Abstract:
A metal oxide semiconductor radio frequency transmit/receive switch may enable lower costs and smaller size. The switch uses an inductor and a capacitor circuit to isolate the power amplifier from the low noise amplifier. Metal oxide semiconductor switches are utilized to switch between transmit and receive modes.

Description:
BACKGROUND 
       [0001]    This relates generally to transceivers for radio frequency communications. 
         [0002]    A transceiver allows both transmission and reception of radio frequency signals. Generally, this bidirectional traffic is facilitated by a radio frequency transmit/receive switch. The switch switches between transmission and reception using the same antenna. In transmission, the signal to the antenna comes from a power amplifier (PA). In reception, the antenna feeds a low noise amplifier (LNA). 
         [0003]    Radio frequency transmit/receive switches may be made with gallium arsenide metal-semiconductor field effect transistor (MESFETs) or Pseudomorphic High Electron Mobility Transistor (PHEMT) devices with superior performance using semi-insulating substrates with high quality passive elements and substrate vias to ground. These devices may have relatively high (greater than 1.5 volts) breakdown voltages in such applications. 
         [0004]    The power amplifier may have relatively high voltage swings, suggesting the use of transistors in series with the antenna node, with large voltage standoffs or voltage blocking. Typically, such transistors may be gallium arsenide MESFETs and PHEMT devices with higher breakdown voltages. The higher breakdown voltage devices have higher on resistance. The losses due to higher on resistance hurt the noise figure and sensitivity of the receiver and the low noise amplifier, in particular. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0005]      FIG. 1  is a circuit schematic for one embodiment of the present invention; and 
           [0006]      FIG. 2  is a more detailed circuit schematic for another embodiment of the present invention. 
       
    
    
     DETAILED DESCRIPTION 
       [0007]    Referring to  FIG. 1 , a power amplifier  11  is coupled to a radio frequency transmit/receive switch  10 . The switch  10  is coupled to an antenna through an antenna node (to ANT) and to a low noise amplifier (to LNA). The transceiver may be part of a mobile radio, a cellular telephone, or a personal computer, to mention a few examples. 
         [0008]    The radio frequency transmit/receive switch  10  includes a pass transistor  12  and a parallel resonant circuit including an inductor  18 , a capacitor  20 , and transistors  22  and  24 . The inductor  18  is in parallel with the capacitor  20  that is in series with the transistor  22 . The parallel resonant network is in series with a low noise amplifier (LNA) and receiver (not shown in  FIG. 1 ). Neither of the transistors  22  or  24  is in the series path to the low noise amplifier when the low noise amplifier is operational. 
         [0009]    All of the transistors may be made with complementary metal oxide semiconductor (CMOS) technology, in one embodiment. In such case, each of the transistors may be an n-channel metal oxide semiconductor field effect transistor (MOSFET). However, in some embodiments, other technologies may be used to form switches, including gallium arsenide MESFET or PHEMT technologies. 
         [0010]    The inductor  18  may be the input matching inductor for the low noise amplifier (LNA). This use of the inductor  18 , as both a switch and a matching element, avoids the need for additional elements that would otherwise require additional die area and would result in additional losses. 
         [0011]    In the transmission mode, the voltage on the nodes Tx_ON and Rx_OFF is high, turning on the transistors  12 ,  22 , and  24 . With the transistor  22  on, the inductor  18  resonates with the capacitor  20  at a specified frequency to form a high impedance, isolating the antenna node (to ANT) from the LNA input (to LNA). 
         [0012]    The transistor  24  is also turned on or in low impedance, acting as a shunt switch. The transistor  24  provides additional attenuation and isolation at the LNA input node which the transistor  24  pulls to ground. If any signal leaks through the parallel resonant circuit, it is attenuated by the switch  24 . Together, the parallel resonant circuit and transistor  24  form a voltage divider that acts as an attenuator. 
         [0013]    The required voltage standoff of transistors  22  and  24  may be small in some embodiments. This small stand off enables the use of short gate length low voltage (i.e., about 1.5 volts or less) devices, which have lower on-resistance and use less die area. 
         [0014]    The transistor  12  may be a pass transistor for the power amplifier (PA)  11  in the transmit mode, whose reliability and linearity may be ensured by using a large gate resistor  14  and floating/remote bulk connection, as indicated, in some embodiments. This may make the AC voltage on the gate of the transistor  12  and the bulk nodes bootstrapped to the voltage on the source and drain nodes of the transistor  12 . The transistor  12  may also be a MOSFET transistor. The transistor  12  may have body contacts that are spaced away from the transistor to form remote body contacts or a floating bulk. The transistors  12 ,  20 , and  24  may be low voltage MOSFET devices having breakdown voltages on the order of 1.5 volts or less and generally of the same magnitude as the supply voltage. 
         [0015]    The resulting peak power handling capability may be 20 dBm in some embodiments. The simulated insertion loss in the transmit mode may be about 0.4 dB in some embodiments. 
         [0016]    In the receive mode, the voltage on nodes Tx_ON and Rx_OFF is low, turning off the transistors  12 ,  22 , and  24 . Thus, there may be no significant losses between the antenna and the low noise amplifier input. Simulated insertion loss and increased noise figure for this embodiment may be 0.1 dB. This is much less and in sharp contrast to conventional radio frequency switches that typically have a series transistor between the low noise amplifier input and the antenna, which have much greater insertion loss, typically on the order of 1 dB. 
         [0017]    Referring to  FIG. 2 , a transceiver includes both the switch  10  and the low noise amplifier  10   a . The floating/remote bulk connection for the transistor  12  is chosen by developing an equivalent model of the substrate from field simulations for various layout conditions. The low noise amplifier design and tuning may be similar to a conventional source degenerated, series-tuned cascode amplifier. 
         [0018]    The low noise amplifier includes the transistors  34  and  36 , a resistance  30 , switches  28  and  38 , a capacitor  26 , and an inductor  32 . When the LNA  10   a  is off, switches  28  and  38  are set to ground to disable transistors  34  and  36 , achieving greater isolation. The transistors  34  and  36 , as well as the switches  28  and  38 , may be NMOS transistors in a CMOS technology in one embodiment. The drain of the transistor  36  may be coupled to the rest of the receiver section that may include another amplifier stage or mixer, as two examples. 
         [0019]    When the LNA  10   a  is off, it is preferable that it does not draw current. If the gate of transistor  34  is DC coupled to ground by the switch  28 , then the drain current of the transistor  34  should be essentially zero. Connecting the gate of transistor  36  to ground helps to ensure that the whole chain of transistors  34  and  36  does not conduct or pass any signal when the switch  10  is in the transmit mode. 
         [0020]    The capacitor  26 , between the parallel resonant network and the low noise amplifier  10   a , provides DC blocking. It is also part of a matching network of the low noise amplifier  10   a , in one embodiment. 
         [0021]    In some embodiments of the present invention, lower cost and lower power consumption, as well as very low insertion loss and large power handling capability, can be achieved. In some embodiments, the need for a front end module may be eliminated or relaxed, lowering costs. In some embodiments, the radio frequency transmit/receive switch may be integrated on the same circuit with all or additional parts of the transceiver, such as the low noise amplifier is depicted in  FIG. 2 . In some embodiments, relatively small, low breakdown voltage (1.5 volts or less), inexpensive, MOSFET transistors may be used to fabricate the switch  10 . In some cases, the entire transceiver can be formed using low voltage CMOS technology. However, PMOS transistors may also be used as well. NMOS and PMOS transistors can be used separately or together to implement any of the switches described herein. 
         [0022]    References throughout this specification to “one embodiment” or “an embodiment” mean that a particular feature, structure, or characteristic described in connection with the embodiment is included in at least one implementation encompassed within the present invention. Thus, appearances of the phrase “one embodiment” or “in an embodiment” are not necessarily referring to the same embodiment. Furthermore, the particular features, structures, or characteristics may be instituted in other suitable forms other than the particular embodiment illustrated and all such forms may be encompassed within the claims of the present application. 
         [0023]    While the present invention has been described with respect to a limited number of embodiments, those skilled in the art will appreciate numerous modifications and variations therefrom. It is intended that the appended claims cover all such modifications and variations as fall within the true spirit and scope of this present invention.