the output port. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0006]    The invention can be more fully understood by the subsequent detailed description and examples with references made to the accompanying drawings, wherein: 
           [0007]      FIG. 1  depicts a multi-band RF receiver according to an embodiment of the invention; 
           [0008]      FIG. 2  depicts the LNA shown in  FIG. 1 ; and 
           [0009]      FIG. 3  shows some resulted signal paths in the LNA of  FIG. 2  when the amplifier stage  26   2  is enabled. 
       
    
    
     DETAILED DESCRIPTION 
       [0010]    It is to be understood that the techniques of the present invention are not limited to the methods and apparatuses shown and described herein. Rather, alternative methods and apparatuses within the scope of the invention will become apparent to those skilled in the art given the teachings therein. 
         [0011]      FIG. 1  depicts a multi-band RF receiver  10  according to an embodiment of the invention. The RF receiver  10  includes an antenna  12 , several impedance matching networks  20   1 - 20   n , a low-noise amplifier (LNA)  14 , a mixer  16 , a baseband circuit  18 , a band selector  22 , and a bias generator  24 , where n is an integer larger than  1 . 
         [0012]    The antenna  12  receives inbound RF signals denoted as inRFi in  FIG. 1 , which might be carried in different RF bands. Each of the impedance matching network  20   i , where i=1, 2, . . . , n, provides impedance matching for inbound RF signals in one RF band. Accordingly, inbound RF signals inRF i  in one frequency band go through a corresponding impedance matching network  20   i , while being rejected by other impedance matching networks. The LNA  14  has several amplifier stages  26   1 - 26   n . Each of the amplifier stage  26   i , if enabled, amplifies corresponding inbound RF signals inRF i , filtered and matched by a corresponding impedance matching network  20   i , and generates a corresponding result in a common output port OUT to drive an inductive load  28 . As the impedance matching networks  20   1 - 20   n  correspond to respective RF bands for communication, so do the amplifier stages  26   1 - 26   n . 
         [0013]    The amplifier stages  26   1 - 26   n  share a common degeneration inductor  29  as shown in  FIG. 2 , which provides a real part of the input impedance to input ports IN 1 -IN n  of the entire amplifier stages  26   1 - 26   n . Referring back to  FIG. 1 , the mixer  16 , coupled to the LNA  14 , down-converts the signal at the output port OUT of the LNA  14  by mixing signal at output with local oscillation signals LO. The mixer  16  might include a pair of mixers if it is required to receive a pair of differential signals. The mixer  16  accordingly provides baseband signals to the baseband circuit  18  for further signal processing, such as analog-to-digital conversion and demodulation. The band selector  22 , based upon an active RF band to be used for receiving RF signals, provides one among the corresponding control signals EN 1 -EN n  to enable corresponding one of the amplifier stages  26   1 - 26   n , meanwhile the other amplifier stages are disabled. Similarly based upon the active RF band, the bias generator  24  provides corresponding bias voltages BI 1 -BI n  to the amplifier stages  26   1 - 26   n , respectively. Except for the bias voltage for an enabled amplifier stage, the remaining bias voltages eliminate the gains of the disabled amplifier stages. 
         [0014]      FIG. 2  depicts detail structure of the LNA  14  shown in  FIG. 1 . The LNA  14  in  FIG. 2  has amplifier stages  26   1 - 26 , each of the amplifier stages  26   i  being a differential amplifier, having two differential input ports (IN_P i  and IN_N i ) for receiving balanced inbound RF signals inRF_P i  and inRF_N i , and sharing two common differential output ports (OUT_P and OUT_N) coupled to the inductive load  28 , which is further coupled to a power supply VCC. The inductive load  28  includes two inductors and two tunable capacitors, whose resonant frequency is tunable for output impedance matching. Amplifier stage  26   1 , for instance, is a differential amplifier with portions  26 _P 1  and  26 _N 1  sharing the common degeneration inductor  29 , which is implemented by two inductors inductively-coupled to each other in  FIG. 2 . All amplifier stages  26   1 - 26   n  are of the same in view of circuitry architecture, such that only the non-inverted portion  26 _P 1  of the amplifier stage  26   1  is detailed and the remaining non-inverted portions of the amplifier stages  26   2 - 26   n  in  FIG. 2  are self-explanatory based on the explanation of the non-inverted portion  26 _P 1 . 
         [0015]    The non-inverted portion  26 _P 1  has a gain stage GS_P 1  and a buffer stage BS_P 1  connected in series between the input port IN_P 1  and the output port OUT_P. The gain stage GS_P 1  includes a common source amplifier, where the source of NMOS N_P 1  is connected to a degeneration inductor  29 , and the gate of NMOS N_P 1  is coupled to the bias voltage BI 1  through resistor RP 1 . The bias voltages BI 1  provided from the bias generator  24  (shown in  FIG. 1 ) substantially determines the transconductance of NMOS N_P 1 , whose gate functions as the input port IN_P 1  to receive the inbound RF signals inRF_P 1  from the impedance matching networks  20   1  (in  FIG. 1 ) to generate output current I_P 1 . The buffer stage BS_P 1  includes a common gate amplifier, for channeling the output current I_P 1  to the output port OUT_P based on the control signal EN 1 . 
         [0016]    When the amplifier stage  26   1  is enabled, the band selector  22  asserts the control signal EN 1  and the bias generator  24  keeps the bias voltages BI 1  at a high level above the threshold voltage of the NMOS N_P l . Thus, the gain stage GS_P 1  is now operating in an ON state, and the output current I_P 1  reflects the amplitude of the inbound RF signals inRF_P i  at the gate of the NMOS N_P l . Because the NMOS in the common gate amplifier is conducted, the output current I_P 1  is then channeled to the output port OUT_P. On the contrary, when the amplifier stage  26   1  is disabled, the band selector  22  disasserts the control signal EN 1  and the bias generator  24  turns the bias voltages BI 1  to be a low level under the threshold voltage of the NMOS N_P 1 . For example, the bias voltages BI 1  could be zero. The gain stage GS_P 1  is now operating in an OFF state as the NMOS N_P 1  is turned off. As a result, the common gate amplifier is turned off and it no longer provides a channel to the output node OUT_P. Also, there is no induced output current I_P 1  because the NMOS N_P 1  is turned off, leaving the output port OUT_P driven by another amplifier stage. 
         [0017]      FIG. 3  depicts signal paths in the LNA  14  of  FIG. 2  when the amplifier stage  26   2  is enabled. If the inbound RF signals inRF 2 , consisting of balanced RF signals inRF_P 2  and inRF_N 2 , are to be used for communication, all the amplifier stages other than amplifier stage  26   2  are disabled because control signals EN 1 , EN 3 -EN n  are off and bias voltages BI 1 , BI 3 -BI n  are at ground level. Any interfering RF signals that go through impedance matching networks  20   1 ,  20   3 - 20   n , other than impedance matching networks  20   2 , are rejected by the NMOSs of which gate is grounded in the gain stages, or have no influence on the output ports OUT_P and OUT_N, which are currently disconnected from the gain stages GS_P 1 , GS_N 1 , GS_P 3 -GS_P n , and GS_N 3 -GS_N n . The enabled amplifier stage  26   2 , amplifies the inbound RF signals inRF 2  as the bias voltages BI 2  is higher than the threshold voltages of the NMOSs N_N 2  and N_P 2 . Furthermore, the control signal EN 2  allows the buffer stages BS_P 2  and BS_N 2  to provide electrical connections from the drain of the NMOS N_P 2  to the output port OUT_P and from the drain of the NMOS N_N 2  to the output port OUT_N. The output currents I_P 2  and I_N 2  are accordingly channeled to the output ports OUT_P and OUT_N, respectively. 
         [0018]    If the inbound RF signals inRF 1 , for instance, are to be amplified, all but the amplifier stage  26   1  are disabled. The control signal EN 1  is asserted while the control signals EN 2 -EN n  are off. The bias voltage BI 1  becomes higher than an NMOS threshold voltage, and the bias voltages BI 2 -BI n  become zero. The present operation of the LNA  14  is analogous to the description in the previous paragraph and is omitted herein for purposes of brevity. 
         [0019]    An advantage of the present invention is the lower semiconductor cost of the single integrated circuit chip embodying the multi-band RF receiver  10  in  FIG. 1 . Unlike the techniques taught in the prior art which needs individual mixers for different LNAs, only one mixer  16  is needed, as shown in  FIG. 1 . Furthermore, instead of employing several source degeneration inductors in different amplifier stages, only one source degeneration inductor  29 , commonly shared by amplifier stages  26   1 - 26   n  is needed. The required number of inductors as well as the semiconductor cost is thus reduced. Nevertheless, the present invention is not limited to  FIG. 1 . Some other embodiments covered by the invention might employ more than one mixer and source degeneration inductor. 
         [0020]    Please also note that the LNA  14  is capable of avoiding impedance node noise pickup. High impedance nodes are notorious for its higher thermal noise and the tendency of capacitive and inductive noise pickup. The drains of NMOSs in any disabled amplifier stages of the LNA  14  are connected to the power supply VCC and are therefore not high impedance nodes. Thus, noise pickup can be avoided. Even though each disabled buffer stage in  FIG. 2  channels an output current to the power supply VCC, a disabled buffer stage in another embodiment could channel an output current to another power supply, such as ground, to avoid impedance node noise pickup. 
         [0021]    While the invention has been described by way of example and in terms of preferred embodiment, it is to be understood that the invention is not limited thereto. To the contrary, it is intended to cover various modifications and similar arrangements (as would be apparent to those skilled in the art) . Therefore, the scope of the appended claims should be accorded the broadest interpretation so as to encompass all such modifications and similar arrangements.