Abstract:
A low noise amplifier ( 110 ) is disclosed that is particularly suitable for ultra wideband telecommunications. The low noise amplifier ( 110 ) provides a variable gain by a current controller ( 115 ) to amplify signals received directly from an antenna ( 140 ) and sends the amplified signal to a receiver ( 111 ). In a transceiver configuration, the low noise amplifier ( 110 ) is further connected to a transmitter ( 133 ) through a switch ( 120 ) which provides zero power consumption which the switch ( 120 ) is turned on and provides high impedance when the switch ( 120 ) is turned off.

Description:
TECHNICAL FIELD 
       [0001]    The present invention relates to a circuit. More particularly, the present invention relates to a circuit implementation of a low noise amplifier (LNA). The low noise amplifier is particularly applicable to perform low noise amplification of ultra wideband (UWB) signal. 
       BACKGROUND 
       [0002]    A low noise amplifier is used to amplify a signal. In general, a low noise amplifier is used to amplify very weak signals captured by an antenna. It is always desirable to have a low noise amplifier which is capable of providing variable gain. For example, a larger gain is required if the signal from the antenna is too weak while a lower gain is enough if the signal from an antenna is too strong. If the gain is not fixed to one or two specific values but has a range of possible values, a higher flexibility is provided. Furthermore, it is desirable to have a low noise amplifier which receive signals from an antenna without introducing any noise or with any noise introduced minimized. 
         [0003]    There is a need in the art for a low noise amplifier with variable gain, and for a low noise amplifier for use in a transceiver. 
       SUMMARY 
       [0004]    Disclosed herein is a low noise amplifier that can vary gain by using a transistor. The use of one transistor saves the silicon area and provides an efficient way to control the gain for the output of the low noise amplifier. The design of a low noise amplifier with a transistor as a current controller enables simple architecture and implementation. Instead of limiting the gain to one or two specific values, the present invention provides the gain with a plurality of values. The transistor is preferably a CMOS (Complementary Metal Oxide Silicon), as CMOS provides the low noise amplifier with higher noise immunity and lower power consumption. 
         [0005]    To eliminate any possible noise introduction to the low noise amplifier, the low noise amplifier is directly connected to the antenna to receive signals from the antenna directly. No external inductor or resonant tank (LC circuit) is connected between the antenna and the low noise amplifier in order to minimize the insertion loss. Furthermore, load matching is performed between the antenna and the low noise amplifier to reduce the mismatch loss for the low noise amplifier. 
         [0006]    The low noise amplifier is also applicable to a transceiver by installing the low noise amplifier along a receiver path to provide the gain as required by the receiver. Furthermore, a switch is provided along a transmitter path to cut the transmitter off from the rest of the transceiver circuit when the transceiver functions as a receiver. The complete isolation of the transmitter path by the switch to eliminate any noise contributed from the transmitter path. 
         [0007]    When the transceiver functions as a receiver, the switch completes the transmitter path and the high impedance of the low noise amplifier ensures that the signal from the transmitter is sent to the antenna without being degraded by the receiver path. If the switch leads to any power loss for a signal from the transmitter, a power amplifier is provided between the transmitter and the switch to compensate for any power loss. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0008]    These and other objects, aspects and embodiments of this present invention will be described hereinafter in more details with reference to the following drawings, in which: 
           [0009]      FIG. 1  shows a block diagram illustrating a low noise amplifier. 
           [0010]      FIG. 2  shows a schematic diagram of a low noise amplifier. 
           [0011]      FIG. 3  shows a schematic diagram of a transceiver. 
       
    
    
     DETAILED DESCRIPTION 
       [0012]      FIG. 1  shows a block diagram illustrating a low noise amplifier. A low noise amplifier  110  includes a transistor amplifier  117  which generate a variable gain. The low noise amplifier further includes a current controller  115  which varies the gain generated by the transistor amplifier  117 . By controlling the current flowing through the transistor amplifier  117 , the current controller  115  allows the transistor amplifier  117  to generate an output signal with a variable gain and the variable gain has a plurality of magnitudes rather than simply a fixed magnitude or two stages of magnitudes. The transistor amplifier  117  is directly connected to an antenna  140  and receives a signal from the antenna  140 . The signal received from the antenna  140  has an ultra wide bandwidth. A receiver path  101  is formed with the flow of signal from the antenna  140  to the low noise amplifier  110  and from the low noise amplifier  110  to a receiver  111 . Since the current controller  115  and the transistor amplifier shares the same voltage source, the larger the current flowing through the current controller  115 , the lower the current that flows through the transistor amplifier  117 . The lower the current, I d3 , flowing through the current controller  115 , the larger the current, I d2 , that flows through the transistor amplifier  117 , whereas the bias current source  235 , I bias , flowing through an inductor L 3    232  is constant as shown in  FIG. 2 . 
         [0000]        I   bias   =I   d3   +I   d2    (1)
 
         [0013]    As a result, the variable gain A v  provided by the transistor amplifier  117  is proportional to the current flowing through the transistor amplifier  117 , I d, transistor amplifier 117 . 
         [0000]        A   v   =I   d, transistor amplifier 117   ×Z   L2, 225    (2)
 
         [0014]    In addition, the low noise amplifier  110  is used in a transceiver. In a transceiver, the receiver  111  shares the antenna  140  with a transmitter  133 . The low noise amplifier  110  uses the antenna  140  to receive a signal and amplifies the received signal before sending the received signal to the receiver  111 . The transmitter  133  uses the antenna  140  to transmit a signal. The low noise amplifier  110  further includes a switch  120  which switches off the low noise amplifier  110  when the transceiver transmits a signal and switches on the low noise amplifier  110  when the transceiver receives a signal. 
         [0015]    When the low noise amplifier  110  is switched off by the switch  120 , the low noise amplifier  110  has high impedance. Because of the high impedance, no signal will leak through the low noise amplifier  110  so that it is not required to have a switch to cut off the path between the low noise amplifier  110  and the antenna  140 . Furthermore, the transmitter  133  is connected to the antenna  140  through the switch  120  and the switch  120  allows the signal to flow through the switch  120  with a minimum power loss. In one embodiment, a power amplifier  130  is used to amplify the output of the transmitter  133  to compensate for any power loss contributed by the switch  120 . A transmitter path  102  is formed by the flow of a signal from the transmitter  133  to the power amplifier  130 , from the power amplifier  130  to the switch  120 , and further from the switch  120  to the antenna  140 . 
         [0016]    When the low noise amplifier  110  is switched on by switch  120 , switch  120  cuts off the path between transmitter  133  and antenna  140 . The direct connection between the low noise amplifier  110  and the antenna  140  eliminates any power loss or noise contributed by the path between the low noise amplifier  110  and the antenna  140 . The signal from the antenna will follow along the receiver path  101  and be amplified with a variable gain provided by the low noise amplifier  110 . 
         [0017]      FIG. 2  shows a schematic diagram of a low noise amplifier. The low noise amplifier includes a current controller which is transistor M 3    210 . By varying the gate voltage V bit    218  of the transistor M 3    210 , transistor M 3    210  controls the current I d3  which flows through the transistor M 3    210  and thus controls the current I d2  which flows through the transistor M 2    220 . In one embodiment, a resistor R 3    212  is provided between the gate terminal of the transistor M 3    210  and the gate voltage V bit    218  and the gate voltage V bit    218  is varied by varying the resistance provided by the resistor R 3    212 . A function of R 3    212  is to provide better isolation between the V bit    218  voltage and high frequency signal leakage caused by M 3    210 . In one embodiment, the gate voltage V bit    218  is varied according to a digital control bit. 
         [0018]    Both transistors M 3    210  and M 2    220  have their drain terminals connected to the same voltage source V dd  so that the current I d3  and the current I d2  share the same source. When the gate voltage V bit    218  is decreased by the transistor M 3    210  is partially on and the current I d3  is low so that a larger current I d2  will flow through the transistor M 2    220 . When the gate voltage V bit    218  is increased by the transistor M 3    210  is more fully on and the current I d3  is high so that a smaller current I d2  will flow through the transistor M 2    220 . By varying the current I d2 , the low noise amplifier provides a variable gain to the signal output RF out    228  of the low noise amplifier. 
         [0019]    In addition to being connected to the voltage source V dd , the drain terminal of the transistor M 2    220  is further connected to a signal input RF in    240  of the low noise amplifier. The signal input RF in    240 , in one embodiment, is a signal received from an antenna. In one embodiment, an inductor L 2    225  is provided between the voltage source V dd  and the drain terminal of the transistor M 2    220 . In yet another embodiment, an inductor L 1    241 , a capacitor C 1    243 , a capacitor C 3    245  and a resistor R f    247  are connected in series between the drain terminal of the transistor M 2    220  and the signal input RF in    240 . 
         [0020]    The drain terminal of the transistor M 2    220  is further connected to the signal output RF out    228  of the low noise amplifier. The signal output RF out    228 , in one embodiment, is provided to a receiver. In one embodiment, a capacitor C 2    222  is provided between the signal output RF out    228  and the drain terminal of the transistor M 2    220 . 
         [0021]    The gate terminal of the transistor M 2    220  is connected to the voltage source V dd  and turns the transistor always on as long as the voltage source V dd  is available. 
         [0022]    The source terminal of the transistor M 2    220  is connected to the source terminal the transistor M 3    210  and both the source terminals are connected to the drain terminal of the transistor M 1    230 . By having the transistor M 2    220  and the transistor M 1    230  connected in series with the source terminal of the transistor M 2    220  connected to the drain terminal of the transistor M 1    230 , the transistor M 2    220  is staggered with the transistor M 1    230 . The transistor M 1    230  is provided with a bias current I bias  by having the source terminal of the transistor M 1    230  connected to a bias current source  235  through an inductor L 3    232 . The gate terminal of the transistor M 1    230  is connected to the signal input RF in    240 . In one embodiment, in addition to being connected to a path to the signal output RF out    228  through the capacitor C 3    245  and the resistor R f    247 , the inductor L 1    241  and the capacitor C 1    243  are connected in series in a path between the gate terminal of the transistor M 1    230  and the signal input RF in    240 . 
         [0023]    In one embodiment, the transistors M 1    230 , M 2    220 , M 3    210  are CMOS transistors. 
         [0024]      FIG. 3  shows a schematic diagram of a transceiver. The receiver path  310  of the transceiver contains the low noise amplifier as illustrated in  FIG. 2 . The signal input RF in  of the low noise amplifier is obtained from an antenna  320 . The antenna  320  is directly connected to the low noise amplifier. The signal output RF out  of the low noise amplifier is sent to a receiver  301 . To switch between the receiver  301  and a transmitter  302  in a transceiver, a switch  330  is provided between a transmitter path  340  and the antenna  320 . In one embodiment, a 50Ω input impedance matching for ultra wideband signals is performed for the low noise amplifier. 
         [0025]    The switch  330  includes a control block  338  which is capable of switching on the low noise amplifier as well as switching off the low noise amplifier. The switch  330  includes two transistors M 4    331  and M 5    332 . When the control block  338  switches off the low noise amplifier by powering off the voltage source V dd , the control block  338  switches on the transmitter path by switching on the transistor M 4    331  and switching off the transistor M 5    332 . The transistor M 4    331  is connected to the output of the transmitter path  340 . The transmitter path  340  includes a power amplifier  345  which amplifies the signal from the transmitter  302 . The voltage source V dd  is provided respectively to the drain terminal of the transistor M 5    332  and the collector terminal of the transistor M 5    332 . It is possible to further add a resistor R 4  between the drain terminal of the transistor M 5    332  and the voltage source V dd . The drain terminal of the transistor M 5    332  is further connected to the path between the transmitter path  340  and the transistor M 4    331 . A capacitor  336  is provided to earth the voltage source V dd  which is supplied at the source terminal of the transistor M 5    332 . 
         [0026]    When the control block  338  switches on the low noise amplifier by powering on the voltage source V dd , the control block  338  switches on the receiver path by switching off the transistor M 4    331  and switching on the transistor M 5    332 . The signal from the transmitter path  340  is no longer able to reach the antenna  320  as the switch  330  has cut off the path between the transistor  302  and the antenna  320  when the transistors M 4    331  is shut down and M 5    332  is power on by shutting off the gate voltage of transistor M 4    331  and power on the gate voltage of transistor M 5    332  respectively. 
       INDUSTRIAL APPLICABILITY 
       [0027]    The low noise amplifier disclosed herein finds particular use in wireless communications, especially ultra wideband applications. The amplifier achieves ultra wideband matching and provides variable gain, making an output with multiple gain stages possible. The low noise amplifier can be implemented in a single chip design solution. In addition, a transceiver for ultra wideband applications may use the low noise amplifier. 
         [0028]    The foregoing description is to be considered as illustrative and not restrictive, and the invention is not to be limited to the details given herein, but may be modified within the scope of the appended claims.