Abstract:
A low noise amplifier (LNA) for filtering an input signal to generate an output signal. The low noise amplifier includes a switched loading circuit having a plurality of loading units, each of the loading units determining a corresponding center frequency of the low noise amplifier. The switched loading circuit selectively enables a loading unit having the desired corresponding center frequency. At least one converters coupled to the switched loading circuit converts a voltage of the input signal into a loading current and passes the loading current through the enabled loading unit to generate the output signal.

Description:
BACKGROUND OF INVENTION 
   1. Field of the Invention 
   The present invention relates to a low noise amplifier, and more particularly to a low noise amplifier and related method in a wideband communication system. 
   2. Description of the Prior Art 
   A low noise amplifier (LNA) is a common device used to filter out the noise of input signals received at the front ends of communication systems. Generally speaking, the LNA is capable of decreasing most of the incoming noise and amplifying a desired signal within a certain frequency range to increase the signal to noise ratio (SNR) of the communication system and improve the quality of received signal as well. Because the desired signal is transmitted via a carrier, the frequency range of the desired signal is supposed to be near the carrier frequency. Therefore, the LNA is designed to magnify an input signal with a frequency close to the carrier frequency and attenuate an input signal with a frequency far away from the carrier frequency through a common gate or common source amplifier with a frequency-dependent loading, such as an inductor connected to a capacitor in parallel. 
   Please refer to  FIG. 1 , which is a schematic diagram illustrating the frequency response  10  of a prior art narrow-band LNA. Taking the narrow-band LNA of a wireless communication system for example, the frequency response  10  of the LNA has a center frequency of 2.43 GHz and a bandwidth about 200 MHz. As one can see, an input signal with a frequency equaling 2.43 GHz is allowed to have a maximum magnitude value processed by the LNA. However, because of the characteristics of the loading formed by the inductor and the capacitor connected in parallel, an input signal with a frequency deviated from 2.43 GHz is attenuated via the LNA, and is completely filtered out if its frequency is located outside the allowed bandwidth 2.42–2.44 GHz. Please note that the combination of the inductor and the capacitor determines the center frequency and the operating bandwidth associated with the frequency response  10  shown in  FIG. 1 . 
   However, this kind of LNA is not applicable in a wideband communication system especially when the wideband communication system uses a high-frequency carrier to transfer data. Concerning an ultra-wideband (UWB) communication system, it requires an LNA having a great bandwidth from 10 GHz to 66 GHz. Therefore, the combination of the inductor and the capacitor according to the prior art narrow-band LNA is unable to generate a frequency response with such a great bandwidth. However, if a prior art wide-band LNA is adopted, much of the undesired noise could not be successfully suppressed through a single filtering bandwidth, and is injected into the ultra-wideband communication system, which degrades the signal quality. 
   SUMMARY OF INVENTION 
   It is therefore one of the objectives of the claimed invention to provide a low noise amplifier with a switched bandwidth instead of a fixed bandwidth, to solve the above-mention problem. 
   According to the claimed invention, a low noise amplifier is disclosed. The low noise amplifier includes a switched loading circuit having a plurality of loading units, each of the loading units determining a corresponding center frequency of the low noise amplifier. The switched loading circuit selectively enables a loading unit having the corresponding center frequency. At least one converter coupled to the switched loading circuit converts the input signal into a loading current and passes the loading current through the enabled loading unit to generate the output signal. 
   In addition, the claimed invention discloses a low noise amplifying method for filtering an input signal to generate an output signal. The low noise amplifying method includes providing a plurality of loading units, each of the loading units determining a corresponding center frequency. A loading unit corresponding to the desired center frequency is selectively enabled. The input signal is converted into a loading current and the loading current is passed through the enabled loading unit to generate the output signal. 
   These and other objectives of the present invention will no doubt become obvious to those of ordinary skill in the art after reading the following detailed description of the preferred embodiment that is illustrated in the various figures and drawings. 

   
     BRIEF DESCRIPTION OF DRAWINGS 
       FIG. 1  is a schematic diagram of a frequency response of a prior art low noise amplifier. 
       FIG. 2  is schematic diagram of a low noise amplifier according to the present invention. 
       FIG. 3  is a schematic diagram of a frequency response of the low noise amplifier shown in  FIG. 2 . 
       FIG. 4  is a schematic diagram of a low noise amplifier according to another embodiment of the present invention. 
   

   DETAILED DESCRIPTION 
   Please refer to  FIG. 2 , which is schematic diagram of a low noise amplifier (LNA)  100  according to an embodiment of the present invention. In this embodiment, the LNA  100  is applied to a wideband communication system (e.g. an ultra-wideband communication system). The LNA  100  includes a switched loading circuit  20 , a voltage to current transformer  40 , and a bias circuit  60 . The switched loading circuit  20  is used to provide a switched loading in order to tune a desired center frequency and a desired bandwidth. As shown in  FIG. 2 , the switched loading circuit  20 , in this embodiment, contains three loading units  22 ,  24 ,  26 , and three switches  32 ,  34 ,  36 . NMOS transistors implement all of the switches  32 ,  34 ,  36 , respectively. However, it is well-known that an NMOS transistor or a PMOS transistor can be adopted to function as a transistor switch. The implementation of the switches  32 ,  34 ,  36  shown in  FIG. 2  is only meant to serve as an example and is not meant to be taken as a limitation. 
   As one can see, the switch  32  is turned on if the logic level of the control signal CS 1  is “high”, and is turned off if the logic level of the signal CS 1  is “low”. Similarly, the switch  34  is turned on if the logic level of the control signal CS 2  is “high” and is turned off if the logic level of the signal CS 2  is “low”. The switch  36  is turned on if the logic level of the control signal CS 3  is “high” and is turned off if the logic level of the signal CS 3  is “low”. Therefore, if the control signals CS 1 , CS 2 , CS 3  are controlled to have logic levels respectively corresponding to “high”, “low”, “low”, the loading unit  22  is selected and enabled to provide a specific loading for the LNA  100 . In the same way, each of the loading units  24 ,  26  is selected and enabled through pulling “high” the corresponding control signal CS 2 , CS 3 . So, the LNA  100  according to the present invention can select a desired loading unit based on the requirements for the gain, the center frequency, and the bandwidth. As for the electrical structure of the loading units  22 ,  24 ,  26 , it is further disclosed in the following paragraph. 
   The loading units  22 ,  24 ,  26  shown in  FIG. 2  correspond to a plurality of impedance Z 1 , Z 2 , Z 3  respectively. As one can see, the loading unit  22  includes a capacitor  222 , an inductor  224 , and a resistor  226 . Similarly, the loading unit  24  includes a capacitor  242 , an inductor  244 , and a resistor  246 , and the loading unit  26  includes a capacitor  262 , an inductor  264 , and a resistor  266 . The inductors  224 ,  244 ,  264  connected to the respective resistor  226 ,  246 ,  266  in series and connected to the respective capacitor  222 ,  242 ,  262  in parallel can implement each of the impedances. Therefore, the mathematical model of the impedance Z 1 , Z 2 , Z 3  is shown as follows: 
   
     
       
         
           
             
               
                 
                   
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   In Equation (1), L i  stands for the inductance of the inductor, C i  stands for the parasitic capacitance of the real inductor, and R i  stands for the internal resistance of the real inductor. 
   Please note that the resistor in each of loading units  22 ,  24 ,  26  is optional. That is, the added resistor is capable of providing the LNA  100  with a greater gain. Generally speaking, any circuit component is sure to have a parasitic capacitor. If the parasitic capacitance of the inductor is big enough, the combination of the inductor and the capacitor shown in  FIG. 2  can be practically implemented by a single inductor. However, if the inductor manufactured by a semiconductor process contributes a small parasitic capacitance to the loading unit  22 ,  24 ,  26 , the loading unit  22 ,  24 ,  26 , as shown in  FIG. 2 , has to include one additional capacitor connected to the inductor in parallel. According to Equation (1), each specific loading unit  22 ,  24 ,  26  corresponds to a specific center frequency (i.e., a specific carrier frequency of the communication system) and a specific operating bandwidth based on the 3-dB frequency of the frequency response associated with the impedance of the specific loading unit  22 ,  24 ,  26 . So, the impedance of each loading unit  22 ,  24 ,  26  has a maximum magnitude if the frequency of the input signal is just the same as the selected center frequency. In other words, the LNA  100  provides a maximum gain upon the incoming input signal. 
   However, the impedance of a loading unit  22 ,  24 ,  26  is much smaller if the frequency of the input signal is outside the operating bandwidth. In addition, the gain of the LNA  100  applied to the input signal is smaller than one for attenuating the magnitude of the input signal, which is treated as the undesired noise. The relationship between the impedance and the gain of the LNA  100  is shown as follows:
 
 A   v   =gm*Z   i   ,i= 1,2,3  Equation (2)
 
   In Equation (2), A v  represents the voltage gain of the LNA  100 , and gm represents the transconductance of the NMOS transistor  42  within the voltage to current transformer  40 . The operation and functionality of the voltage to current transformer  40  is detailed in the following paragraph. 
   The voltage to current transformer  40  shown in  FIG. 2  is implemented by an n-channel Metal-Oxide Semiconductor (NMOS) transistor  42  and an inductor  44 , and is used for transforming the voltage of the input signal V in  into a loading current I. As for the ultra-wideband communication system, the carrier frequency is from 10 GHz to 66 GHz, and the magnitude of input signal is quite small. As a result, the maximum transmitted power level is lower than 15 dBm (measured at the antenna port of the transmitter), so it is reasonable to make use of a small signal model of the NMOS  42  to illustrate the operation of the voltage to current transformer  40 . In this preferred embodiment, the NMOS  42  is implemented to function as a common gate amplifier, so the relationship between the voltage of the input signal V in  and the induced loading current I is shown via the following equation:
 
 I=gm* (− V   in )  Equation (3)
 
   In Equation (3), gm stands for the transconductance of the NMOS  42 . According to Equation (3), the output voltage V out  of the LNA is equal to the loading current I multiplied by the impedance Z i  provided by an enabled loading unit  22 ,  24 ,  26 . The relationship is shown as follows:
 
 V   out   =I*Z   i   Equation (4)
 
   According to Equations (3) and (4), the voltage gain A v  of the LNA  100  is easily computed via Equation (2). That is, 
             A   v     =     Vout   Vin           
As for the bias circuit  60 , it is a typical current mirror for providing a bias current I bias  for the voltage to current transformer  40 . Please note that because the current mirror is known to anyone skilled in the art, the description for the bias circuit  60  is omitted for the sake of brevity.
 
   Please refer to  FIG. 3 , which is a schematic diagram illustrating a plurality of frequency responses  120 ,  140 ,  160  provided by the LNA  100  shown in  FIG. 2 . As one can see, the frequency response  120  having a center frequency f c1  is formed due to the enabled loading unit  22 , the frequency response  140  having a center frequency f c2  is formed due to the enabled loading unit  24 , and the frequency response  160  having a center frequency f c3  is formed due to the enabled loading unit  26 . In addition, the frequency response  180  shown in  FIG. 3  is a property possessed of the prior art wideband LNA used in the ultra-wideband communication system, wherein the center frequencies f c1 , f c2 , f c3  correspond to three different carrier frequencies, respectively. In this preferred embodiment, only one of the switches  32 ,  34 ,  36  is selectively turned on, so only one of the frequency responses  120 ,  124 ,  126  on the spectrum is selected. Therefore, the LNA  100  according to the present invention is capable of switching on another switch when the wideband communication system uses another carrier frequency to deliver data. Instead of adopting a single wide bandwidth to cover these three carrier frequencies, the LNA  100  uses a switched bandwidth to better eliminate the undesired noise. Consequently, the wideband communication system is capable of being more resistive to noise through the LNA  100 . 
   Please note that the number of enabled loading units is not limited to one. That is, the LNA  100  according to the present invention can be properly designed to enable a plurality of loading units at the same time, so these enabled loading units are connected in parallel to provide new impedance Z eq  and shift the operating bandwidth of the LNA  100 . Assume that two loading units are enabled simultaneously, and the impedance Z eq  is shown as follows: 
   
     
       
         
           
             
               
                 
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   In Equation (5), Z 1  and Z 2  represent the impedance values of two enabled loading units. 
   Please refer to  FIG. 4  which is a schematic diagram of a low noise amplifier (LNA)  300  according to another embodiment of the present invention. The difference between the LNA  300  and the LNA  100  is the LNA  300  can adjust the voltage gain through the voltage to current transformer  320  and the gain controller  340 . The gain controller  340  is used to control the voltage to current transformers  360  and  320  according to a desired voltage gain. The voltage to current transformer  320  is implemented by using NMOS transistor  322  and  324 . When the maximum voltage gain is adopted, the voltage to current transformer  320  is enabled to convert the voltage of the input signal V in  into a loading current I 2 , and the voltage to current transformer  360  is also able to converting the voltage of the input signal V in  into the loading current I 1  at the same time as the voltage to current transformer  40  in  FIG. 2 . That is the voltage to current transformers  360  and  320  provide the LNA  300  with a bigger voltage gain than the LNA  100 . However, when a smaller voltage gain is adopted, the NMOS transistor  322  is disabled (i.e., turn off) and the NMOS transistor  324  is enabled (i.e., turn on) at the same time for matching the input impedance to 50 ohms. Therefore, the LNA  300  is able to provide two kinds of voltage gain according the preferred embodiment. 
   In contrast to the prior art, the LNA according to the present invention selectively enables the appropriate loading unit(s) according to the carrier frequency used by a wideband communication system, that is, the LNA according to the present invention has a plurality of candidate bandwidths, which are narrow bands with specific center frequencies, for amplifying signals transmitted via different carrier frequencies in the wideband communication system. To sum up, with the help of the LNA according to the present invention, the wideband communication system is provided with a better gain characteristic to eliminate the unwanted noise when receiving the input signals with different carrier frequencies. 
   Those skilled in the art will readily observe that numerous modifications and alterations of the device and method may be made while retaining the teachings of the invention. Accordingly, the above disclosure should be construed as limited only by the metes and bounds of the appended claims.