Abstract:
A stacked pre-driver stage and a power amplifier including the stacked pre-driver stage are described. The stacked pre-driver stage comprises stacked pre-drivers arranged in series between a supply voltage and a reference voltage. Each pre-driver includes a pre-driving amplifier, together with MOS transistors. Each pre-driver is subject, in operation, to a voltage difference which is inferior to a maximum allowed use voltage of the MOS transistors with a largely reduced voltage drop across the regulator included in the power amplifier.

Description:
BACKGROUND 
     1. Field 
     The present disclosure relates to amplifiers. More specifically, it relates to a stacked pre-driver amplifier and to devices using a stacked pre-driver amplifier, such as power amplifiers having a stacked pre-driver amplifier. 
     2. Description of Related Art 
     Radio frequency power amplifiers are commonly used in many applications. Often, they are used in consumer battery-powered applications such as mobile telephones, for which power efficiency is a very important attribute. 
     As semiconductor processes shrink to smaller dimensions in order to achieve increased performance, reliability criteria cause a reduction in the maximum allowed voltage across the terminals of a transistor. This maximum allowed voltage is defined as Vuse. Common battery voltages can exceed 3.5V, while maximum transistor voltages may be 2.5V or even less. The voltage imposed on a pre-driver stage of a power amplifier must be reduced from the battery voltage down to Vuse. This may be accomplished by several means known to one skilled in the circuit design arts. 1) DC-DC voltage converters can be used to provide a lower voltage, but power is lost in the conversion efficiency and there is significant cost in creating a DC-DC converter. 2) A resistor divider can be used to reduce the voltage from the battery voltage to the use voltage, but the power dissipated in the resistor divider is wasted as heat, resulting in reduced efficiency of the overall amplifier circuit. 3) Similarly, a transistor having a variable bias can be used to provide a voltage drop that can be changed as desired. However, the power dissipated in the transistor is also wasted as heat, resulting in reduced efficiency. 
     The above problem is illustrated in  FIG. 1 , which shows a block diagram of a prior art radio frequency power amplifier (PA) with a pre-driver circuit ( 102 ). The pre-driver circuit ( 102 ) comprises a pre-driver amplifier ( 104 ) and transistors ( 106 ), ( 108 ). Regulator ( 110 ) is in series with transistors ( 106 ) and ( 108 ) between a power supply voltage Vbat and a reference voltage Vref. The function of regulator ( 110 ) is familiar to one skilled in the circuit design arts and could be any of several different designs. For example, it could be a stack of p-channel transistors, or any of several other circuit topologies. 
     Vbat can be, for example, 3.5V (as is common with mobile telephones). On the other hand, as also described above, a typical 0.25 μm CMOS process has a maximum voltage (Vuse) allowed across any two terminals of a transistor of 2.5V. Such limitation is due to physical degradation effects known to one skilled in the art of semiconductor device design and may include gate oxide breakdown due to so-called time-dependent dielectric breakdown (TDDB) or so-called hot carrier injection (HCI). These physical phenomena cause degradation of device performance of the system or even failure of the system. The rule for the maximum voltage between any two terminals of a transistor should therefore be strictly followed in order to meet reliability requirements of the system. 
     Referring again to the circuit in  FIG. 1 , if Vbat is 3.5V and Vref is electrical ground, the power supply differential voltage is 3.5 Volts. Vin ( 112 ) is an external source signal that increases and decreases in value over time and is capacitively coupled to the driver amplifier ( 104 ). The driver amplifier ( 104 ) outputs a signal that is high when Vin ( 112 ) is high and low when Vin ( 112 ) is low. The highest voltage that can be output from the driver amplifier ( 104 ) is Vpre. The lowest voltage that can be output from the driver amplifier ( 104 ) is Vref. 
     As is typical with CMOS inverter arrangements, when the output of the driver amplifier ( 104 ) is close to Vpre, the p-channel transistor ( 106 ) is turned OFF and the n-channel transistor ( 108 ) is turned ON. As a consequence of this, the input voltage Vg 1  to the PA ( 114 ) is pulled down close to Vref. 
     At this point, if it is assumed that Vdrop of regulator ( 110 ) is close to zero, then the voltage present at the source of the p-channel transistor ( 106 ) is approximately Vbat and the voltage present at the drain of the same p-channel transistor ( 106 ) is approximately Vref. Therefore, if Vbat is assumed to be equal to 3.5V and Vref is assumed to be equal to electrical ground, then Vds=Vdrain−Vsource of the p-channel transistor ( 106 ) is approximately 3.5V. If the above mentioned rule for Vuse sets a maximum amount of 2.5V, then this situation violates the rule for Vuse. 
     On the other hand, when the output of the driver amplifier ( 104 ) is close to Vref, the p-channel transistor ( 106 ) is turned ON and the n-channel transistor ( 108 ) is turned OFF. As a consequence, the input voltage Vg 1  to the PA is pulled up close to Vpre. 
     If Vdrop of the regulator ( 110 ) is assumed to be close to zero, then Vds of the n-channel transistor ( 108 ) is close to Vbat−Vref. Therefore, if Vbat is 3.5V and Vref is 0V, then Vds of the n-channel transistor ( 108 ) is close to 3.5V, which violates the Vuse rule. 
     As noted above, one possible solution is to increase Vdrop, for example to 1.3 V, by changing the Vbias of the regulator ( 110 ). Should this happen, the Vds of the p-channel transistor ( 106 ) or the n-channel transistor ( 108 ) is reduced to 2.2V, thus satisfying the Vuse rule. However, such a possible solution increases power dissipation in the regulator ( 110 ). Such power would be wasted as heat, reducing the overall efficiency of the power amplifier (PA). 
     SUMMARY 
     According to a first aspect, an amplifier for amplifying an input signal is provided, comprising: a first amplifying stage connected with a second amplifying stage, wherein: the first amplifying stage comprises at least two amplifying circuits connected in series, each amplifying circuit receiving as input the input signal, each amplifying circuit having an output, and the outputs of the amplifying circuits of the first amplifying stage are input to the second amplifying stage. 
     According to a second aspect, a power amplifier for amplifying an input carrier signal is provided, comprising: a stacked pre-driver stage, the stacked pre-driver stage comprising a plurality of stacked pre-drivers arranged in series between a supply voltage and a reference voltage, each pre-driver comprising a pre-driving amplifier, the input carrier signal being fed, in operation, to each pre-driver, outputs of the stacked pre-drivers being combined to form an output of the stacked pre-driver stage; a driver stage receiving the output of the stacked pre-driver stage; and a regulator located between a power supply supplying the supply voltage and the stacked pre-driver stage, wherein each pre-driver comprises MOS transistors and is subject to a voltage difference which is inferior to a maximum allowed use voltage of the MOS transistors and wherein voltage drop across the regulator is small when compared to the voltage difference to which the stacked pre-driver stage is subjected. 
     According to a third aspect, a power amplifier for amplifying an input carrier signal is provided, comprising: a stacked pre-driver stage, the stacked pre-driver stage comprising a plurality of stacked pre-drivers arranged in series between a supply voltage and a reference voltage, each pre-driver comprising a pre-driving amplifier, the input carrier signal being fed, in operation, to each pre-driver, outputs of the stacked pre-drivers forming outputs of the stacked pre-driver stage; a driver stage receiving the outputs of the stacked pre-driver stage, the driver stage comprising a plurality of power amplifiers, each power amplifier being in correspondence of a pre-driver and receiving a respective output from a respective pre-driver, outputs of the power amplifiers forming outputs of the driver stage; a regulator located between a power supply supplying the supply voltage and the stacked pre-driver stage, and a power combiner that combines the outputs of the driver stage, wherein each pre-driver comprises MOS transistors and is subject to a voltage difference which is inferior to a maximum allowed use voltage of the MOS transistors and wherein voltage drop across the regulator is small when compared to the voltage difference to which the stacked pre-driver stage is subjected. 
     According to a fourth aspect, a power amplifier for amplifying an input carrier signal is provided, comprising: a stacked pre-driver stage, the stacked pre-driver stage comprising two stacked pre-drivers arranged in series between a supply voltage and a reference voltage, the input carrier signal being fed, in operation, to each pre-driver, each pre-driver comprising a pre-driving amplifier, outputs of the stacked pre-drivers forming outputs of the stacked pre-driver stage, the output of a first pre-driver being opposite in phase to the output of the second pre-driver; a driver stage receiving the outputs of the stacked pre-driver stage, the driver stage comprising two power amplifiers, a first power amplifier being in correspondence of the first pre-driver and receiving the output of the first pre-driver, the second power amplifier being in correspondence of the second pre-driver and receiving the output of the second pre-driver; a regulator located between a power supply supplying the supply voltage and the stacked pre-driver stage, and a power combiner that combines the outputs of the first power amplifier and second power amplifier, wherein each pre-driver comprises MOS transistors and is subject to a voltage difference which is inferior to a maximum allowed use voltage of the MOS transistors and wherein voltage drop across the regulator is small when compared to the voltage difference to which the stacked pre-driver stage is subjected. 
     According to a fifth aspect, a power amplifier for amplifying an input carrier signal is provided, comprising: a stacked pre-driver stage, the stacked pre-driver stage comprising two stacked pre-drivers arranged in series between a supply voltage and a reference voltage, each pre-driver comprising a pre-driving amplifier, the input carrier signal being fed, in operation, to each pre-driver, outputs of the stacked pre-drivers forming outputs of the stacked pre-driver stage; a driver stage receiving the outputs of the stacked pre-driver stage, the driver stage comprising two power amplifiers, a first power amplifier being in correspondence of the first pre-driver and receiving the output of the first pre-driver, the second power amplifier being in correspondence of the second pre-driver and receiving the output of the second pre-driver, each of the first and second power amplifier being powered with the supply voltage and the reference voltage; a regulator located between a power supply supplying the supply voltage and the stacked pre-driver stage, and a power combiner that combines the outputs of the driver stage, wherein each pre-driver comprises MOS transistors and is subject to a voltage difference which is inferior to a maximum allowed use voltage of the MOS transistors and wherein voltage drop across the regulator is small when compared to the voltage difference to which the stacked pre-driver stage is subjected. 
     According to a sixth aspect, a stacked pre-driver stage adapted to be used in a power amplifier circuit for amplification of an input signal is provided, the power amplifier comprising a regulator connected with the stacked pre-driver stage, the stacked pre-driver stage comprising: a plurality of stacked pre-drivers to be arranged in series between a supply voltage and a reference voltage, each pre-driver comprising a pre-driving amplifier, the input signal being fed, in operation, to each pre-driver, wherein each pre-driver comprises MOS transistors and is subject, in operation, to a voltage difference which is inferior to a maximum allowed use voltage of the MOS transistors and wherein voltage drop across the regulator is small when compared to the voltage difference to which the plurality of stacked pre-drivers is subjected. 
     The above described circuits reduce or eliminate power wasted in reducing voltage from Vbat to Vuse, resulting in improved efficiency. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a block diagram of a prior art power amplifier with pre-driver. 
         FIG. 2  is a block diagram of a power amplifier with split pre-driver in accordance with a first embodiment of the present disclosure. 
         FIG. 3  is a block diagram of a power amplifier with split pre-driver in accordance with a second embodiment of the present disclosure. 
         FIG. 4  is a block diagram of a power amplifier with split pre-driver in accordance with a third embodiment of the present disclosure. 
         FIG. 5  is a block diagram of a power amplifier with split pre-driver in accordance with a fourth embodiment of the present disclosure. 
     
    
    
     DETAILED DESCRIPTION 
       FIG. 2  shows a block diagram of a power amplifier with a split pre-driver arrangement in accordance with an embodiment of the present disclosure. As shown in  FIG. 2 , a first pre-driver circuit ( 202   a ) is connected to the power supply voltage Vbat in series with a second pre-driver circuit ( 202   b ). Therefore, in accordance with this embodiment, the power supply voltage Vbat−Vref is dropped across Vdrop, Vpre 1 , and Vpre 2 . In particular, Vbat−Vref=(Vbat−Vpre 1 )+(Vpre 1 −Vpre 2 )+(Vpre 2 −Vref). By way of example and not of limitation, Vref can be assumed to be equal to electrical ground and Vdrop of regulator ( 210 ) is assumed to be equal to zero. In such exemplary embodiment, it follows that Vpre 1 =Vbat. 
     An external source signal Vin ( 212 ) is, for example, a RF signal that is capacitively coupled to pre-driver amplifiers ( 204   a ) and ( 204   b ). In the embodiment of  FIG. 2 , pre-driver amplifiers ( 204   a ) and ( 204   b ) amplify the signal Vin ( 212 ) without inverting it. Therefore, when Vin ( 212 ) is high, the outputs of the pre-driver amplifiers ( 204   a ) and ( 204   b ) are also both high. 
     The output of the pre-driver amplifier ( 204   a ) feeds into the gates of the p-channel transistor ( 206   a ) and the n-channel transistor ( 208   a ). When the output of the pre-driver amplifier ( 204   a ) is high, the p-channel transistor ( 206   a ) is turned OFF, and the n-channel transistor ( 208   a ) is turned ON. Therefore, substantially all of the voltage Vpre 1 −Vpre 2  is dropped across the p-channel transistor ( 206   a ). If the pre-driver amplifiers ( 204   a ) and ( 204   b ) are designed to have similar electrical characteristics, and the transistors ( 206   a ), ( 208   a ) are designed to have similar characteristics to the transistors ( 206   b ), ( 208   b ), the voltage difference Vpre 1 −Vref divides evenly, meaning that Vpre 1 −Vpre 2 =Vpre 2 −Vref or, alternatively stated, Vpre 2 =(Vpre 1 −Vref)/2. Assuming that Vdrop=0, Vref=0, and Vbat=3.5, Vpre 1 =3.5V and Vpre 2 =1.75V. Thus, the voltage across the transistors ( 206   a ), ( 208   a ) is 1.75V and the voltage across the transistors ( 206   b ), ( 208   b ) is 1.75V. Therefore, none of the transistors ( 206   a ), ( 208   a ), ( 206   b ), ( 208   b ) can have a Vds voltage higher than 1.75V. It follows that all voltages on those transistors are within the Vuse reliability rules. 
     Referring again to  FIG. 2 , the output of the first pre-driver stage ( 202   a ) is capacitively coupled through a capacitor C 3  to the output of the second pre-driver stage ( 202   b ). The coupled output Vg 1  is connected to the input of a final stage PA ( 214 ). In other words, the pre-driver stages ( 202   a ), ( 202   b ) drive the final stage PA ( 214 ). The capacitor C 3  allows RF coupling of the two pre-driver outputs while different DC potentials are present at the outputs of the first and second pre-driver stages ( 202   a ), ( 202   b ). While a capacitive coupling of the outputs is shown in the embodiment of  FIG. 2 , those skilled in the art will understand that alternative methods can be used to combine the RF power of the two pre-driver outputs. For example, power combiners can be used in some embodiments to combine RF power of the two pre-driver outputs. 
     Also shown in  FIG. 2  is a capacitor Cf 1 , which can be used to filter the voltage Vpre 2  in order to hold it approximately constant across the RF signal swing. As shown in the embodiment of  FIG. 2 , the use of two stacked pre-drivers ( 202   a ), ( 202   b ) allows the use of a Vbat that is higher than the Vuse of the circuit, without any wasted power in voltage scaling. While two pre-drivers are shown in the embodiment of  FIG. 2 , the person skilled in the art will appreciate that a stack of n&gt;2 pre-drivers can also be provided, thus allowing the use of a Vbat equal to Vuse×n. 
     In the embodiment shown in  FIG. 2 , the regulator ( 210 ) can turn OFF the pre-driver stages ( 202   a ), ( 202   b ) by preventing current flow from Vbat to the pre-driver stages ( 202   a ), ( 202   b ). In particular, the input Vbias is operatively coupled to the regulator ( 210 ). The Vbias acts as an externally-supplied control signal. When Vbias is high, for example, the Vdrop across the regulator ( 210 ) may be very small, and the pre-driver stages ( 202   a ), ( 202   b ) are powered. On the other hand, when Vbias is low, the Vdrop is equal to Vbat, and the pre-driver circuitry does not consume any power. 
     The regulator ( 210 ) can also be used to separately control the power supply voltage presented to the pre-driver stages ( 202   a ), ( 202   b ) and the final PA stage ( 214 ). Moreover, control of the Vpre 1  and Vpre 2  voltages by way of the regulator ( 210 ) enables higher efficiency of the power amplifier ( 214 ) at maximum power by limiting the loss of power across the regulator ( 210 ), given that only a very small voltage drop is present across the regulator ( 210 ) compared to the voltage difference to which the stacked pre-driver stage is subjected. 
     By way of example, assuming typical nominal battery and maximum RF power conditions for the circuit in question with battery voltage of 3.5V, required pre-driver current to drive the input of the RF amplifier output stage of 200 mA, maximum voltage across devices of 2V, the voltage drop across the pre-driver regulator should be 1.5V to maintain device save operating conditions, resulting in a power loss of 1.5*0.2 A=0.3 W. With a circuit solution of two stacked pre-drivers, the full battery voltage could be applied to the pre-driver stack through a regulator with minimum voltage drop. For example, a typical regulator for this application would have a minimum resistance of 0.5 Ohms resulting in a voltage drop of 0.2 A*0.5 Ohms=0.1V. The 0.1V voltage drop across the regulator is negligible as compared to the voltage drop across the stack of pre-drivers that is 3.4V. The power loss in the regulator would be only 0.2 A*0.2 A*0.5 Ohms=0.02 W in this case. 
     Power savings can be also realized when a larger battery voltage is present, for example during battery charging, where the voltage could be as high as 4.5V. In this case the regulator should drop the voltage by 0.5V to maintain save operating conditions for the stack of two pre-drivers. The voltage drop across the regulator is still small when compared to the voltage difference to which the stack of pre-drivers is subjected. 
     As a consequence of the embodiment described above, the split topology shown in  FIG. 2  allows use of transistors having a breakdown voltage that is less than Vbat−Vref. The efficiency of the PA is maintained at a high level. In particular, because the power used in both of the pre-driver stages ( 202   a ), ( 202   b ) is used for amplification in the pre-driver, there is no power wasted in reducing the voltage from Vbat to Vuse. This results in improved power efficiency of the amplifier. 
     Those skilled in the electronic design arts will note that, due to the fact that Vdrop can be essentially equal to zero, approximately the entire battery voltage Vbat is present at a node ( 216 ). In particular, the regulator ( 210 ) exhibits only a small voltage drop due to its ON resistance, usually on the order of about 100 mV. Therefore, a higher percentage (as compared to the circuit of  FIG. 1 ) of the power that is consumed by the pre-drivers ( 202   a ), ( 202   b ) is used to drive the input of the PA ( 214 ), resulting in a higher overall efficiency of the PA ( 214 ) calculated with the inclusion of pre-driver power. 
       FIG. 3  shows a block diagram of a further embodiment of the present disclosure, where stacked pre-drivers drive separate power amplifiers. In particular, in the embodiment of  FIG. 3 , a pre-driver stage ( 302   a ) is connected with a PA 1  ( 314   a ), while a pre-driver stage ( 302   b ) is connected with a PA 2  ( 314   b ). As shown in the figure, an output Vg 1  of the pre-driver stage ( 302   a ) is connected to the input of the PA 1  ( 314   a ), while an output Vg 2  of pre-driver stage ( 302   b ) is connected to the input of the PA 2  ( 314   b ). 
     Those skilled in the electronic design arts will note that the PA 1  ( 314   a ) and the PA 2  ( 314   b ) are stacked across the power supply in the same manner as are the pre-driver stages ( 302   a ), ( 302   b ). Therefore, no voltage level shifting is necessary from each pre-driver to the respective PA. However, if required, DC blocking capacitors can be connected between the output of each pre-driver stage and the input of the respective PA. 
     As further shown in the embodiment of  FIG. 3 , the outputs of the power amplifiers ( 314   a ), ( 314   b ) are combined in a power combiner ( 324 ). The power combiner ( 324 ) has an output ( 326 ) that comprises the power amplifier output. RF power combiners are well known to those skilled in the art of RF circuit design. Any suitable on-chip or off-chip power combiner technology can be used. 
     By way of example and not of limitation, in the simplest case embodiment, the PA outputs are connected to each other, for example through a DC blocking capacitor, since they are in the same phase. In this simplest case embodiment, the output impedance of the two amplifiers is half that of a single, comparable PA. Such an embodiment has better impedance match between the RF output and the impedance of the device receiving the RF power if the receiving device has lower input impedance than the output impedance of a single PA. Better impedance match allows for higher power efficiency of the circuit. 
     In yet another embodiment, an off-chip power combiner with very high efficiency can be used to combine the PA outputs and convert the output impedance at high efficiency to a different desired output impedance, for example, 50 ohms. 
       FIG. 4  shows a block diagram of another embodiment of the present disclosure, where stacked differential pre-drivers are included. In particular, outputs Vg 1 , Vg 2  of pre-driver stages ( 402   a ), ( 402   b ) are designed to be opposite in phase, forming a differential amplifier. As shown in  FIG. 4 , the output of a driver amplifier ( 404   a ) is inverted compared to the output of driver amplifier ( 404   b ). Therefore, the outputs of a PA 1  ( 414   a ) and a PA 2  ( 414   b ) are in opposite phase to each other. As a consequence, the outputs of the PA 1  ( 414   a ) and the PA 2  ( 414   b ) can be used to drive, for example, a differential device. 
     The embodiment shown in  FIG. 4  shows, by way of example, a differential power combiner. It will be obvious to those skilled in the RF circuit design arts that other differentially-driven devices can be substituted for the differential power combiner such as a transformer, antenna, or any other convenient differential device. By way of example, the outputs of pre-driver stages ( 402   a ) and ( 402   b ) could drive a single differential driver stage or output stage having a (+) input connected to one of the two outputs and a (−) input connected to the other output. 
       FIG. 5  shows a block diagram of yet another embodiment of the present disclosed power amplifier, where separate power amplifiers PA 1  ( 514   a ), PA 2  ( 514   b ) are powered individually. In particular, as shown in  FIG. 5 , each PA 1  ( 514   a ) and PA 2  ( 514   b ) is powered with Vbat and Vref. Therefore, in the embodiment of  FIG. 5 , both PAs have the full power supply available for amplification of an input signal. 
     An output of a pre-driver stage ( 502   a ) is capacitively coupled, through a capacitor C 3 , to an input of the PA 1  ( 514   a ). The capacitor C 3  acts as a DC blocking capacitor, allowing the output of the pre-driver stage ( 502   a ), which swings from Vpre 1  to Vpre 2 , to input power into the PA 1  ( 514   a ). Similarly, the output of a pre-driver stage ( 502   b ) is capacitively coupled, through a capacitor C 4 , to the input of PA 2  ( 514   b ). The capacitor C 4  acts as a DC blocking capacitor, allowing the output of the pre-driver stage ( 502   b ), which swings from Vpre 2  to Vref, to input power into the PA 2  ( 514   b ). Those skilled in the electronics art will understand that in most cases a connection between the pre-driver stage ( 502   b ) and PA 2  ( 514   b ) is also possible without DC blocking capacitor C 4 , due to similar DC voltage requirements at the connection point between pre-driver stage ( 502   b ) and PA 2  ( 514   b ). 
     In the embodiment shown in  FIG. 5 , an amplifier driver ( 504   a ) has an output that is inverted compared to an amplifier driver ( 504   b ). In the embodiment shown in  FIG. 5 , the output of the circuit is differential. Those skilled in the electronics art will understand that the output of the amplifier driver ( 504   a ) can also be made non-inverting with respect to the amplifier driver ( 504   b ). In that case, the outputs of the PA 1  ( 514   a ) and the PA 2  ( 514   b ) can be summed directly through a power combiner (e.g., the power combiner  524 ) or through direct connection. 
     A number of embodiments of the present inventive concept have been described. Nevertheless, it will be understood that various modifications may be made without departing from the scope of the inventive teachings. 
     Accordingly, it is to be understood that the inventive concept is not to be limited by the specific illustrated embodiments, but only by the scope of the appended claims. The description may provide examples of similar features as are recited in the claims, but it should not be assumed that such similar features are identical to those in the claims unless such identity is essential to comprehend the scope of the claim. In some instances the intended distinction between claim features and description features is underscored by using slightly different terminology.