Abstract:
Proposed is a bias circuit for a transistor in a C class amplifier. The bias circuit comprises: a class AB amplifier bias voltage generating means adapted to generate a bias voltage at an output terminal; and a transistor connected between the output terminal and a first reference voltage, the control terminal of the transistor being connected to a second reference voltage via a switch. Closure of the switch connects the second reference voltage to the control terminal of the transistor to cause a shift in the bias voltage generated by the class AB amplifier bias voltage generating means to achieve a predetermined class C bias voltage at the output terminal.

Description:
FIELD OF THE INVENTION 
       [0001]    This invention relates to a bias circuit for a Radio Frequency (RF) power transistor. 
       BACKGROUND OF THE INVENTION 
       [0002]    In the field of RF power amplification, with the increasing complexity of signals to be amplified (such as GSM, WCDMA, LTE, etc . . . ) and the demand for efficiency, the preferred mode of operation of a RF transistor is a common source (emitter) class AB linear amplifier. 
         [0003]    In order to maintain linearity, it is known to use a bias circuit to provide thermal compensation. This has been implemented in MMIC&#39;s using the well-known current mirror arrangement of transistors. When integrated on the same die as a RF power transistor, this provides thermal tracking and process compensation. 
         [0004]    With the continued drive to further increase efficiency and reduce the size and cost of amplifiers, Doherty amplifiers are now widely used because of their advantages in efficiency in back-off. In its simplest configuration, a Doherty amplifier consists of two amplifiers, namely a main (or carrier) amplifier and an auxiliary (or peaking amplifier) connected in parallel with their outputs joined by a quarter-wave transmission line. The main amplifier is typically a Class B or Class AB type linear power amplifier and the auxiliary amplifier is typically a Class C type non-linear power amplifier. 
         [0005]    Thus, to be suitable for Doherty amplifier, a bias circuit has to be able to provide a Class C bias level, which corresponds to a class AB level from which a fixed voltage is subtracted. A conventional current mirror (like that shown in  FIG. 1  for example) is therefore not suitable for Class C. 
       BRIEF SUMMARY OF THE INVENTION 
       [0006]    According to an aspect of the invention there is provided a bias circuit according to claim  1 . 
         [0007]    There is proposed a bias circuit that employs a transistor connected between the output of the bias circuit and a reference voltage (such as ground, for example). The transistor may be adapted to act as a non-linear load and may allow quick discharge of load capacitance in a switching mode. Thus, embodiments may provide fast thermal tracking and process compensation together with low video impedances. Embodiments may also be compact in size, exhibit lower power consumption, and provide a reduced switching time (such as below 100 ns for example). Embodiments may therefore be employed in MMIC&#39;s or discrete transistors, and will be suitable for Doherty amplifiers and Time Division (TD) amplifiers. 
         [0008]    Embodiments may comprise class AB amplifier bias voltage generating means employing a current mirror. The current mirror may comprise first and second MOS transistors that are provided in a looped arrangement, wherein the drain of the first MOS transistor is connected to a current source (which could be a voltage source connected via a reference resistor), and wherein the gate of the second MOS transistor is connected to the drain of the first MOS transistor, such that the source of the second MOS transistor provides the bias voltage. 
         [0009]    Embodiments may be formed in an integrated circuit, and may further be integrated on the same circuit as a transistor to be biased. 
         [0010]    Embodiments may be employed in a Doherty amplifier. 
         [0011]    A radio frequency amplifier circuit may employ an embodiment. 
         [0012]    According to another aspect of the invention, there is provided a method of biasing a transistor in a C class amplifier according to claim  11 . 
         [0013]    According to another aspect of the invention, there is provided a computer program product for biasing a transistor in a C class amplifier according to claim  12 . 
         [0014]    Embodiments may exhibit low impedance to prevent parasitic modulation appearing at the output (e.g. at the input of a transistor to be biased), thus restricting the generation of undesirable memory effects and non-linearity. Embodiments may compensate for part-to-part variation in the threshold voltage of an amplifier&#39;s transistor, thus reducing a need for adjustment of the bias circuit. 
         [0015]    A bias circuit according to an embodiment may be included in various types of power amplifiers, such as those for cellular radio base-stations for example. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0016]    Preferred embodiments of the present invention will now be described, by way of example only, with reference to the following drawings in which: 
           [0017]      FIG. 1  shows an example of a conventional current mirror arrangement; 
           [0018]      FIG. 2  is a schematic diagram of a bias circuit according to an embodiment; 
           [0019]      FIG. 3A  illustrates an exemplary layout of an embodiment within a ten cells discrete die; 
           [0020]      FIG. 3B  illustrates an exemplary layout of a conventional sense FET within a ten cells discrete die; 
           [0021]      FIG. 4  illustrates a transistor topology including a bias circuit according to an embodiment; 
           [0022]      FIG. 5  shows the implementation of an embodiment within a regular discrete package; 
           [0023]      FIG. 6A  shows a conventional bias arrangement using a sense FET; and 
           [0024]      FIG. 6B  shows a bias arrangement according to a proposed embodiment. 
       
    
    
     DETAILED DESCRIPTION OF THE EMBODIMENTS 
       [0025]    There is proposed a bias circuit which employs a current mirror circuit (similar to the conventional circuit arrangement shown in  FIG. 1 ) to generate a bias voltage at an output terminal. The bias circuit also employs an additional transistor connected between the output terminal of the bias circuit and ground. This additional transistor can be controlled via a switch that is adapted to supply a control voltage to the control terminal (e.g. the gate) of the transistor. 
         [0026]    An embodiment of a bias circuit  10  is shown in  FIG. 2 . The bias circuit  10  comprises first T 1  and second T 2  transistors which are scaled versions of a RF transistor  12  to be biased. The first T 1  and second T 2  transistors are provided in a looped arrangement, similar to that shown in  FIG. 1 , wherein the drain of the first transistor T 1  is connected to a current source Iref. The voltage source Vdd is connected to the drain of the second transistor T 2  and the gate of the second transistor T 2  is connected to the drain of the first transistor T 1 . The source of the second transistor T 2  provides the output Vbias of the bias circuit  10 . 
         [0027]    In use, the first transistor T 1  acts as a reference, which has its bias point fixed by the reference current source IREF. The second transistor T 2  operates as a voltage buffer for the first transistor T 1 , and provides current to the output (i.e. the RF transistor  12  to be biased) when necessary. For example, since the RF transistor  12  behaves like a capacitor, the bias circuit  10  may need to charge the RF transistor  12  in order to increase the bias voltage Vbias. 
         [0028]    The looped arrangement of the first and second transistors T 1  and T 2  enables the voltage on the first transistor T 1  to control the output bias voltage Vbias. Thus, the voltage output from the gate of the first transistor T 1  is the class AB amplifier bias voltage (i.e. a reference voltage Vref). 
         [0029]    In other words, the bias circuit  10  employs a conventional current mirror arrangement (like that shown in  FIG. 1 ) wherein the reference transistor is the first transistor T 1  in which a current Iref is injected corresponding to the desired class AB bias level. Like in a conventional current mirror (similar to that shown in  FIG. 1 ), current is supplied to the output by the second transistor T 2 . It will be appreciated that in the case of MOS transistors, there is no DC current. 
         [0030]    Having established a reference or class AB bias voltage (with the looped arrangement of the first and second transistors T 1  and T 2 ), it is necessary to subtract a voltage (Vdelta) from the reference voltage Vref for biasing a class C amplifier. To achieve this, a third transistor T 3  is connected between bias circuit output and ground, with the control terminal (i.e. the gate) of the third transistor T 3  being connected to the gate of the first transistor T 1 . Also, the gate of the first transistor T 1  is connected to a switch via a first resistor R 1 , and connected to the output terminal (i.e. the source of the second transistor T 2  via a second resistor R 2 . The switch  14  is adapted to selectively disconnect/connect the first resistor R 1  to control voltage Vctrl. 
         [0031]    When used in class AB, the switch  14  is in an open configuration and the control voltage Vctrl is not connected to the gate of the first transistor T 1  via the first resistor R 1 , and the third transistor T 3  then acts as an active load for the current mirror. 
         [0032]    When used in class C, the switch  14  is in a closed configuration and the control voltage Vctrl is connected to the gate of the first transistor T 1  via the first resistor R. The control voltage Vctrl translates through the first R 1  and second R 2  resistors into shift in voltage at the output terminal of the bias circuit (Vbias) and the third transistor T 3  acts as a non-linear load. 
         [0033]    When used in a switching mode, the third transistor T 3  allows quick discharging of the load capacitance without affecting the bias circuit consumption in steady state mode. 
         [0034]    Implementation of the third transistor T 3  for generating the Class C bias voltage does not affect the functionality of the first T 1  and second T 2  transistor looped arrangement for generating the Class AB bias voltage. 
         [0035]    An exemplary layout of an embodiment within a ten cells discrete die is shown in  FIG. 3A , and the layout of a so called “sense FET” (widely used across base-station devices) within a ten cells discrete die is shown in  FIG. 3B . 
         [0036]    As can be seen from a comparison of  FIGS. 3A and 3B , the overall size of the bias circuit and the drain filter is very similar to the commonly used “sense FET”. It is noted that, in case of a discrete die, increasing the size of the bias circuit translates to a decrease in the available space for the RF transistor, which in turn translates to a reduction of power density. 
         [0037]    Implementation within an MMIC is straightforward for a skilled reader and external connections (to class AB and Vctrl signals for examples) are easy to implement. Detailed description of such details is therefore omitted. 
         [0038]    For the example of a discrete transistor, an example of transistor topology including a bias circuit according to an embodiment is illustrated in  FIG. 4 , wherein input prematch circuit includes a shunt inductance decoupled by a high value capacitor and a low-pass filter. Note that use of the shunt inductance with decoupling capacitor allows increasing RF bandwidth of the transistor, but is currently not used because it prevents fast switching of the bias (overall input capacitance too high). Embodiments allow use of wideband matching while maintaining fast switching capability. 
         [0039]    Implementation of an embodiment within a regular discrete package is shown in  FIG. 5 . Here, the leads usually dedicated to the sense FET connections are used as the input for the Class AB and control voltage Vctrl connections. The output leads are free for any kind of Video Bandwidth optimization at the output (the input Video Bandwidth being assured by the low impedance of the integrated bias). 
         [0040]    On top of the fact that providing low video impedance allows for improved “linearizability” and that “switchability” is preferred for many applications, the space savings provided by embodiment can be significant. Such potential space savings are is illustrated in  FIG. 6 , wherein  FIG. 6   a  shows a conventional bias arrangement using a sense FET, and  FIG. 6   b  shows a bias arrangement according to a proposed embodiment. 
         [0041]    While one or more embodiments have been illustrated in detail, one of ordinary skill in the art will appreciate that modifications and adaptations to those embodiments may be made. 
         [0042]    Other variations to the disclosed embodiments can be understood and effected by those skilled in the art in practising the claimed invention, from a study of the drawings, the disclosure, and the appended claims. In the claims, the word “comprising” does not exclude other elements or steps, and the indefinite article “a” or “an” does not exclude a plurality. A single processor or other unit may fulfil the functions of several items recited in the claims. The mere fact that certain measures are recited in mutually different dependent claims does not indicate that a combination of these measured cannot be used to advantage. Any reference signs in the claims should not be construed as limiting the scope.