Abstract:
Switchmode DC-DC power converters using one or more non-Silicon-based switching transistors and a Silicon-based (e.g. CMOS) controller are disclosed. The non-Silicon-based switching transistors may comprise, but are not necessarily limited to, III-V compound semiconductor devices such as gallium arsenide (GaAs) metal-semiconductor field effect transistors (MESFETs) or heterostructure FETs such as high electron mobility transistors (HEMTs). According to an embodiment of the invention, the low figure of merit (FoM), τ FET , of the non-Silicon-based switching transistors allows the converters of the present invention to be employed in envelope tracking amplifier circuits of wireless devices designed for high-bandwidth technologies such as, for example, EDGE and UMTS, thereby improving the efficiency and battery saving capabilities of the wireless devices.

Description:
FIELD OF THE INVENTION  
       [0001]     The present invention relates generally to power conversion. More particularly, the present invention relates to extremely high-speed switchmode DC-DC converters.  
       BACKGROUND OF THE INVENTION  
       [0002]     Switchmode DC-DC power converters are commonly used in electronic devices. They operate by converting the voltage of an available voltage source to a voltage that complies with the voltage requirements of one or more components of the device. More particularly, switchmode DC-DC power converters operate to convert a direct current (DC) input voltage to a DC output voltage that is higher (boost converter) or lower (buck converter) than the DC input voltage. A common application of a buck converter is the conversion of an available voltage source in a personal computer (PC) to a voltage compatible with the voltage source requirements of devices and integrated circuits (e.g. the central processing unit (CPU)) on the motherboard of the PC.  
         [0003]     A conventional DC-DC buck converter  10  is shown in  FIG. 1 . A first Silicon-based n-channel metal-oxide-semiconductor field effect transistor (MOSFET)  12 , commonly referred to as the “high-side switch” of the converter  10  is coupled to a second Silicon-based MOSFET  14 , which is referred to as the “low-side switch”. Together, high-side switch  12  and low-side switch  14  control current flow through an inductor  16 . During a charging phase of operation of converter  10 , a controller  18  maintains high-side switch  12  in an ON condition and maintains low-side switch  14  in an OFF condition, thereby coupling a DC input voltage, VIN, to inductor  16 . This charging phase energizes inductor  16 , which stores energy in its magnetic field. Following the charging phase, converter  10  enters a discharging phase, during which time controller  18  maintains high-side switch  12  in an OFF condition and maintains low-side switch  14  in an ON condition, thereby decoupling VIN from inductor  16 . During this discharging phase inductor  16  operates as a current source, supplying current from the energy stored in its magnetic field into capacitor  22  and load  24 . Schottky diode  20  clamps any negative voltage from inductor  16  that may occur between the turn-OFF of switch  12  and the turn-ON of switch  14 .  
         [0004]     During the charging and discharging phases of operation of converter  10 , current through inductor  16  rises and falls linearly, resulting in a triangular-shaped current signal. Capacitor  22  filters the inductor current so that the output voltage, VOUT, of converter  10  is essentially DC. It can be shown that the average value of VOUT over time is equal to the product of the duty cycle, D, of the high-side switch switching period and the value of VIN. By way of a feedback loop  26 , VOUT is fed back to controller  18 , which dynamically compares VOUT to a reference voltage VREF and modifies D depending on whether the value of VOUT is higher than the desired output voltage level or lower than the desired output voltage level.  
         [0005]     In addition to switchmode power converters being of widespread use in the PC market, they are also prevalent in the wireless device industry. In this technology sector, switchmode power converters are used to not only provide efficient conversion for powering the baseband portion of the wireless device, but are also used to improve the efficiency of the power amplifier (PA) of the radio frequency (RF) transmitter portion of the wireless device. (The PA is usually the dominant power consumer of a wireless device.)  
         [0006]     The PA of a wireless device is designed so that the battery voltage supplied is large enough to permit maximum linear output voltage swing for the largest RF signal present at the PA RF input. However, because smaller RF input levels (i.e. lower PA drive levels) require less DC power for the same gain, the PA becomes inefficient at lower drive levels. To improve efficiency at lower drive levels, a dynamic control technique known as “envelope tracking” has been developed. According to this technique, the envelope of the PA RF input signal is tracked and used to regulate the battery voltage into a dynamically variable voltage source. The envelope tracking technique thereby improves PA efficiency. When applied to a conventional linear amplifier this technique tends to degrade linearity, as it varies the bias of the active devices. However, when applied to polar transmitters there is no sacrifice of linear performance, and the desired efficiency improvement is more readily realized.  
         [0007]     Accurate envelope tracking requires that the switching frequency of the switchmode DC-DC power converter be about 20 to 50 times higher than the required signal envelope bandwidth. For a signal such as EDGE (Enhanced Data GSM (Global System for Mobile Communications) Environment) this envelope bandwidth is 1 MHz, whereas for UMTS (Universal Mobile Telecommunications System) this envelope bandwidth expands to 10 MHz. For EDGE, this means that the DC-DC power converter must switch at a 20-50 MHz rate. This switching frequency requirement increases to 200-500 MHz for UMTS application. Unfortunately, most DC-DC power converters operate with a switching frequency below 1 MHz, and a 2 MHz switching frequency is considered to be extremely high. To meet this efficiency need, therefore, there is a need to increase the switching frequency of DC-DC converters by a factor of about 20 to 200.  
         [0008]     To evaluate the applicability of a variety of transistor types to the high-speed, high-current applications described above, a figure-of-merit (FoM) defined by the product of the transistor switch on-resistance RDSon and the average input capacitance CGS of the transistor (defined as the ratio of the gate-charge QG required to turn on the FET to the gate-source voltage VGS required to set up the controlling electric field, so that here CGS=QG/VGS) may be used. This FoM has units of time and may be expressed as τ FET =RDSon CGS. The validity of this approach is seen in  FIG. 2 , which plots several combinations of gate-charge and the corresponding channel resistance achieved for Silicon MOSFETs available commercially. The best-fit τ FET  model is drawn among the points as a continuous curve. Different values of τ FET  are needed for the PMOS and NMOS devices, which in this instance are 580 nanoseconds (ns) and 80 ns, respectively. It is also found that different manufacturers have slightly different values of τ FET  for their competitive devices, which demonstrates another intention for this FoM in that it should allow comparison of devices across process types.  
         [0009]     Since the conducting channel of a FET is controlled by the electric field between the gate and source terminals, the turning ON and OFF of this conducting channel depends on repeatedly moving this gate-charge into and out of the transistor. The value of the gate current required to move this charge depends on the amount of time allowed to move the charge. Clearly, to achieve high operating speed it is strongly desired to have a minimum amount of gate-charge necessary to control the channel. Assuming that no more than 40% of the operating time is spent in switching transitions (a very generous assumption) then it is possible to determine practical maximum values for τ FET , depending on the desired operating frequency f CLK . This determination is presented in  FIG. 3 . Commonly available values of τ FET  for modern Silicon MOSFET switching transistors range between 0.025 and 0.09 ns. As  FIG. 3  shows, these values limit the operating frequency of the Silicon MOSFET switching transistor (using CMOS driver circuitry) to under 1 MHz. To exceed this operating frequency it is necessary to use bipolar-based drive circuitry, a technique that is widely used in industry today. This is undesirable from both cost and integration compatibility points of view. It is strongly desired to design using only CMOS technology for cost reasons. Any use of bipolar transistors on a chip forces the use of a more expensive process. For most DC-DC switching converters the power switch transistors are external already, so process compatibility with these huge transistors is not an issue anyway. How to control and drive these huge transistors is an extremely important issue.  
         [0010]     An alternative figure of merit, called FET-FOM, can be considered which emphasizes the joint desirability of low switch on-resistance RDSon, low gate-charge (QG), and low gate-source voltage (VGS). This is defined as the product of these three parameters: FET-FOM=RDSon QG VGS. To compare various devices of different technologies, this alternative method has merit in that when gate-charge and gate-source voltage scale down together, the value of τ FET  will not change but the value of FET-FOM will fall on the fact that both parameters are now lower.  
         [0011]     To realize the desired efficiency improvements discussed above, the switching transistors of the DC-DC power converter must be capable of switching ampere-scale currents within a small fraction of the period of switching frequency. For example, to use a switching frequency of 100 MHz, the transistors must switch the supply currents ON or OFF in typically under one nanosecond. These requirements demand that the driving circuitry in the controller of the converter be robust enough to translate the low-level CMOS logic outputs of the controller  18  into drive signals capable of driving switching transistors  12  and  14 . Due to the large gate capacitances of the switching transistors  12  and  14 , however, the required size of the drivers could be prohibitively large and in many instances, irrespective of size, simply unable to transfer the gate charge QG fast enough to switch the switching transistors at the desired speed. This driver problem, in addition to the limits on the achievable τ FET S of Silicon-based switching devices, renders the conventional converter  10  in  FIG. 1  of no practical use for many applications including, for instance, use in the envelope tracking wireless device application described above.  
         [0012]     There is prior art where GaAs technology is used to build the entire DC-DC converter, including GaAs device technology to drive the switching transistors of the converter. See, G. Hanington, A. Metzger, P. Asbeck and H. Finlay, “Integrated DC-DC Converter having GaAs HBT Technology”, Electronics Letters, vol. 35, pp. 2110-2112, 1999; M. Ranjan, K. H. Koo, C. Fallesen, G. Hanington and P. Asbeck, “Microwave Power Amplifiers with Digitally-Controlled Power Supply Voltage for High Efficiency and Linearity”, 2000  IEEE MIT-S International Microwave Symposium Digest,  pp. 495, June 2000; S. Ajram, R. Kozlowski, H. Fawaz, D. Vandermoere and G. Salmer, “A fully GaAs-based 100 MHz,  2 W DC-to-DC Power Converter”,  Proceedings of the  27 th European Solid - State Device Research Conference,  Stuttgart, Germany, 22-24 September 1997. However, in addition to these prior art approaches using all-GaAs technology (e.g. GaAs MESFETs and/or HBTs), all of the prior art in which HBTs are employed is applicable to the boost (higher output voltage than input voltage) converter configuration only. That prior art is not applicable to the buck (lower output voltage than input voltage) converter configuration. HBT devices (or other bipolar devices) cannot be used for the shunt element (synchronous rectifier) because current flows in the reverse direction through these types of elements.  
         [0013]     In summary, the gate-charge required by Silicon MOSFET transistors is simply too high to achieve the operating frequencies necessary to support EDGE, UMTS and other high-frequency envelope following DC-DC converter applications. It would be desirable, therefore, to find an alternative approach, which can both meet the desired operating frequencies and reduce the costs of driver circuitry by allowing standard CMOS technology to be used.  
       SUMMARY OF THE INVENTION  
       [0014]     Switchmode DC-DC power converters employing one or more non-Silicon-based switching transistors and a Silicon-based (e.g. CMOS) controller are disclosed. The non-Silicon-based switching transistors may comprise, but are not necessarily limited to, III-V compound semiconductor devices such as gallium arsenide (GaAs) metal-semiconductor field effect transistors (MESFETs) or heterostructure FETs such as high electron mobility transistors (HEMTs). According to an embodiment of the invention, the low figure of merit (FoM), τ FET , of the non-Silicon-based switching transistors allows the converters of the present invention to be employed in envelope tracking amplifier circuits of wireless devices designed for high-bandwidth technologies such as, for example, EDGE and UMTS, thereby improving the efficiency and battery saving capabilities of the wireless devices.  
         [0015]     Further aspects of the invention are described and claimed below, and a further understanding of the nature and advantages of the inventions may be realized by reference to the remaining portions of the specification and the attached drawings. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0016]      FIG. 1  is a schematic diagram of a conventional synchronous DC-DC buck converter;  
         [0017]      FIG. 2  is a graph illustrating the τ FET  characteristics of various Silicon-based transistor switches of the prior art;  
         [0018]      FIG. 3  is a graph illustrating the minimum τ FET  values required for desired operating frequencies with direct drive from a standard CMOS digital integrated circuit;  
         [0019]      FIG. 4  is a schematic diagram of an exemplary DC-DC power converter, according to an embodiment of the present invention;  
         [0020]      FIG. 5  is a schematic diagram of an alternative exemplary DC-DC power converter, according to an embodiment of the present invention;  
         [0021]      FIG. 6  is a graph illustrating the τ FET  characteristics of a particular exemplary switching transistor type, the GaAs MESFET, which can be used in the DC-DC power converters shown in  FIG. 4 ,  FIG. 5 , or similar converter;  
         [0022]      FIG. 7A  is a table comparing FoM, i.e. τ FET , values of particular GaAs MESFET and pHEMT transistor switches to the τ FET  values of various Silicon-based transistor switches of the prior art, when the transistors are biased to their final gate-source voltages;  
         [0023]      FIG. 7B  is a graph comparing multiple transistor technologies using the alternative FET FoM;  
         [0024]      FIG. 8  is a graph showing the ratio of a CMOS driver size to Si NMOS switching transistor size as a function of switching transition time for cascaded CMOS gates made using transistors meeting the τ FET  values shown in  FIG. 7A ;  
         [0025]      FIG. 9  is a graph relating the number of CMOS driver stages required vs. operating frequency for Si NMOS and GaAs pHEMT switching transistor types;  
         [0026]      FIG. 10  is a schematic block diagram of an envelope tracking amplifier circuit, which may employ the DC-DC converter shown in  FIG. 4 , the DC-DC converter shown in  FIG. 5 , or similar converter, so as to improve the efficiency of a power amplifier of a wireless device, according to an embodiment of the present invention;  
         [0027]      FIG. 11  is a schematic block diagram of a direct polar transmitter circuit, which may employ the DC-DC converter shown in  FIG. 4 , the DC-DC converter shown in  FIG. 5 , or similar converter, so as to improve the efficiency of a power amplifier of a wireless device, according to an embodiment of the present invention; and  
         [0028]      FIG. 12  is a schematic block diagram of a high efficiency video amplifier/driver circuit, which may employ the DC-DC converter shown in  FIG. 4 , the DC-DC converter shown in  FIG. 5 , or similar converter, according to an embodiment of the present invention. 
     
    
     DETAILED DESCRIPTION  
       [0029]     Referring to  FIG. 4 , there is shown a schematic diagram of an exemplary synchronous DC-DC power converter  40 , according to an embodiment of the present invention. Converter  40  comprises a Silicon-based (e.g. CMOS) controller  41 , non-Silicon-based switching transistors  42  and  44 , a Schottky diode  46 , an inductor  48 , and a capacitor  49 .  FIG. 5  shows a schematic diagram of an alternative topology of a DC-DC converter  50 , according to an alternative embodiment of the present invention. Converter  50  comprises a Silicon-based (e.g. CMOS) controller  52 , a non-Silicon-based switching transistor  54 , a diode  55 , an inductor  56 , and a capacitor  58 . Unlike prior art power converters (e.g. the power converter  10  in  FIG. 1 ), converters  40  and  50  are capable of following wide-bandwidth envelope variations of wide-bandwidth technologies such as, for example, EDGE and UMTS. Accordingly, as described in more detail below, the power converters of the present invention may be used to, for example, improve the efficiencies and prolong the battery life of wireless devices.  
         [0030]     The ability to achieve the desired efficiencies and operating frequencies described above relates, at least in part, to the inventor&#39;s determination that Silicon-based devices lack the necessary FoM, i.e. the τ FET , required to achieve such efficiencies and operating frequencies.  FIG. 6  shows the τ FET  characteristics of a GaAs MESFET device, which is a transistor type that may be used for switching transistors  42  and  44  of converter  40  or for switching transistor  54  of converter  50  in the embodiments shown in  FIGS. 4 and 5 . When the gate of the GaAs MESFET is charged to its final gate-source voltage, the value of τ FET  is approximately 0.0005 nanoseconds (ns). This τ FET  value of the GaAs MESFET and the τ FET  value of a GaAs pseudomorphic high electron mobility transistor (pHEMT) are compared to models of various other Silicon transistor device technologies in  FIG. 7A . From  FIG. 7A  it is seen that the GaAs MESFET has a τ FET  value that is a factor of 160 times better (400 times better for the pHEMT) than the lowest τ FET  achieved by the other Silicon devices. This means that for a particular transistor ON resistance (RDSon) required by the load application, the GaAs MESFET and pHEMT technologies exhibits a lower switch input capacitance and, therefore, require a lower amount of gate charge of the Silicon-based FET technologies evaluated in  FIG. 7A . This distinction is further highlighted in  FIG. 7B , where the alternative FET-FOM (RDSon QG VGS) is plotted for many instances of switching FETs of multiple technologies. The Silicon-based devices all bunch together at the top, with values above unity. The GaAs MESFET devices have FET-FOM values approximately two orders of magnitude lower, due primarily to this technology&#39;s much lower gate-charge requirements compared to Silicon. The GaAs pHEMT devices are yet another two orders of magnitude lower, due to their lower VGS voltages and slightly lower gate-charge. Clearly the GaAs devices are far more desirable than the Silicon-based transistors for this high speed switch application.  
         [0031]     GaAs EpHEMT devices have achieved operating frequencies of over 250 MHz. This is two orders of magnitude greater than frequencies considered extremely high for all-Silicon switching transistor designs, placing this approach well beyond evolutionary improvement status of conventional designs. These results are achieved using pHEMT devices that are not specifically designed for this use, so further improvements are expected as the inventions set forth in this disclosure are capitalized upon by industry.  
         [0032]     Those skilled in the art will understand that the particular GaAs MESFET and pHEMT devices described above are only exemplary and that other non-Silicon-based device technologies using materials similar to or other than GaAs (e.g. other III/V compound semiconductors, high-temperature superconductors, etc.) may be used, so long as such device technologies are capable of functioning as bi-directional switches and are able to exhibit superior τ FET  and/or gate-charge characteristics compared to the Silicon-based MOSFET switches described in relation to the prior art or Similar τ FET  and/or gate-charge characteristics as that of the GaAs MESFET and pHEMT devices described in the context of the various embodiments of the present invention.  
         [0033]     Even if Silicon-based transistors could switch currents at the rate required for a particular application (which as described above is not necessarily the case), the driver circuitry required to switch the Silicon transistor would be large and costly, especially where the driver circuitry comprises part of the Silicon-based controller of the converter. The reason for this is that the gate capacitances of Silicon-based MOSFETs are much larger than the gate capacitances of non-Silicon-based devices like the GaAs MESFET and pHEMT switches described above, for example.  
         [0034]     When minimum transition time is the dominant performance metric, driver gates are sized to charge/discharge the load capacitance presented by the power switching transistor. Large gate capacitances require smaller driver on-resistances, with correspondingly larger driver devices, to achieve high switch transition speed. Therefore, faster switch operation requires larger driver devices, for the same switching transistor. If the speed requirement is fast enough, the driver devices may actually be no smaller than the actual power switch, and no gain is available to scale up from standard logic drive signals. Clearly this is a limiting case of no value.  
         [0035]     Consider, as an example, the drive requirements a one ampere switch made using GaAs MESFETs compared to the drive requirements of a switch made from Silicon-based MOSFETs.  FIG. 8  shows that for cascaded CMOS gates made using Silicon transistors meeting the FoM shown in  FIG. 7A , this unity ‘gain’ point (where the switch driver transistor is the same size as the switch transistor itself) occurs at about 75 MHz operating frequency.  FIG. 9  shows that when a Si NMOS switching transistor is used, the number of driver stages needed to attain this speed increases dramatically as the operating frequency passes above 1/10 of this limit. (In this example, the power switch has an ON channel resistance of 0.05 ohms, and the driving IC is limited to 30 ohms of drive resistance. These parameters are included not in any way to limit the scope of the inventions disclosed herein, but are included merely as a means of highlighting the difference in drive requirements between Silicon-based switching transistors and non-Si-based switching transistors.) These stages, while increasing in number, also increase in individual size, leading to an increase in controller die size, and ultimately an increase in cost.  
         [0036]     The power switch input capacitance is dramatically lower for GaAs MESFET and pHEMT switches than it is for a Silicon-based switches of equivalent current capability. For example, the ratio between Silicon NMOS and GaAs pHEMT input capacitances, found from  FIG. 7A , is approximately 0.08:0.0002˜400:1. The substantial drop in switch device equivalent input capacitances renders the CMOS driver stages from the CMOS lineup in  FIG. 9  no longer necessary. As shown in  FIG. 9 , the driver lineup for switches made from GaAs pHEMT devices is essentially eliminated. Indeed, when the switching transistor is changed to a GaAs pHEMT device, the CMOS digital controller can readily drive the power switch alone, even at a rate beyond 100 MHz. Accordingly, additional costs and Silicon area are saved by the embodiments of the present invention. Avoiding use of GaAs drive and control technology also saves costs and reduces complexities. The cost of GaAs materials is much higher (on the order of 5-10 times) than for Silicon. If GaAs is to be used, cost issues dictate that a minimum amount of GaAs be allowed. Addressing these cost concerns and other performance demanding concerns, embodiments of the present invention allow only the power switches of the converter to be in a non-Silicon technology. All other circuitry, including driver circuitry, is implemented in Silicon technologies such as, for example, CMOS.  
         [0037]     According to another embodiment of the invention, either of the DC-DC power converters in  FIGS. 4 and 5  (or similar power converter employing non-Silicon-based switching transistors) may be used in an envelope tracking amplifier circuit of a wireless device transmitter to improve the efficiency of the power amplifier (PA) of a radio frequency (RF) transmitter. An envelope tracking amplifier circuit  100  for improving the efficiency of a PA, according to these embodiments, is shown in  FIG. 10 . An RF input signal is applied to an input port of an RF coupler  102 , which operates to direct the RF input signal to an input port of a PA  104  and couple the RF input signal to an input of an envelope detector  106 , as is known in the art. PA  104  produces an RF output signal that is radiated by an antenna  107 . Envelope detector  106  operates to track the envelope variation of the coupled RF input signal and provide a control signal to a control input of a DC-DC power converter  108 , which as mentioned above may comprise either one of the power converters shown in  FIG. 4  or  5  or similar power converter employing non-Silicon-based switching transistors. Based on the envelope variation, the control signal regulates a battery  110  into a dynamically variable voltage that is used to power PA  104 . Because non-Silicon-based, low τ FET  devices are used in DC-DC power converter  108 , envelope tracking amplifier circuit  100  may be used to improve the efficiency and prolong the battery life of wireless devices that use UMTS, EDGE and other high-bandwidth technologies.  
         [0038]     According to another embodiment of the invention, either of the DC-DC power converters in  FIGS. 4 and 5  (or similar power converter employing non-Silicon-based switching transistors) may be used in a polar envelope modulator circuit of a wireless device transmitter to improve the efficiency of the PA of the transmitter. A polar transmitter circuit  200  for using an RF power generation stage (e.g. a PA) in its most efficient manner, being fully compressed, according to these embodiments, is shown in  FIG. 11 . RF power generation stage  204  is a three port device comprising two input ports and one output port. A first input port  215  is configured to receive an RF carrier signal with all angle modulation (if any). A second input port  217  is configured to receive an envelope control signal from DC-DC converter  208 . Modulation symbols are applied to an input port of a polar modulator  205 , which operates to convert the modulation symbols into their corresponding phase modulation (PM) and amplitude modulation (AM) components. The AM component is applied as a control signal to a control input of a DC-DC power converter  208 , which as mentioned above may comprise either one of the power converters shown in  FIG. 4  or  5  or similar power converter employing non-Silicon-based switching transistors. RF power generation stage  204  produces an RF output signal at RF output port  219 , which is then radiated by an antenna  207 . Based on the amplitude modulation signal received from polar modulator  205 , the DC-DC converter  208  regulates a battery  210  into a dynamically variable voltage that is used to power RF power generation stage  204 . Because non-Silicon-based, low τ FET  devices are used in DC-DC power converter  208 , polar transmitter circuit  200  may be used to provide long battery life for wireless devices that use UMTS, EDGE and other high-bandwidth technologies.  
         [0039]     According to yet another embodiment of the invention, either of the DC-DC power converters in  FIGS. 4 and 5  (or similar power converter employing non-Silicon-based switching transistors) may be used as a video driving amplifier to provide unusually high efficiency in such a broadband driver. A video amplifier/driver circuit  300  for providing high efficiency, according to these embodiments, is shown in  FIG. 12 . A video input signal is applied to both a non-inverting input port of an operational amplifier  322  and a voltage summer  324 . An offset voltage is added to the video signal by the voltage summer  324 , which provides a control signal to a control input of a DC-DC power converter  308 , which as mentioned above may comprise either one of the power converters shown in  FIG. 4  or  5  or similar power converter employing non-Silicon-based switching transistors. Based on the offset video signal, the control signal regulates a battery  310  into a dynamically variable voltage that is applied to pass transistor  320 . The output terminal of pass transistor  320  is applied to the video load  330 , shown in  FIG. 12  as a resistor. The signal on the output terminal of pass transistor  320  is also fed back to the inverting input of operational amplifier  322 . The output of operational amplifier  322  is connected to the control terminal of pass transistor  320 . Because non-Silicon-based, low τ FET  devices are used in DC-DC power converter  108 , envelope tracking amplifier circuit  100  may be used to improve the efficiency of video driving amplifiers, thereby reducing the heat they dissipate.  
         [0040]     Whereas the above is a complete description of the preferred embodiments of the invention, various alternatives, modifications, and equivalents may be used. Therefore, the above description should not be taken as limiting the scope of the invention as it is defined by the appended claims.