Abstract:
A power amplifier circuit includes an amplifying transistor and a dc bias circuit for biasing the amplifier transistor to obtain a conduction angle of at least about 180°. The dc bias circuit includes a self-bias boosting circuit for initially decreasing and then increasing the dc bias voltage provided to a control terminal of the amplifying transistor by the dc bias circuit as the input signal provided to the power amplifier increases. The self-bias boosting circuit is extremely simple and compact in design, and permits the power amplifier circuits to operate in Class B or Class AB with improved power output characteristics.

Description:
BACKGROUND OF THE INVENTION  
         [0001]    The invention is in the field of transistor amplifier circuits, and relates more particularly to a power amplifier circuit having a self-bias boosting circuit for increasing maximum power output and reducing power dissipation at low power levels.  
           [0002]    Amplifiers of this general type are frequently used in high-frequency RF amplifiers, as well as in audio amplifiers and other applications. In order to obtain a linear input-output relationship and high operating efficiency, such amplifiers are typically operated with a conduction angle of about 180° (Class B) or slightly greater (Class AB) to avoid crossover distortion.  
           [0003]    Typically, amplifiers of this type require a dc bias circuit to establish the quiescent bias current in the amplifier circuit to ensure operation in the Class B or Class AB mode. In the prior art, bias is typically provided by a fixed current source, as shown in U.S. Pat. No. 5,844,443, or else by an external supply, which can be set to a desired constant value to secure the quiescent current necessary to operate in the desired mode, as shown in U.S. Pat. No. 5,548,248.  
           [0004]    However, in amplifiers of the type described above the average current drawn from the supply depends upon the input signal level. As the output power increases so does the average current in both the emitter and the base of the power transistor. This increased average current causes an increased voltage drop in the biasing circuitry and in ballast resistors (which are used to avoid hot-spotting and thermal runaway in transistors using an interdigitated design). This in turn reduces the conduction angle (i.e. the number of degrees out of 360° that the amplifier is conducting), and forces the amplifier deep into Class B or even Class C operation, thereby reducing the maximum power output. To avoid this power reduction, the amplifier must have a larger quiescent bias. In prior-art circuitry this inevitably leads to a higher power dissipation at low power output levels and therefore an undesirable tradeoff in operating characteristics.  
           [0005]    A recent improvement in this art is disclosed in co-pending U.S. patent application Ser. No. 09/536,946, entitled Dynamic Bias Boosting Circuit For A Power Amplifier, filed on Mar. 28, 2000 by Sowlati and Luo, two of the present inventors. This application discloses a solution to the problem discussed above which entails providing the power amplifier circuit with a dynamic bias boosting circuit to dynamically increase the bias of the power transistor as the output power increases by using a circuit that senses the input voltage to the amplifier and generates a dynamic bias boost as a function of the amplitude of this signal. The drawback to this solution is that it employs numerous active and passive components, thus not maximizing simplicity, compactness and economy of manufacture.  
           [0006]    Accordingly, it would be desirable to have a power amplifier circuit which offers the advantages of optimum maximum output power and reduced power dissipation at low power levels. Additionally, it would be desirable for such a circuit to be extremely simple and compact in design, and very economical to manufacture.  
         SUMMARY OF THE INVENTION  
         [0007]    It is therefore an object of the present invention to provide a power amplifier circuit which provides improved maximum output power and less power dissipation at low power levels. It is a further object of the invention to provide a power amplifier circuit which is both extremely simple and compact in design and which is very economical to manufacture.  
           [0008]    In accordance with the invention, these objects are achieved by a new power amplifier circuit for amplifying an input signal and having a conduction angle of at least about 180°, the amplifier circuit including an amplifying transistor and a dc bias circuit for biasing the amplifier transistor to obtain the desired conduction angle. The dc bias circuit includes a self-bias boosting circuit for initially decreasing and then increasing the dc bias voltage provided to a control terminal of the amplifying transistor by the dc bias circuit as the input signal provided to the power amplifier increases.  
           [0009]    In a preferred embodiment of the invention, the amplifier circuit is either a Class B or a Class AB amplifier circuit.  
           [0010]    In a further preferred embodiment of the invention, the self-bias boosting circuit includes a capacitor coupled from an output terminal of the self-bias boosting circuit to a common terminal, and a resistor coupled between the output terminal and the control terminal of the amplifying transistor.  
           [0011]    In yet a further preferred embodiment of the invention, the self-bias boosting circuit also includes a switch coupled in series with the capacitor for enabling the power amplifier circuit to operate in either of two output power modes.  
           [0012]    A power amplifier circuit in accordance with the present invention offers a significant improvement in that a particularly advantageous combination of features, including increased maximum output power and reduced power dissipation at low power levels, can be obtained in an extremely simple, compact and economical configuration.  
           [0013]    These and other aspects of the invention will be apparent from and elucidated with reference to the embodiments described hereinafter.  
       
    
    
     BRIEF DESCRIPTION OF THE DRAWING  
       [0014]    The invention may be more completely understood with reference to the following description, to be read in conjunction with the accompanying drawing, in which:  
         [0015]    [0015]FIG. 1 shows a simplified schematic diagram of a power amplifier circuit in accordance with a first embodiment of the invention; and  
         [0016]    [0016]FIG. 2 shows a simplified schematic diagram of a power amplifier circuit in accordance with a second embodiment of the invention. 
     
    
       [0017]    In the drawing, like reference numerals are generally used to designate like components.  
       DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0018]    A simplified schematic diagram of a high-frequency power amplifier circuit  1  is shown in FIG. 1 of the drawing. The amplifier circuit includes an amplifying transistor  10  and a bias circuit  2  coupled to the base of the amplifying transistor  10  by a resistor  11 . The bias circuit  2  includes bipolar transistors  12  and  13 , coupled in series between V cc  and a common terminal (GND), with the transistors being coupled to bias voltage supplies shown in simplified form by bias voltage supplies  14  and  15  coupled to the bases of transistors  12  and  13 , respectively, by resistors  16  and  17 . The basic circuit configuration is completed by an input coupling capacitor  18  for coupling an input signal Vin to the base of amplifying transistor  10 , with the transistor  10  being connected in a common-emitter configuration and coupled between V cc  and GND by an inductor  19 . The output of power amplifier circuit  1  is taken from the collector of transistor  10  and is designated as V out  in the drawing.  
         [0019]    In connection with the circuit as so far described, it should be understood that although the active components are shown as bipolar transistors for illustrative purposes, field effect transistors or a combination of bipolar and field effect transistors may alternatively be used within the scope of the invention. Additionally, it is to be understood that the power amplifier circuit  1  and bias circuit  2  may differ in form and detail from the simplified, illustrative depictions shown in the drawing. Furthermore, it is to be understood that the bias supply may be configured and adjusted to permit the amplifier circuit to operate in either Class B or Class AB mode.  
         [0020]    In accordance with the invention, capacitor  20  is coupled from an output terminal  21  of the bias circuit  2  to a common terminal (here GND) in order to achieve a self-bias boost feature in an extremely simple, compact and economical manner. In an illustrative but nonlimiting example, in a circuit suitable for use at wireless telephone communication frequencies, capacitor  20  may have a value of about 2.2 pF and resistor  11  may have a value of about 15 ohms. Additionally, in a practical implementation, the bias voltages supplied to transistors  12  and  13  by components  14 ,  15 ,  16  and  17  could be supplied by other bias circuits already existing in other portions of a device incorporating the power amplifier circuit of the invention, thus further simplifying the circuit and reducing cost and size.  
         [0021]    As opposed to more complex and costly prior-art bias control circuits, the present invention provides an extremely simple and automatic improvement in circuit performance by using capacitor  20  together with the other circuit components, with the capacitor acting as a self-bias booster in the amplifier circuit, in a manner which will be explained in further detail below.  
         [0022]    A further preferred embodiment of the invention is shown in the simplified schematic diagram of FIG. 2. The circuit shown in FIG. 2 incorporates all of the components of FIG. 1, with like reference numerals designating like components, and accordingly these components will not be described further here. The circuit of FIG. 2 differs from FIG. 1 in that it additionally includes a switch  22  coupled in series with capacitor  20 , so that the two series-connected components are connected between terminal  21  and GND. In this manner, with the addition of only a single additional component, the power amplifier circuit can operate in either of two different output power modes, namely a higher power mode (in which capacitor is connected to ground) and a lower output power mode (in which switch  22  is opened to disconnected capacitor  20  from the circuit). When the circuit is used in wireless communication applications, for example, this dual power mode embodiment may be employed to switch power levels in order to enable operation in both analog and digital modes, which typically employ different power levels. Although circuit element  22  is shown in FIG. 2 as a simple switch, it will be understood that this component may be implemented in practice by either a mechanical switch or a semiconductor device such as a transistor or a PIN diode.  
         [0023]    In the foregoing circuits, the capacitor  20  acts as a self-bias booster in the amplifier circuit. This feature, when used with proper input and output matching networks, will provide improved maximum output power and less power dissipation at low power levels.  
         [0024]    In order to better understand the operation of the instant invention, consider the operation of the circuit of FIG. 2 with switch  22  opened so that capacitor  20  is effectively disconnected. In this mode, the average voltage drop across resistor  11  will increase as the RF input level (Vin) increases. This will in turn cause a reduction in the base-emitter voltage of transistor  10 , thus pushing this transistor into saturation. When switch  22  is closed, thus connecting capacitor  20  into the circuit, the base-emitter voltage of transistor  10  will initially decrease and then subsequently increase as the input signal provided to the power amplifier circuit increases, thus helping transistor  10  to stay within the desired operating region and permitting improved maximum output power within the desired mode of operation.  
         [0025]    It is to be emphasized that although the improvement in the invention is extremely simple, compact and economical to implement, it is based upon a recognition of certain operating conditions that are not apparent or obvious. More particularly, the invention is based upon a recognition that, at lower input power levels, the discharging rate of capacitor  20  through transistor  12  is equal to or faster than its charging rate through transistors  10  and  13 . At higher power levels, on the contrary, the discharging rate of capacitor  20  becomes slower than its charging rate. Accordingly, as the input signal provided to the power amplifier circuit is increased, the DC bias voltage provided to the base of transistor  10  will initially decrease and then increase, thus providing the desired self-bias boosting effect in an extremely simple and economical manner. Computer simulations indicate that the provision of this single component (capacitor  20 ) can improve the maximum output power available by up to about 40%.  
         [0026]    While the invention has been particularly shown and described with reference to several preferred embodiments thereof, it will be understood by those skilled in the art that various changes in form and detail, some of which have been suggested above, may be made without departing from the spirit or scope of the invention. Thus, for example, different types of transistors may be employed, and alterations to the circuit configuration may be made to suit particular design requirements.