Abstract:
A device for switching radio frequency signals that includes at least a first MOS transistor for selectively interrupting or transmitting the signals between at least first and second terminals as a function of a control signal. The gate of the first MOS transistor is connected to the first terminal, and both the drain and source of the first MOS transistor are connected to the second terminal. In a preferred embodiment, the first MOS transistor selectively interrupts or transmits the signals between the first terminal and the second terminal, and the device also includes a second MOS transistor for selectively interrupting or transmitting the signals between the first terminal and a third terminal. A system for processing radio frequency signals is also provided. The system includes at least one device for switching radio frequency signals, and the device includes at least a first MOS transistor for selectively interrupting or transmitting the signals between at least first and second terminals of the system as a function of a control signal. The gate of the first MOS transistor is connected to the first terminal, and both the drain and source of the first MOS transistor are connected to the second terminal.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is based upon and claims priority from prior French Patent Application No. 98-09930, filed Aug. 3, 1998, the entire disclosure of which is herein incorporated by reference. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to electronic devices, and more specifically to devices for switching radio frequency signals. 
     2. Description of Related Art 
     Devices for switching radio frequency signals are very important components in radio frequency systems that have multiple operating modes. Exemplary systems include reception and transmission systems, such as GSM telephones and radio frequency head remote controls. Such switching devices can be used to specify which path a radio frequency signal should follow according to the selected mode of operation. They can also be used to connect or disconnect system components to and from one another (e.g., switching an antenna between the receiver and the transmitter of a radio frequency device). They can also be used to enable or disable certain components as a function of the selected mode (e.g., an attenuator or an amplifier). 
     Conventional mechanical switching devices have satisfactory isolation performance but suffer from extremely slow switching speeds and very high costs. Conventional switching devices using gallium arsenide components perform satisfactorily but also suffer from very high costs. On the other hand, conventional switching devices based on MOS transistors integrated on an electronic chip occupy a considerable amount of space on the chip and also suffer from high costs while providing relatively poor performance. 
     A MOS transistor acting as a switch is normally connected so as to transmit signals between its drain and source, while being controlled by a voltage applied to its gate. If the voltage V gs  between the gate and the source is below a threshold voltage V t , the transistor is referred to as “off” and there is no conduction between the drain and source. If the voltage V gs  is above the threshold voltage V t , a conducting channel forms between the drain and the source to allow a signal to pass. However, the resistance between the drain and source of such a MOS transistor is proportional to the distance between the drain and source, and inversely proportional to the product of the width times the thickness between the drain and source. In order to obtain a drain-source resistance of low value, in particular the reference value of 50 ohms that constitutes the typical input resistance of the downstream circuits, it is necessary to provide a very wide channel zone. 
     SUMMARY OF THE INVENTION 
     In view of these drawbacks, it is an object of the present invention to overcome the above-mentioned drawbacks and to provide a switching device that has a more efficient and improved performance. 
     It is another object of the present invention to provide a switching device that is inexpensive. 
     Still another object of the present invention is to provide a switching device that takes up little space on an electronic chip. 
     One embodiment of the present invention provides a device for switching radio frequency signals. The device includes at least a first MOS transistor for selectively interrupting or transmitting the signals between at least first and second terminals as a function of a control signal. The gate of the first MOS transistor is connected to the first terminal, and both the drain and source of the first MOS transistor are connected to the second terminal. In a preferred embodiment, the first MOS transistor selectively interrupts or transmits the signals between the first terminal and the second terminal, and the device also includes a second MOS transistor for selectively interrupting or transmitting the signals between the first terminal and a third terminal. 
     Another embodiment of the present invention provides a system for processing radio frequency signals. The system includes at least one device for switching radio frequency signals, and the device includes at least a first MOS transistor for selectively interrupting or transmitting the signals between at least first and second terminals of the system as a function of a control signal. The gate of the first MOS transistor is connected to the first terminal, and both the drain and source of the first MOS transistor are connected to the second terminal. 
    
    
     Other objects, features, and advantages of the present invention will become apparent from the following detailed description. It should be understood, however, that the detailed description and specific examples, while indicating preferred embodiments of the present invention, are given by way of illustration only and various modifications may naturally be performed without deviating from the present invention. 
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a schematic diagram showing operation of a MOS transistor in the “on” mode according to a preferred embodiment of the present invention; 
     FIG. 2 is a schematic diagram showing operation of the MOS transistor in the “off” mode according to the preferred embodiment; 
     FIG. 3 is a schematic diagram of a switching device according to one embodiment of the present invention; 
     FIG. 4 is a schematic diagram of a switching device according to another embodiment of the present invention; 
     FIG. 5 shows a schematic diagram of a switching device according to yet another embodiment of the present invention; and 
     FIG. 6 shows a schematic diagram of a switching device according to still another embodiment of the present invention. 
    
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
     Preferred embodiments of the present invention will be described in detail hereinbelow with reference to the attached drawings. 
     FIG. 1 shows a MOS transistor for use in a switching device according to an embodiment of the present invention. The drain D and the source S are short-circuited. When an illustrative DC voltage of 3 volts is applied to the gate G and a zero voltage is applied to the source S, the voltage V gs  is above the threshold voltage V t , (which is in the order of 0.7-0.8 volts), so the MOS transistor is “on”. Furthermore, because the voltage V ds  between the drain D and source S is zero, the voltage V gs  is greater than the sum of the voltage V ds  and the voltage V t , so the MOS transistor operates in the “ohmic” region and behaves as a voltage-controlled resistor. 
     If this MOS transistor in the on state is modeled by a series RC circuit, the resistance is equal to the sum of the gate resistance and one quarter of the channel resistance. This is because the channel that is formed between the drain D and the source S and lying under the oxide layer separating the channel from the gate G has, in the on state, a resistance distributed linearly between the drain D and the source S. So long as this resistance is distributed, it can be modeled by assuming that the contact with the oxide takes place only in the middle between the drain D and source S, and that this midpoint sees a resistance value of R/2 to the source. 
     Because of the short-circuiting of the drain D and source S, the value of the resistance is therefore one quarter of the value of the resistance of the channel. Thus, the resistance in the on state is of low value. The capacitance in the on state essentially consists of the capacitance of the oxide layer arranged between the gate G and the channel, and thus is of high value. The result is that the impedance between the gate G and the source S is low and decreases with frequency. 
     FIG. 2 shows the transistor of FIG. 1 with a zero voltage applied to both the gate G and the source S. The voltage V gs  is zero and therefore below the threshold voltage V t , so the MOS transistor is “off.” Because the channel is not formed between the drain D and source S, the resistance in the off state is very high. The capacitance is formed only by the gate G on drain D and gate G on source S overlap capacitances, and is therefore of low value. The MOS transistor in the off state is therefore equivalent to an open switch. 
     FIG. 3 illustrates one embodiment of a switching device according to the present invention. A MOS transistor  1  has its gate G connected to a capacitor  2 , which is connected to a radio frequency signal source  3  (e.g., a portable telephone antenna). The gate G is also connected to a control circuit  4  containing an inductor  5  and a variable voltage source  6  arranged in series. The drain D and source S of the MOS transistor  1  are short-circuited and connected to another capacitor  8 , which is connected to a load (not shown) that is to receive the radio frequency signal originating from the source  3 . The drain D and source S are also connected to another control circuit  9  containing another inductor  10  and another variable voltage source  11  arranged in series. 
     The control circuits  4  and  9  operate in the following way. The generator  6  delivers either a zero voltage or a voltage of 3 volts. The generator  11  delivers either a zero voltage or a voltage such that the voltage V gs  is above the threshold voltage of the MOS transistor. When the two generators  6  and  11  deliver a zero voltage, the voltage V gs  is zero and the MOS transistor  1  is off. The MOS transistor  1  then has a high impedance to separate the radio frequency signals originating from the generator  3  from the load that is to receive such a signal. The capacitors  2  and  8  make it possible to isolate the other elements in the layout from the DC voltages delivered by the control circuits  4  and  9 . 
     When the generator  6  delivers a DC voltage of 3 volts and the generator  11  delivers a zero voltage, the voltage V gs  is equal to 3 volts and is therefore above the threshold voltage of the MOS transistor  1  to put it in the on state. The MOS transistor  1  then has a high capacitance and a low resistance so its impedance is low and decreases with the frequency of the signal passing through it. The radio frequency signal originating from the generator  3  (e.g., with a frequency of a few gigahertz) reaches the gate of the MOS transistor  1  via the capacitor  2 , which is of low impedance in relation to such a signal. 
     The signal is not attenuated by the inductor  5  because it behaves as an open circuit to a high frequency signal. The signal then passes through the MOS transistor  1  and the capacitor  8 , and is connected to the load. The inductor  10  at the output of the MOS transistor  1  also behaves as an open circuit to the radio frequency signal. Because of the difficulties of integrating inductors  5  and  10 , these elements can advantageously be replaced by resistors of sufficient values as to not attenuate the radio frequency signal. 
     FIG. 4 shows another embodiment of a switching device according to the present invention. Two MOS transistors  12  and  13  are provided, making it possible to direct the radio frequency signal originating from an antenna  14  to either a transmission circuit  15  or a reception circuit  16 . The gate of transistor  12  and the drain and the source of transistor  13  are connected to the antenna  14 . The drain and source of transistor  12  are connected to the transmission circuit  15 . The gate of transistor  13  is connected to the reception circuit  16 . 
     Transistors  12  and  13  are controlled by a control circuit  17  having two MOS transistors  18  and  19  that are mounted in series to form an inverter. The entire device is supplied by a DC voltage source  20 . The drain of transistor  18  is connected to the DC voltage source  20 . The source of transistor  18  and the drain of transistor  19  are connected together. The source of transistor  19  is connected to ground. The gates of transistors  18  and  19  are connected together and receive a control signal from a terminal  21  that selects either the transmission mode or the reception mode. The gates of transistors  18  and  19  are also connected via a resistor  22  to the antenna  14 . 
     The source of transistor  18  and the drain of transistor  19  are connected to the gates of two MOS transistors  23  and  24 . The drain of MOS transistor  23  is connected to the drain and source of transistor  12 . The source of transistor  23  is connected to ground. The drain of transistor  24  is connected to the DC voltage source  20 . The source of transistor  24  is connected to the gate of transistor  13 . A resistor  25  is arranged between the drain and source of transistor  12  and ground, and a resistor  26  is arranged between the gate of transistor  13  and the DC voltage source  20 . Optionally, the drain and source of transistor  12  and the gate of transistor  13  can be connected to offset circuits (not shown) via resistors  27  and  28  in order to make it possible to adjust the biassing of the transistors  12  and  13 . 
     This switching device operates in the following way. When a zero voltage is applied to terminal  21 , the gates of transistors  18  and  19  are subjected to a zero voltage, the gate of transistor  12  is subjected to a zero voltage and the drain and source of transistor  13  are subjected to a zero voltage. The voltage V gs  of transistor  19  is zero so it is off. Via resistor  22 , the gate of transistor  12  and the drain and source of transistor  13  are also subjected to a zero voltage. The voltage V gs  of transistor  12  is therefore zero so long as its source is connected to ground via resistor  25 . Transistor  12  is therefore off. Via resistor  26 , the gate of transistor  13  is subjected to the DC voltage from the source  20 . The voltage V gs , of transistor  13  is therefore above its threshold voltage so it is in the on state. 
     Because transistors  18  and  19  operate as an inverter, transistor  18  is on and its drain and source receive the same positive voltage originating from the source  20 . The gates of transistors  23  and  24  therefore also receive this positive voltage. The voltage V gs , of transistor  23  is above its threshold voltage so transistor  23  is on. The voltage V gs  of transistor  24  is zero so it is off. The result is that the radio frequency signal arriving via the antenna  14  can be transmitted by transistor  13  to the reception circuit  16 . Conversely, the signal is interrupted by transistor  12  between the antenna  14  and the transmission circuit  15 . The values of the various resistors  22  and  25  through  28  are selected with values high enough (e.g., above 1 kΩ) so as not to significantly attenuate the radio frequency signal. 
     When a positive voltage is applied from terminal  21  (e.g., a voltage equal to the supply voltage provided by the source  20 ), the operation is reversed. Transistor  19  is on and transistor  18  is off. The gates of transistors  23  and  24  therefore see a zero voltage, from which it follows that transistor  23  is off and transistor  24  is on. Because transistor  23  is off and the gate of transistor  12  receives a positive voltage via resistor  22 , the voltage V gs  of transistor  12  is positive and above its threshold voltage. Transistor  12  is therefore on. 
     Conversely, transistor  24 , whose gate is subjected to a zero voltage, is on. The gate of transistor  13  therefore receives a positive voltage, &amp; its source and drain receive a positive voltage. The voltage V gs  of transistor  13  is zero so it is off. The radio frequency signal originating from the transmission circuit  15  is therefore transmitted via the transistor  12  to the antenna  14 , whereas any signal transmission between the antenna  14  and the reception circuit  16  is prevented by the fact that transistor  13  is off. 
     FIG. 5 shows a variant of the switching device of FIG.  4 . Here, coupling capacitors (similar in their principle to capacitors  2  and  8  in FIG. 3) are provided. One capacitor  29  is provided between the antenna  14  and the point common to resistor  22 , the gate of transistor  12 , and the drain and source of transistor  13 . Another capacitor  30  is provided between the transmission circuit  15  and the point common to the drain and source of transistor  12 , the drain of transistor  23 , and resistor  25 . Another capacitor  31  is provided between the reception circuit  16  and the point common to the gate of transistor  13 , the source of transistor  24 , and resistor  26 . 
     Furthermore, resistors (e.g., of 50 ohms) are provided so that the circuits (not shown) connected to the switching device see such a (50 ohm) resistance. One resistor  32  is arranged between capacitor  29  and the antenna  14 , another resistor  33  is arranged between the common point of capacitor  30  and the transmission circuit  15  and ground and another resistor  34  is arranged between the point common to capacitor  31  and the reception circuit  16  and ground. 
     FIG. 6 shows a variant of the switching device of FIG. 5 that differs insofar as resistor  32  is arranged between the point common to capacitor  29  and the antenna  14  and ground, and resistor  33  is arranged in series between capacitor  30  and the transmission circuit  15 . 
     By virtue of such layouts of the MOS transistors, it is possible to use MOS transistors as controllable capacitors. Thus, it is possible to increase the switching speed in comparison with conventional circuits. In the on mode, only one quarter of the channel resistance is seen, so it is possible to either profit from a low value resistance or to reduce the area of silicon used. 
     While in the illustrated embodiments transistors  23  and  24  are presented in a conventional configuration, in further embodiments they could be connected as controlled capacitors in a similar fashion to transistors  12  and  13 . Furthermore, because either their drain or source is grounded, one of the two equivalent diodes formed between the drain and the substrate and between the source and the substrate is not involved, so there is a reduction in stray capacitances. Further, it is particularly beneficial to use a MOS transistor as a controlled capacitor insofar as it has a capacitance per unit area on the order of five times greater than that of an integrated capacitor. 
     Accordingly, the present invention provides a switching circuit for radio frequency signals that has a high switching speed, is inexpensive in terms of integrated circuit area, has performance that improves as the frequency of the switched signal increases, and can be adapted to improvements in MOS transistor technology. 
     While there has been illustrated and described what are presently considered to be the preferred embodiments of the present invention, it will be understood by those skilled in the art that various other modifications may be made, and equivalents may be substituted, without departing from the true scope of the present invention. Additionally, many modifications may be made to adapt a particular situation to the teachings of the present invention without departing from the central inventive concept described herein. Furthermore, an embodiment of the present invention may not include all of the features described above. Therefore, it is intended that the present invention not be limited to the particular embodiments disclosed, but that the invention include all embodiments falling within the scope of the appended claims.