Patent Publication Number: US-2016241231-A1

Title: RF Switch

Description:
TECHNICAL FIELD 
     The present application relates to a radio frequency (RF) switch and to corresponding devices. 
     BACKGROUND 
     Radio frequency (RF) switches are used to selectively open and close electrical connections used for radio frequency signals, sometimes also referred to as high frequency signals. Such radio frequency signals, for example, in mobile communication applications, may have frequencies exceeding 100 MHz, for example, in a range between 600 MHz and 5 GHz. 
     As RF switches, in many applications field effect transistors (FETs) are used. Also, PIN diodes are sometimes used. For various reasons, it may be desirable to also use bipolar junction transistors (BJTs) as RF switches. Previous approaches, for example, used a base emitter or base collector coupling for such a bipolar transistor based switch, i.e., an RF signal source and an RF signal destination to be selectively coupled via the switch were coupled to base and emitter or base and collector of a BJT, respectively. However, at least in some applications such a coupling via a base emitter diode or a base collector diode of a BJT may have a comparatively high damping and/or a comparatively low linearity. 
     SUMMARY 
     In the following, various embodiments will be described in detail referring to the attached drawings. It is to be noted that these embodiments serve illustrative purposes only and are not to be taken in a limiting sense. For example, while embodiments may be described as comprising a plurality of features or elements, in other embodiments some of these features or elements may be omitted, and/or may be replaced by alternative features or elements. In yet other embodiments, additional features or elements in addition to the ones explicitly described herein or shown in the drawings may be provided. Furthermore, features or elements from different embodiments may be combined to form further embodiments. Variations and modifications discussed with respect to one of the embodiments may also be applicable to other embodiments. 
     Any direct connection or coupling between elements or components shown in the drawings or described herein, i.e., a connection or coupling without intervening elements, may also be implemented by an indirect connection or coupling, i.e., a connection or coupling comprising one or more additional intervening elements, and vice versa, as long as the general function and/or purpose of the connection or coupling, for example, to transmit a certain kind of signal or to transmit a certain kind of information, is essentially maintained. Any directional references made when describing the figures like “left”, “right” etc. are given merely for ease of reference to various parts of the figures and is not to be construed as implying any particular spatial arrangement of the elements or components described. 
     In some embodiments, a collector-emitter coupling of a bipolar junction transistor (BJT) is used for switching radio frequency (RF) signals, for example, RF signals having a frequency exceeding 100 MHz, for example, between 600 MHz and 5 GHz. 
     In some embodiments, a closing and opening of the switch may be controlled by supplying a base current to a base terminal of the bipolar junction transistor. 
     In some embodiments, capacitances may be coupled to the collector and emitter terminals to block direct current (DC) components. 
     In some embodiments, the BJT may be operated in a forward reverse saturation region. 
     Generally, a BJT in the context of the present application may be described as “open” or “off” when it is essentially non-conducting between its collector and emitter terminals, and may be described as “closed” or “on” when it is conducting RF signals between its collector and emitter terminals. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       For a more complete understanding of the present invention, and the advantages thereof, reference is now made to the following descriptions taken in conjunction with the accompanying drawings, in which: 
         FIG. 1  is a block diagram illustrating a device according to an embodiment; 
         FIG. 2  is a diagram illustrating a bipolar junction transistor usable in embodiments; 
         FIGS. 3 and 4  show characteristic curves for bipolar junction transistors to illustrate operation of some embodiments; 
         FIG. 5  illustrates a small signal equivalent circuit of a bipolar junction transistor in an off state; 
         FIG. 6  illustrates a small signal equivalent circuit of a bipolar junction transistor in an on state; 
         FIG. 7  illustrates a circuit diagram of a device according to an embodiment; 
         FIG. 8  illustrates a circuit diagram of a device according to an embodiment; 
         FIG. 9  illustrates a circuit diagram of a device according to an embodiment; 
         FIG. 10  illustrates a circuit diagram of a device according to an embodiment; and 
         FIG. 11  illustrates a circuit diagram of a device according to an embodiment. 
     
    
    
     DETAILED DESCRIPTION OF ILLUSTRATIVE EMBODIMENTS 
     Turning now to the figures,  FIG. 1  illustrates a device according to an embodiment. The device  10  of  FIG. 1  comprises a bipolar switch device  11 , the bipolar switch device comprising a bipolar junction transistor (BJT) and optionally additional elements like capacitors or resistors coupled to the BJT. 
     An RF signal source  12  is coupled to one of a collector (C) or emitter (E) terminal of the BJT of bipolar switch device  11 , and an RF signal destination  13  is coupled to the other one of collector and emitter of the BJT of bipolar switch device  11 . RF signal source  12  may be any kind of circuit generating an RF signal. By selectively opening and closing bipolar switch device  11  and in particular the BJT thereof, the RF signal may be selectively provided to RF signal destination  13 . RF signal destination  13  may, for example, be a circuit receiving the RF signal, but may also be, for example, a fixed potential like ground. In the latter case, bipolar switch device  11  may serve to selectively shunt the RF signal to ground, just to give an example. 
     Bipolar switch device  11  is controlled by a controller  14 . In embodiments, controller  14  may serve as a base current supply circuit to selectively provide a base current to a base terminal (B) of the BJT of bipolar switch device  11 . In some embodiments, as will be explained later using examples, for enabling flowing of the base current an emitter terminal of the BJT of bipolar switch device  11  may be coupled to a reference potential like ground via a resistor or another impedance. 
     Example implementations of bipolar switch device  11  usable in some embodiments will be discussed later with reference to  FIGS. 7-11 . For a better understanding, before describing various example implementations in detail, with reference to  FIGS. 2-6  various properties of bipolar junction transistors usable in embodiments will be explained. 
       FIG. 2  illustrates a bipolar junction transistor (BJT)  26  used for illustration of various features of various embodiments later. The bipolar junction transistor  26  in the example represented in  FIG. 2  is an NPN transistor. However, concepts and techniques disclosed herein may also be applied to PNP transistors. NPN and PNP may also be referred to as polarities of the transition. 
     BJT  26  in the embodiment of  FIG. 2  comprises a collector terminal  20 , a base terminal  21  and an emitter terminal  22 . An arrow  23  represents a collector-emitter voltage V CE , an arrow  25  represents a base-emitter voltage V BE  and an arrow  25  represents a base current I B . Voltages V CE , V BE  and base current I B  will be used later for explanatory purposes. 
     In some embodiments, to be used as an RF switch a bipolar transistor like the bipolar transistor shown in  FIG. 2  is operated in a forward saturation range or reverse saturation range. In some embodiments, a current consumption in the reverse saturation range may be lower than in the forward saturation range. To improve understanding of the embodiments described later, these modes of operation will be discussed later. 
     As already mentioned, embodiments use a collector-emitter coupling for switching, for example, as illustrated in  FIG. 1 , where an RF signal source is coupled to one of a collector terminal or emitter terminal, and an RF signal destination is coupled to the other one of collector terminal and emitter terminal. A base current may be used to control the coupling, for example, to open and close the switch. In embodiments, using such a collector-emitter coupling may enable a realization of a highly linear and/or low loss switch in bipolar technology. 
     In embodiments, an emitter terminal of a BJT used (for example, emitter terminal  22  of  FIG. 2 ) may be coupled with a reference potential (for example, ground) via a resistor. Optionally, such a coupling to a reference potential may also be made for the collector terminal. In other embodiments, a coupling may be made to another blocking impedance like an external blocking coil. An RF signal may be coupled to collector and/or emitter terminals via capacitances blocking DC components. In such a case, the above-mentioned coupling of the emitter to a reference potential may enable the base current to flow via the base to the reference potential. Depending on a base current I B  used, the BJT (for example, BJT  26 ) may be set to a forward or reverse saturation mode of operation. An operating point in this respect may depend on external circuitry coupled to the bipolar transistor. For example, in a case where the collector is not coupled to further circuitry, for example, a forward saturation mode of operation may be obtained. In case the collector is coupled to further circuitry, a reverse saturation mode of operation may be obtained where base emitter and base collector diodes are forward biased diodes and a negative collector current results. 
     BJT  26  may be implemented based on silicon, but may be also implemented based on other materials and/or using heterostructures, for example, heterostructures comprising at least two materials selected from the group of Si, SiGe, SiC, and SiGeC. BJT  26  may, for example, be implemented as a heterojunction bipolar transistor (HBT). 
     A low ohmic connection between collector and emitter terminals of a BJT in a reverse saturation mode of operation, i.e., a closing of the switch, may be realized as follows: 
     A certain base current I B  is provided to the base-emitter diode of the transistor (for example, transistor  26 ). This base current results from an injection of minority carriers, i.e., of holes injected from base to emitter and from electrons injected from emitter to base. Such a base current may, for example, be caused by applying a certain base-emitter voltage V BE , as indicated by arrow  24  of  FIG. 2 . In embodiments, a base region of the transistor (for example, in case of a HBT) is so thin that the injected electrons may diffuse to a space charge region of a collector-base diode before recombining in the base. 
     In a scenario as described above, as an operating point a collector-emitter voltage V CE  is established, which in embodiments may be smaller than 10 mV. In the situation described so far, a collector current does not necessarily flow. However, in reverse operation a direct current smaller than 0 occurs. 
     In the use as an RF switch as an embodiment, when, for example, an alternating current (AC) signal (for example, an RF signal) is applied to the collector (for example, collector terminal  20  of  FIG. 2 ), because of the potential difference between collector and emitter electrons are provided from a collector-base space charge region to the collector. Due to this, the emitter follows the collector potential. In a reverse situation, an AC signal at the emitter leads to a collector potential (voltage) following this RC signal. Therefore, AC signals like RF signals may be transmitted from collector to emitter and vice versa. 
     The base current I B  which is a DC current, determines the properties of the collector-emitter coupling. The more the transistor is operated in saturation (irrespective of forward- or reverse operation), the more low ohmic is the collector-emitter coupling. 
     To illustrate this behavior further,  FIGS. 3 and 4  show characteristic curves of a heterojunction bipolar transistor usable in embodiments.  FIGS. 3 and 4  illustrate an collector current I C  in mA versus a collector-emitter voltage V CE  in V for different base currents ranging from 50 μA to 200 μA.  FIG. 4  shows an enlarged view of a part of  FIG. 3 , in particular a part around 0 V/0 A. 
     As can be seen, a higher base current leads to a lower ohmic collector-emitter coupling (higher current I C  for the same voltage V CE ). The behavior in forward saturation may be seen in the first quadrant (positive V CE , positive I C ); saturation starts at between about 0.2 V and 0.5 V, depending on the base current. Reverse saturation may be seen between about −0.1 V and −0.7 V, before the onset of reverse breakthrough. 
     In summary, both modes of operation (forward saturation and reverse saturation) which may be used in embodiments may be described as follows: A base emitter and a base collector diode are operated in forward bias, and between collector and emitter there is a low ohmic coupling. 
     In many applications, a collector-emitter voltage (V CE ) may be small, for example, smaller 10 mV. In such a case, for simplification purposes as an approximation a parallel coupling of the base-collector diode and base-emitter diode for reverse saturation mode may be assumed. 
     To illustrate this further,  FIGS. 5 and 6  illustrate small signal equivalent circuits of bipolar junction transistors like heterojunction bipolar transistors usable in some embodiments. 
       FIG. 5  illustrates a small signal equivalent circuit for an off state (open state) of the transistor. In this case, only depletion layer capacitances  53  and  54  of the base-collector diode and the base-emitter diode, respectively, are essentially to be taken into account. Numeral  50  designates a collector terminal, numeral  51  designates an emitter terminal and numeral  52  designates a base terminal of the transistor. In many applications, a capacitance CBC 0  of capacitance  53  representing the base collector diode is smaller than a capacitance CBE 0  of the base emitter diode  54 . For good isolation properties in the off state for RF applications, a low capacitance is desirable. Therefore, in embodiments the base collector diode and its capacitance  53  predominantly contribute to the isolation properties in the off state in embodiments. 
       FIG. 6  illustrates a small signal equivalent circuit for a bipolar junction transistor in an on state (closed state).  60  designates a collector terminal,  61  designates an emitter terminal and  62  designates a base terminal. 
     A base-collector diode in a closed state is represented by a non-linear diffusion capacitor  62  (CBCd), a depletion layer capacitor  63  (CBCi) and a non-linear current source  64  (ibc). Similarly, a base-emitter diode is represented by a non-linear diffusion capacitor  67  (CBEd), a depletion layer capacitor  66  (CBEi) and a non-linear current source  65  (ibe). Furthermore, the equivalent circuit of  FIG. 6  comprises a resistor  68  coupled between collector terminal  60  and emitter terminal  61 . A conductance value gce of resistor  68  is a function of the base current I b , as can be seen from  FIGS. 3 and 4 . 
     In forward saturation, essentially only the base emitter diode is active. In reverse saturation, both diodes are active. 
     For example, based on the small signal equivalent circuit of  FIGS. 5 and 6 , in some embodiments a highly linear switch for RF signals may be realized which has low losses. Such a switch may, for example, be adapted to RF signals having comparatively low power. In such embodiments, a collector-emitter path of a bipolar junction transistor is used for a selective coupling, for example, as illustrated with reference to  FIG. 1 . The small signal circuit of  FIG. 6  also illustrates the operation of the collector-emitter coupling, e.g., that a signal at emitter terminal  61  follows a signal at collector terminal  60  and vice versa (e.g., due to resistor  68 ). 
     The collector terminal of such a transistor in embodiments may be coupled to a remaining circuit at a location where the remaining circuit is least loaded. When the transistor is switched off, referring to  FIG. 5 , for example, only capacitor  53  acts as a load to the remaining circuit, capacitor  53  as explained having a lower capacitance in embodiments than capacitor  54 . This may, for example, reduce an overall load on the circuit. 
       FIG. 7  illustrates a circuit diagram of a switch device according to an embodiment. The switch device of  FIG. 7  comprises a first terminal  70  and a second terminal  76 . The switch device of  FIG. 7  is adapted to selectively provide a radio frequency coupling between terminals  70 ,  76  (i.e., to selectively provide either a low ohmic path for radio frequency signals or a high ohmic, essentially isolating, path for radio frequency signals). To provide such a switching, the switch device of  FIG. 7  comprises a bipolar junction transistor  74 , for example, a heterojunction bipolar transistor. An emitter terminal of transistor  74  is coupled to terminal  70  via a capacitor  71 , and a collector terminal of transistor  74  is coupled to terminal  76  via a capacitor  75 . Capacitors  71 ,  75  serve to block, for example, DC components of signals at terminal  70  or  76 . Therefore, in a DC case transistor  74  is essentially floating between terminals  70 ,  76  and receives only AC signals, in particular RF signals, from terminal  70  or terminal  76 . 
     Furthermore, the emitter terminal of transistor  74  is coupled to ground via a resistor  72 . A base terminal of transistor  74  is coupled to a positive supply voltage  78  via a resistor  73  and a switch  77 . Resistor  73  and switch  77  are examples of a base current supply circuit. When switch  77  is closed, a base current Ibias flows setting the transistor  74  to an on state (closed state), thus enabling the transmission of RF signals from terminal  70  to terminal  76  or vice versa. When switch  77  is open, no base current flows, which effectively decouples terminal  70  from terminal  76 . 
     Resistors  73 ,  72  may set an operation point, in particular may determine a magnitude of a base current. Furthermore, resistors  72 ,  73  serve as blocking resistors that prevent that a significant portion of the RF signal is coupled to ground, thus keeping losses of the switch low in embodiments. A resistance value of resistor  72 ,  73  each may be 50Ω or more, but is not limited thereto. 
     In addition to the resistors shown, in further embodiments, also a further resistor coupling a collector terminal of transistor  74  to ground may be provided. In other embodiments, instead of one or more of the resistors, other impedances like a blocking inductivity may be used. 
     A magnitude of the base current Ibias of  FIG. 7  may be 5 mA or less, for example, 100 μA or less, but is not limited thereto. While  FIG. 7  shows a switch device using an NPN transistor  74 , in other embodiments a PNP transistor may be used, for example, by reversing the polarities involved. 
     In some embodiments, to improve transmission behavior of the switch device, a capacitive base-emitter coupling may be used. An example for such a capacitive base-emitter coupling will be illustrated later with respect to  FIG. 9 . 
     Further elements which are not explicitly shown in  FIG. 7  may also be used, for example, a biasing or clamping to increase the isolation in an off state of the switch device. 
     Next, with reference to  FIGS. 8-11 , further switch devices will be used, which at least in part have additional elements or features compared to the embodiment of  FIG. 7 . 
       FIG. 8  illustrates a switch device according to a further embodiment which may, for example, be used as a bypass switch. A bypass switch is generally to be understood as a switch which selectively couples two nodes of a circuit, thus bypassing circuitry provided between the two nodes when the switch is closed. 
     The switch device of  FIG. 8  comprises a first terminal  80  and a second terminal  81  which by means of the switch device are selectively coupled with each other. As switching elements, the switch device of  FIG. 8  comprises two bipolar transistors  83 ,  84 . Base terminals of transistors  83 ,  84  may be provided a base current Ibias via a resistor  82 , resistor  82  having essentially the same function as resistor  73  of  FIG. 7 . A collector terminal of transistor  83  is coupled with terminal  80 , and a collector terminal of transistor  84  is coupled with terminal  81 . Emitter terminals of transistors  83 ,  84  are coupled with each other. Furthermore, the emitter terminals of transistors  83 ,  84  are coupled to ground via a resistor  87 , which essentially has the same function as resistor  72  of  FIG. 7 . 
     Furthermore, collector terminal of transistor  83  is coupled to ground via a resistor  85 , and the collector terminal of transistor  84  is coupled to ground via a resistor  86 . Resistors  85 ,  86  may be dimensioned similar to resistor  87  and serve for adjusting a point of operation and as blocking resistors, similar as explained for resistors  72 ,  73  of  FIG. 8 . 
     By providing two transistors  83 ,  84 , a damping introduced by the switch device may be increased compared to a case where one transition is used. On the other hand, by providing two transistors  83 ,  84  with a coupling as shown, in some embodiments a linearity may be increased. For example, some bipolar transistors like heterojunction bipolar transistors may have an asymmetric structure, thus leading to different transfer behavior from collector to emitter and from emitter to collector. With a coupling as illustrated in  FIG. 8 , symmetry is increased. Furthermore, in some embodiments, by coupling collector terminals of transistors  82 ,  84 , to terminals  80 ,  81 , a load to a circuit connected to the switch device may be decreased, as explained before a capacitance of the base-collector diode in an off state may be lower than a capacitance of a base-emitter diode. 
     While not explicitly shown in  FIG. 8 , similar to the embodiment of  FIG. 7  capacitors may be provided between terminal  80  and the collector terminal of transistor  83  and/or between terminal  81  and the collector terminal of transistor  84 . 
       FIG. 9  illustrates a further embodiment of a switch device. The switch device of  FIG. 9  comprises two input terminals  90 ,  99  and an output terminal  911 . Via bipolar transistors  913 ,  914 , input terminals  90 ,  99  may selectively be coupled to output terminal  911  for transmitting RF signals. 
     An emitter terminal of transistor  913  is coupled to terminal  90  via a capacitor  91 , capacitor  91  serving to block DC components (similar to capacitors  71 ,  75  of  FIG. 7 ). An emitter terminal of transistor  914  is coupled to terminal  99  via a capacitor  98 , also to block DC components. Collector terminals of transistors  913 ,  914  are coupled to output terminal  911 . 
     A base terminal of transistor  913  is coupled to a supply voltage VCC via a resistor  93  and a switch  94 , which have the same function as resistor  73  and switch  77 , respectively, of  FIG. 7 , i.e., to selectively supply a base current to transistor  913  to switch transistor  913  on and off. Likewise, a base terminal of transistor  914  is coupled to a supply voltage VCC via a resistor  96  and a switch  95  to selectively supply a base current to transistor  914 , to selectively switch transistor  914  on and off. 
     Furthermore, the emitter terminal of transistor  913  is coupled to ground via a resistor  910 , and the emitter terminal of transistor  914  is coupled to ground via a resistor  912 . Resistors  910 ,  912  essentially serve the same function as already explained for resistor  72  of  FIG. 7  and may be dimensioned in a similar manner, for example, they have a resistance value greater than 50Ω Optionally (not shown in  FIG. 9 ) the collector terminals of transistors  913 ,  914  may be coupled to ground via a further resistor (like resistors  85 ,  86  of  FIG. 8 ). 
     Additionally, in the embodiment of  FIG. 9 , the base terminal and emitter terminal of transistor  913  are coupled by a capacitor  92 , and the base terminal and the emitter terminal of transistor  914  are coupled via a capacitor  97 . Capacitors  92 ,  97  in some embodiments may serve to optimize a transmission behavior of the respective transistor, for example, to reduce a non-linearity. In particular, capacitors  92 ,  97  may improve a large signal behavior of the switch device. In other embodiments, capacitors  92 ,  97  may be omitted. 
       FIG. 10  illustrates a further embodiment of a switch device. The switch device of  FIG. 10  selectively provides a coupling between terminals  100 ,  108 . As switching elements, two bipolar transistors  105 ,  106  having an anti-parallel coupling as shown are provided. An emitter terminal of transistor  105  and a collector terminal of transistor  106  are coupled to terminal  100  via a capacitor  101 , and a collector terminal of transistor  105  and an emitter terminal of transistor  106  are coupled to terminal  108  via a capacitor  107 . Capacitors  101 ,  107  serve to block DC components, similar to capacitors  71 ,  75  of  FIG. 7 . 
     Moreover, the emitter terminal of transistor  105  and the collector terminal of transistor  106  are coupled to ground via a resistor  109 , and the collector terminal of transistor  105  and the emitter terminal of transistor  106  are coupled to ground via a resistor  1010 . Resistors  109 ,  1010  essentially serve the same function as resistor  72  of  FIG. 7  and may have a resistance value exceeding 100Ω. 
     A base terminal of transistor  105  is coupled to a supply voltage VCC via a resistor  103  and a switch  102 , and a base terminal of transistor  106  is coupled to the positive supply voltage VCC via a resistor  104  and switch  102 . 
     By closing switch  102 , transistors  105 ,  106  are supplied with a base current Ibias via resistors  103 ,  104 , respectively, thus switching transistors  105 ,  106  on. Resistors  103 ,  104  essentially serve the same function as resistor  73  of  FIG. 7 . While two resistors  103 ,  104  are shown in  FIG. 10 , in other embodiments transistors  105 ,  106  may receive a bias current via the same resistor. 
     By providing two transistors  105 ,  106  with an anti-parallel coupling as illustrated in  FIG. 10 , in embodiments a large signal behavior and a symmetry may be improved, as essentially each of the transistors is “responsible” for transmission of a half wave. For example, by providing two transistors in case of asymmetrically implemented transistors (like some HBTs) such an asymmetry may be compensated. 
       FIG. 11  illustrates a switch device usable as a transfer gate. The embodiment of  FIG. 11  comprises a first terminal  110  and a second terminal  118 . As switching elements, an NPN bipolar junction transistor  114  and a PNP bipolar junction transistor  115  are provided. Emitter terminals of transistors  114 ,  115  are coupled to terminal  110  via a capacitor  111 . Collector terminals of transistors  114 ,  115  are coupled to terminal  118  via a capacitor  117 . Capacitors  111 ,  117  may serve to block DC components. 
     Furthermore, a base terminal of transistor  114  is coupled to a positive supply voltage VCC via a resistor  113  and a switch  112 . A base terminal of transistor  115  is coupled to ground via a resistor  116 . When switch  112  is closed, a base current Ibias flows via resistor  113  to the base terminal of NPN transistor  114  and from the base terminal of resistor  115  via resistor  116  to ground, thus switching transistors  114 ,  115  to an on state, enabling RF signal transmission from terminal  110  to terminal  118  and vice versa. 
     It should be noted that depending on a transfer frequency of PNP transistor  115 , an operating frequency of the device of  FIG. 11  may be limited. The embodiment of  FIG. 11  may have a good large signal behavior, for example, high linearity. Transistors  114 ,  115  may be implemented by stacking the two transistors, which, for example, may lead to a stacking of two base emitter diodes. 
     In view of the many variations and modifications of switch device described above, it is apparent that the techniques disclosed herein are not limited to any particular embodiment, and the embodiments illustrated are given by way of example only. 
     While this invention has been described with reference to illustrative embodiments, this description is not intended to be construed in a limiting sense. Various modifications and combinations of the illustrative embodiments, as well as other embodiments of the invention, will be apparent to persons skilled in the art upon reference to the description. It is therefore intended that the appended claims encompass any such modifications or embodiments.