Patent Publication Number: US-2011070859-A1

Title: Low noise amplifier and radio frequency signal receiver

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     This application claims priority of Taiwan Patent Application No. 98217228, filed on Sep. 18, 2009, the entirety of which is incorporated by reference herein. 
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The invention relates to radio frequency signal receivers, and more particularly to low noise amplifiers (LNA) of radio frequency signal receivers. 
     2. Description of the Related Art 
     A low noise amplifier is an electronic amplifier used in a communications system for amplifying feeble radio frequency signals captured by an antenna. Conventionally, the low noise amplifier is disposed close to the antenna to reduce signal attenuation due to signal transmission between the low noise amplifier and the antenna. A radio frequency signal receiver ordinarily comprises a low noise amplifier located in the front-end circuit of the receiver. For example, a Bluetooth system comprises a low noise amplifier for signal amplification. 
     A noise figure of a low noise amplifier determines noise amplitude of a radio frequency signal received by a radio frequency signal receiver. A low noise amplifier therefore must have a high voltage gain and a low noise figure to amplify the small signal components within the operative range of a radio frequency signal to improve quality of the output signal. Circuit properties of a low noise amplifier determine quality of a radio frequency signal received by a radio frequency signal receiver. 
     Because a radio frequency signal receiver is often installed in a portable device, and the power of the portable device is supplied by batteries, a radio frequency signal receiver must be designed with low power consumption to extend a lifespan of the batteries. When a conventional low noise amplifier receives a large signal, the power consumption of the conventional low noise amplifier is far greater than that of the conventional low noise amplifier for amplifying a small signal. If the current flowing through the conventional low noise amplifier is reduced, the performance of the conventional low noise amplifier is degraded. Thus, a low nose amplifier which has both reduced power consumption and good performance is required. 
     BRIEF SUMMARY OF THE INVENTION 
     The invention provides a low noise amplifier. The low noise amplifier comprises a first transistor, a second transistor, and a first resistor. The first transistor has a gate to receiving a radio frequency input signal, wherein the source of the first transistor is coupled to a ground voltage. The second transistor has a drain to output a radio frequency output signal, wherein the gate of the second transistor is coupled to a first reference voltage. The first resistor is coupled between the drain of the first transistor and the source of the second transistor. 
     The invention provides a low noise amplifier. In one embodiment, the low noise amplifier comprises a first transistor, a first resistor, a second transistor, and a switchable load element. The first transistor is coupled between a first node and a ground voltage, wherein the gate of the first transistor is coupled to a second node for receiving a radio frequency input signal. The first resistor is coupled between the first node and a third node. The second transistor is coupled between a fourth node and the third node, wherein the gate of the second transistor is coupled to a first reference voltage, wherein the fourth node outputs a radio frequency output signal. The switchable load element is coupled between a voltage source and the fourth node, wherein the impedance of the switchable load element is adjustable. 
     The invention provides a radio frequency signal receiver. In one embodiment, the radio frequency signal receiver comprises an antenna, a matching circuit, and a low noise amplifier. The antenna receives a first radio frequency signal. The matching circuit adjusts impedance thereof to transmit the first radio frequency signal as a second radio frequency signal without attenuation. The low noise amplifier amplifies the second radio frequency signal to generate a third radio frequency signal and comprises a first transistor, a second transistor, and a first resistor. The first transistor has a gate receiving the second radio frequency signal, and a source coupled to a ground voltage. The second transistor has a drain outputting the third radio frequency signal, and a gate coupled to a first reference voltage. The first resistor is coupled between a drain of the first transistor and a source of the second transistor. 
     A detailed description is given in the following embodiments with reference to the accompanying drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The invention can be more fully understood by reading the subsequent detailed description and examples with references made to the accompanying drawings, wherein: 
         FIG. 1  is a block diagram of a radio frequency signal receiver according to the invention; 
         FIG. 2  is a circuit diagram of a first embodiment of a low noise amplifier according to the invention; 
         FIG. 3  is a circuit diagram of a second embodiment of a low noise amplifier according to the invention; 
         FIG. 4  is a schematic diagram of a relationship between a voltage gain and a noise figure of the low noise amplifiers shown in  FIGS. 2 and 3 ; and 
         FIG. 5  is a schematic diagram of power consumption of the low noise amplifiers shown in  FIGS. 2 and 3 . 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     The following description is of the best-contemplated mode of carrying out the invention. This description is made for the purpose of illustrating the general principles of the invention and should not be taken in a limiting sense. The scope of the invention is best determined by reference to the appended claims. 
     Referring to  FIG. 1 , a block diagram of a radio frequency signal receiver  100  according to the invention is shown. In one embodiment, the radio frequency signal receiver  100  comprises an antenna  102 , a matching circuit  104 , a low noise amplifier  106 , an image rejection filter  108 , a mixer  110 , a local oscillator  116 , a channel selection filter  112 , and a demodulator  114 . The antenna  102  receives a radio frequency signal S 1 . The impedance of the antenna  102  does not always match that of the low noise amplifier  106 . When the impedance of the antenna  102  does not always match that of the low noise amplifier  106 , a power of a signal transmitted between the antenna  102  and the low noise amplifier  106  is reduced. The matching circuit  104  therefore adjusts impedance thereof to transmit the radio frequency signal S 1  as a radio signal frequency signal S 2  to the low noise amplifier  106  without signal loss. The low noise amplifier  106  then amplifies in-band components of the radio frequency signal S 2  to obtain a radio frequency signal S 3 . 
     The image rejection filter  108  then filters out image components from the radio frequency signal S 3  to obtain a radio frequency signal S 4 . The local oscillator  116  generates a frequency signal F. The mixer  110  then mixes the radio frequency signal S 4  with the frequency signal F to obtain a signal S 5  with an increased frequency or a reduced frequency. The channel selection filter  112  then filters out out-band components from the signal S 5  to obtain a signal S 6 . Finally, the demodulator  114  demodulates the signal S 6  to obtain a data signal S 7 . 
     Referring to  FIG. 2 , a circuit diagram of a first embodiment of a low noise amplifier  200  according to the invention is shown. In the first embodiment, the low noise amplifier  200  comprises transistors  202  and  204 , a switchable load element  220 , and a resistor  214 . The switchable load element  220  comprises a switch  206 , a low-gain load element  208 , and a high-gain load element  210 . The NMOS transistor  202  has a source coupled to a ground voltage V GND , a gate coupled to a reference voltage V g1  via the resistor  214 , and a drain coupled to a source of the NMOS transistor  204 . The NMOS transistor  204  has a gate coupled to a reference voltage V g2 , and a drain coupled to the switch  206 . 
     The low-gain load element  208  and the high gain load element  210  are coupled between the switch  206  and a voltage source V DD . The high-gain load element  210  has resistance higher than that of the low-gain load element  208 . The switch  206  couples the drain of the NMOS transistor  204  to the low-gain load element  208  or the high gain load element  210  according to the gain required by the low noise amplifier  200 . A radio frequency input signal received by an antenna is transmitted to a gate  216  of the NMOS transistor  202  through the matching circuit  212  as an input voltage Vx. A current I flowing through the transistors  202  and  204  is controlled by the voltage Vx on the gate node  216 . When the voltage Vx increases, the current I, correspondingly increases. When a large current I flows through the switchable load element  220 , a voltage drop across the switchable load element is induced, thus generating a radio frequency output signal V Y  on a drain  218  of the transistor  204 . Thus, the voltage signal Vx on the gate node  216  of the transistor  202  is amplified by the low noise amplifier  200  to generate a radio frequency output signal V Y  on a drain node  218  of the transistor  204 . 
     The low noise amplifier  200  shown in  FIG. 2  can properly amplify a radio frequency input signal. The high-gain load element  210 , the low-gain load element  208 , and the NMOS transistors  202  and  204  are coupled between the voltage source V DD  and the ground voltage V GND . When the low noise amplifier operates, if the input voltage signal Vx is a large input signal, the current I flowing through the high-gain load element  210 , the low-gain load element  208 , and the NMOS transistors  202  and  204  is large, inducing large power consumption. 
     Referring to  FIG. 3 , a circuit diagram of a second embodiment of a low noise amplifier  300  according to the invention is shown. In the second embodiment, the low noise amplifier  300  comprises transistors  302  and  304 , a switchable load element  320 , and a resistor  314 . The switchable load element  320  and the NMOS transistors  302  and  304  are coupled between a voltage source V DD  and a ground voltage V GND . The NMOS transistor  302  is coupled between a node  331  and the ground voltage V GND , and has a gate coupled to a node  332 . The resistor  314  is coupled between the node  332  and a reference voltage V g1 . A radio frequency signal is transmitted to the node  332  via a matching circuit  312 . A resistor  305  is coupled between a node  333  and the node  331 . 
     The NMOS transistor  304  is couple between the nodes  334  and  333  and has a gate coupled to a reference voltage V g2 . The switchable load element  320  is coupled between a high voltage source V DD  and the node  334  and has switchable resistance. In one embodiment, the switchable load element  320  comprises a high-gain load element  310 , a low-gain load element  308 , and a switch  306 . The resistance of the high-gain load element  310  is greater than that of the low-gain load element  308 . The switch  306  couples the node  334  to the high-gain load element  310  or the low-gain load element  308  according to a gain of the low noise amplifier  300 . 
     A radio frequency input signal received by an antenna is transmitted to the node  332  via the matching circuit  312  as an input voltage Vx. A current I flowing through the transistors  302  and  304  is controlled by the voltage Vx at the node  332 . When the input voltage signal Vx is large, the current I increases. When a large current I flows through the switchable load element  320 , the current I induces a large voltage drop across the switchable load element  320 , generating a radio frequency output voltage V Y  at the node  334 . The input voltage signal Vx on the gate  332  of the transistor  302  is therefore amplified to generate the radio frequency output voltage V Y  on the drain  334  of the transistor  304 . In comparison with the low noise amplifier  200  of the first embodiment, the low noise amplifier  300  of the second embodiment further comprises a resistor  305  cascaded between the transistors  302  and  304 . When the input voltage signal Vx is a large input signal, the current I flowing through the NMOS transistors  302  and  304  and the resistor  305  is reduced, thus lowering power consumption of the low noise amplifier  300 . 
     Referring to  FIG. 4 , a schematic diagram of a relationship between a voltage gain and a noise figure of the low noise amplifiers  200  and  300  shown in  FIGS. 2 and 3  is shown. Assume that a frequency band of a radio frequency input signal at 355 MHz requires amplification. The low noise amplifiers  200  and  300  therefore must amplify the signal components with frequencies approximate to 315 MHz. Referring to  FIG. 4 , both the low noise amplifier  200  of the first embodiment and the low noise amplifier  300  of the second embodiment have gain peaks at the vicinity of 315 MHz, and both the low noise amplifier  200  of the first embodiment and the low noise amplifier  300  of the second embodiment have noise valleys of 315 MHz. Thus, even if a resistor  305  is added to the low noise amplifier  300 , the low noise amplifier  300  of the second embodiment still has the same voltage gain and noise figure as those of the low noise amplifier  200  of the first embodiment. The low noise amplifier  300  therefore has the same performance as that of the low noise amplifier  200 . 
     Referring to  FIG. 5 , a schematic diagram of power consumption of the low noise amplifiers  200  and  300  shown in  FIGS. 2 and 3  is shown. Because the low noise amplifier  200  does not comprises a resistor coupled between the transistors  202  and  204 , the current I flowing through the transistors  202  and  204  is large, inducing high power consumption. Thus, batteries supplying power to the low noise amplifier  200  have a short lifespan. On the contrary, because the low noise amplifier  300  comprises a resistor  305  coupled between the transistors  302  and  304 , the current I flowing through the transistors  302  and  304  is small, inducing low power consumption. Thus, batteries supplying power of the low noise amplifier  300  have a long lifespan. 
     Referring to  FIG. 5 , when the radio frequency input signal has large signal power, the power consumption of the low noise amplifier is reduced by a large amount. The batteries of a portable device comprising the low noise amplifier  300  of the second embodiment, therefore have longer lifespan, and the dynamic range of the low noise amplifier  300  is therefore extended. In addition, the low noise amplifier  300  of the second embodiment has a longer dynamic range than that that of the low noise amplifier  200  of the first embodiment. Thus, the low noise amplifier  300  of the second embodiment can reduce power consumption without degrading signal quality. 
     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.