The present application relates to a radio frequency (RF) switch and to corresponding devices.
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.
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.
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:
Turning now to the figures,
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
BJT 26 in the embodiment of
In some embodiments, to be used as an RF switch a bipolar transistor like the bipolar transistor shown in
As already mentioned, embodiments use a collector-emitter coupling for switching, for example, as illustrated in
In embodiments, an emitter terminal of a BJT used (for example, emitter terminal 22 of
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 IB 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 VBE, as indicated by arrow 24 of
In a scenario as described above, as an operating point a collector-emitter voltage VCE 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
The base current IB 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,
As can be seen, a higher base current leads to a lower ohmic collector-emitter coupling (higher current IC for the same voltage VCE). The behavior in forward saturation may be seen in the first quadrant (positive VCE, positive IC); 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 (VCE) 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,
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
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
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
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
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
Further elements which are not explicitly shown in
Next, with reference to
The switch device of
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
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
While not explicitly shown in
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
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
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
Additionally, in the embodiment of
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
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
By providing two transistors 105, 106 with an anti-parallel coupling as illustrated in
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
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.