Abstract:
Embodiments of circuits, methods and systems for a voltage-controlled current source are disclosed. In some embodiments, the voltage-controlled current source may be a three-terminal device having separated gate structures. Other embodiments may also be described and claimed.

Description:
FIELD 
       [0001]    Embodiments of the present disclosure relate generally to the field of current sources, and more particularly to linear voltage-controlled current sources. 
       BACKGROUND 
       [0002]    In semiconductor integrated circuits, a current source provides a reference current in order to design transistor bias networks that are insensitive to supply voltage, temperature, and process variations. If the current through a current source can be specified independently of any other variable in a circuit, it is called an independent current source. Conversely, if the current through a current source is determined by some other voltage in a circuit, it is called a voltage-controlled current source (VCCS). 
         [0003]    In most applications, it is desirable to have a linear relationship between the current and the control voltage. However, stand-alone conventional devices do not function as linear VCCSs. For example, in a bipolar junction transistor (BJT) device, the collector current is an exponential function of the base voltage, while in a field-effect transistor (FET) device, the drain current is a power function of the gate voltage. In both cases, the current is a strong function of the control voltage, which makes it unsuitable for linear applications. 
         [0004]    Conventionally, complicated circuits using operational amplifiers are required to make a linear VCCS. Such circuits consume extra direct-current (DC) power and are typically large in size. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0005]    Embodiments are illustrated by way of example and not by way of limitation in the figures of the accompanying drawings, in which like references indicate similar elements. 
           [0006]      FIGS. 1 and 2  respectively illustrate a top view and a cross-sectional side view of a voltage-controlled current source (VCCS) in accordance with an embodiment. 
           [0007]      FIG. 3  is a chart that plots drain-to-source (DTS) current as a function of DTS voltage in a VCCS of an embodiment. 
           [0008]      FIG. 4  is a chart that plots DTS current as a function of control voltage for a number of VCCSs of an embodiment. 
           [0009]      FIG. 5  is a chart that presents statistical analyses of the data of  FIG. 4  in accordance with an embodiment. 
           [0010]      FIG. 6  is a power amplifier duplexer incorporating a VCCS in accordance with an embodiment. 
           [0011]      FIG. 7  is a wireless communication device incorporating a power amplifier duplexer in accordance with an embodiment. 
       
    
    
     DETAILED DESCRIPTION 
       [0012]    Various aspects of the illustrative embodiments will be described using terms commonly employed by those skilled in the art to convey the substance of their work to others skilled in the art. However, it will be apparent to those skilled in the art that alternate embodiments may be practiced with only some of the described aspects. For purposes of explanation, specific devices and configurations are set forth in order to provide a thorough understanding of the illustrative embodiments. However, it will be apparent to one skilled in the art that alternate embodiments may be practiced without the specific details. In other instances, well-known features are omitted or simplified in order not to obscure the illustrative embodiments. 
         [0013]    Further, various operations will be described as multiple discrete operations, in turn, in a manner that is most helpful in understanding the present disclosure; however, the order of description should not be construed as to imply that these operations are necessarily order dependent. In particular, these operations need not be performed in the order of presentation. 
         [0014]    The phrase “in various embodiments” is used repeatedly. The phrase generally does not refer to the same embodiments; however, it may. The terms “comprising,” “having,” and “including” are synonymous, unless the context dictates otherwise. 
         [0015]    In providing some clarifying context to language that may be used in connection with various embodiments, the phrases “NB” and “A and/or B” mean (A), (B), or (A and B); and the phrase “A, B, and/or C” means (A), (B), (C), (A and B), (A and C), (B and C), or (A, B and C). 
         [0016]    As used herein, “coupled with” may mean either one or both of the following: a direct coupling or connection, where there is no other element coupled or connected between the elements that are said to be coupled with each other; or an indirect coupling or connection, where one or more other elements are coupled or connected between the elements that are said to be coupled with each other. 
         [0017]      FIGS. 1 and 2  respectively illustrate a top view and a cross-sectional side view of a voltage-controlled current source (VCCS)  100  in accordance with an embodiment. The VCCS  100  may be a field-effect transistor (FET) having a drain  104  and a source  108  coupled with a channel  112  that is formed over a semi-insulating substrate  202  (shown in  FIG. 2 ). The semi-insulating substrate  202  may be composed of, e.g., gallium arsenide (GaAs), silicon (Si), silicon germanium (SiGe), germanium (Ge), indium phosphide (InP), gallium nitride (GaN), aluminum gallium arsenide (AlGaAs), indium gallium arsenide (InGaAs), etc. 
         [0018]    While embodiments of the disclosure are described with reference to a general FET device, various embodiments may be practiced with respect to any of a variety of FET devices. Such devices may include transistors such as, but not limited to, metal semiconductor field-effect transistors (MESFETs), pseudomorphic high electron mobility transistors (pHEMTs), junction field-effect transistors (jFETs), metal insulator semiconductor field-effect transistors (MISFETs), modulation doped field-effect transistors (MODFETs), bipolar field-effect transistors (BiFETs), bipolar high electron mobility transistor (BiHEMTs), or any other suitable compound semiconductor FET technology. In addition or as an alternative, the VCCS  100  may be formed with a complementary metal oxide semiconductor (CMOS) technology. 
         [0019]    The channel  112  may be an isolation implant channel that operates as an active semiconductor area through which current may flow. The current may flow between the drain  104  and the source  108  as shown by arrow  116 . 
         [0020]    The drain  104  may include a drain terminal  120  configured to be coupled with other components of a circuit into which the VCCS  100  is integrated. Similarly, the source  108  may include a source terminal  124  configured to be coupled with other components of the circuit into which the VCCS  100  is integrated. The drain terminal  120  and the source terminal  124  may be considered ohmic contacts that are characterized by linear and symmetric characteristics of a current-voltage (I-V) curve across the contact interface. 
         [0021]    The VCCS  100  may also include a gate  128  having separated gate structures  132 , including a first gate structure  132   a  and a second gate structure  132   b . In some embodiments, the gate structures  132  may be surrounded by a wide recess  134  to enhance the breakdown voltage. At least a portion of the separated gate structures  132 , hereinafter also referred to as “gate structures  132 ,” may be adjacent to and, in some cases, coupled with the channel  112 . The gate structures  132  may be disposed relative to one another in a manner to define a gap region  136  between tips  140 , including a first tip  140   a  and a second tip  140   b , of the gate structures  132 . While  FIG. 1  shows a particular geometrical configuration of the gate structures  132  (and tips  140 ) and the gap region  136 , other embodiments may have other configurations that operate in a manner similar to the manner described with respect to VCCS  100 . 
         [0022]    Each of the gate structures  132  may be coupled with a gate terminal  144  through vias  148   a  and  148   b . The gate structures  132 , the gate terminal  144 , and the vias  148  may be composed of a conductive material, e.g., gold, to provide a conductive path from the gate terminal  144  to the tips  140 . The coupling arrangement of the components of the gate  128  may be seen with reference to the cross-sectional side view of the VCCS  100  shown in  FIG. 2 . 
         [0023]    The gate terminal  144  may be coupled with other components of a circuit into which the VCCS  100  is integrated. In particular, the gate terminal  144  may be configured to receive a control voltage, to control the current flow between the drain  104  and the source  108  through the channel  112 . The current flowing through the channel  112  may predominantly flow through the gap region  136 ; however, at least some current may flow along the gap-side edges  146 , including gap-side edge  146   a  and  146   b , of the tips  140 . The current flowing through the gap region  136  may be predominately determined by the geometrical properties of the gap region  136  and the implantation properties of the channel  112 , while the current along the gap-side edges  146  may be determined by the control voltage, in addition to the geometrical properties of the gap region  136  and the implantation properties of the channel  112 . The relationship between the control voltage and the current may be a linear relationship, as will be shown below. 
         [0024]    In some embodiments, the gate  128  may be biased below a pinch-off (or threshold) voltage, to restrict current flow through the gap region  136  and along the gap-side edges  146 . 
         [0025]      FIG. 3  is a chart  300  that plots drain-to-source (DTS) current as a function of DTS voltage in the VCCS  100  in accordance with an embodiment. In this embodiment, the control voltage may be fixed at −1.3 volts (V). Line  304  of chart  300  shows that DTS current increases rapidly from 0 to approximately 400 microamps (uA) over a first range of DTS voltage values, e.g., 0 to 1 V. However, after a certain DTS voltage, referred to as the knee voltage, the DTS current is substantially independent of the DTS voltage. The knee voltage is shown to be approximately 1 V in chart  300 . Thus, as can be seen, the DTS current through the VCCS  100  is substantially independent of the DTS voltage over a significant range of the DTS voltage. Therefore, the VCCS  100  shows desirable current-source characteristics. 
         [0026]      FIG. 4  is a chart  400  that plots DTS current as a function of control voltage in accordance with some embodiments. Chart  400  shows lines  404  that correspond to measurements from 60 VCCS devices, having the same structure as VCCS  100 , across one wafer with a DTS voltage fixed at 2 V. The absolute value of the DTS current across the 60 devices may have a standard deviation that is approximately 7%, which may be similar to conventional gap-current sources. However, the slope of the lines barely change. Therefore, chart  400  also shows that current through VCCSs of embodiments of this disclosure, e.g., VCCS  100 , are controlled by the voltage in a desirable manner. 
         [0027]      FIG. 5  is a chart  500  that presents statistical analyses of the lines  404  in accordance with some embodiments. In particular, line  504  represents the slopes of the lines  404 , by reference to the left side of chart  500 , and line  508  represents coefficients of determination, R 2 , by reference to the right side of the chart  500 . Each of the measurements of lines  504  and  508 , determined by using a least squares method, shows characteristics of a straight line that best fits the data presented in chart  400 . The coefficients of determination of measurements of line  508  are close to 1 and thereby indicate a desirable linear fit. The slopes of measurements of line  504  range from about 60 to 70, which is less than 3% standard deviation, and thereby indicate a very small process variation. Thus, chart  500  shows that the VCCS  100  may be considered a linear VCCS  100  in which the control voltage controls the DTS current in a desired linear manner. 
         [0028]    The VCCS  100  may be incorporated into any of a variety of apparatuses and systems. A block diagram of a module  600  incorporating VCCS  100  is shown in  FIG. 6  in accordance with an embodiment. The module  600  may be, for example, a power amplifier (PA) duplexer module. The module  600  may include a filter  604  configured to receive and filter a transmit radio-frequency (RF) signal. The filter  604  may provide the filtered transmit RF signal to power amplification circuitry  608 , including one or more power amplifiers, for amplification. A match circuit  612  may be coupled with the power amplification circuitry  608  to match a source impedance to a load impedance to facilitate efficient amplification of the transmit RF signal. The amplified transmit RF signal may be filtered by filter  616  and transmitted over the air by antenna  620 . 
         [0029]    The module may include a power detector  624  that is coupled with a bias controller  628  and/or controller  636 . The power detector  624  may be configured to obtain RF power measurements associated with the amplified transmit RF signal that is provided to filter  616 . The power detector  624  may obtain the RF power measurements by having line  626  adjacent to, but not connected with, line  630 . 
         [0030]    The antenna  620  may also receive RF signals over the air and couple the received RF signals to filter  632 . The filter  632  may filter and output the received RF signals. 
         [0031]    The bias controller  628  may include the VCCS  100 . The bias controller  628  may use the DTS current through the VCCS  100  as a reference current to set a DC bias current of the power amplification circuitry  608  and also the power detector  624 . The DTS current through the VCCS  100  may be based on a received control voltage, which may be a voltage bias adjust (VBA) signal. The control voltage may be received from the controller  636 , which is either external or internal to the module  600 . The controller  636  may generate the control voltage based on, e.g., feedback measurements such as temperature, pinch-off voltage compensation, RF power measurements, etc. The controller  636  may receive the feedback measurements as a detector voltage, VDET, from the power detector  624 ; as an internal measurement of the bias controller  628 , e.g., from a tap point of a current mirror circuit within the bias controller  628 ; etc. 
         [0032]    In some embodiments, the controller  636  may operate as an open loop control system. For example, the temperature and pinch-off voltage of the entire GaAs chip may be fairly uniform. Therefore, the controller  636  could be a stand-alone circuit, e.g., a gate-source diode with a drain that is either floating or connected to gate. The voltage across the gate-source diode may track the pinch-off voltage and may, therefore, serve as a basis for generating the control voltage in an embodiment. 
         [0033]    The VCCS  100  may not draw current from the controlling circuit, e.g., controller  636 , through the gate terminal  144 . Therefore, embodiments of this disclosure provide the flexibility of a linear VCCS without disturbing or otherwise negatively affecting operation of a controlling circuit. 
         [0034]      FIG. 7  illustrates a wireless communication device  700  in accordance with an embodiment. The wireless communication device  700  may have an antenna  704  and a PA duplexer  708  that is similar to module  600  described above. The wireless communication device  700  may further include a transceiver  712 , a main processor  716 , and a memory  720  coupled with each other at least as shown. The wireless communication device  700  may also include a power source  724  coupled with the electrical components of the wireless communication device  700  in accordance with known electrical principles. While the wireless communication device  700  is shown with transmitting and receiving capabilities, other embodiments may include devices with only transmitting or only receiving capabilities. 
         [0035]    In various embodiments, the wireless communication device  700  may be, but is not limited to, a mobile computing device (e.g., a mobile telephone, a smartphone, a paging device, a personal digital assistant, a text-messaging device, etc.), a portable computing device (e.g., a laptop computing device, a tablet computing device, etc.), a desktop computing device, a base station, a subscriber station, an access point, a radar, a satellite communication device, or any other device capable of wirelessly transmitting/receiving RF signals. 
         [0036]    The main processor  716  may execute a basic operating system program, stored in the memory  720 , in order to control the overall operation of the wireless communication device  700 . For example, the main processor  716  may control the reception of signals and the transmission of signals by transceiver  712 . The main processor  716  may be capable of executing other processes and programs resident in the memory  720  and may move data into or out of memory  720 , as desired by an executing process. 
         [0037]    The transceiver  712  may receive outgoing data (e.g., voice data, web data, e-mail, signaling data, etc.) from the main processor  716 , may generate the transmit RF signal(s) to represent the outgoing data, and provide the transmit RF signal(s) to the PA duplexer  708 . In some embodiments, the transceiver  712  may also include a controller, similar to controller  636 , to generate a control voltage for a VCCS included in a bias controller of the PA duplexer  708  as described above with respect to module  600 . 
         [0038]    The PA duplexer  708  may filter, amplify and transmit the transmit RF signal(s) over the air via the antenna  704  as described above with respect to module  600 . 
         [0039]    In a manner similar, but converse, to the transmitting operations, the transceiver  712  may receive an RF signal from the antenna  704  through the PA duplexer  708 . The transceiver  712  may process and send the receive RF signal to the main processor  716  for further processing. 
         [0040]    In various embodiments, the antenna  704  may include one or more directional and/or omnidirectional antennas, including, e.g., a dipole antenna, a monopole antenna, a patch antenna, a loop antenna, a microstrip antenna or any other type of antenna suitable for OTA transmission/reception of RF signals. 
         [0041]    Those skilled in the art will recognize that the wireless communication device  700  is given by way of example and that, for simplicity and clarity, only so much of the construction and operation of the wireless communication device  700  as is necessary for an understanding of the embodiments is shown and described. Various embodiments contemplate any suitable component or combination of components performing any suitable tasks in association with wireless communication device  700 , according to particular needs. Moreover, it is understood that the wireless communication device  700  should not be construed to limit the types of devices in which embodiments may be implemented. 
         [0042]    Although the present disclosure has been described in terms of the above-illustrated embodiments, it will be appreciated by those of ordinary skill in the art that a wide variety of alternate and/or equivalent implementations calculated to achieve the same purposes may be substituted for the specific embodiments shown and described without departing from the scope of the present disclosure. Those with skill in the art will readily appreciate that the teachings of the present disclosure may be implemented in a wide variety of embodiments. This description is intended to be regarded as illustrative instead of restrictive.