Patent Publication Number: US-10333470-B2

Title: Apparatus and methods for envelope tracking systems

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     The present application is a continuation of U.S. patent application Ser. No. 15/336,311, filed Oct. 27, 2016, titled “APPARATUS AND METHODS FOR ENVELOPE TRACKING SYSTEMS” which is a continuation of U.S. patent application Ser. No. 14/805,343, filed Jul. 21, 2015, titled “APPARATUS AND METHODS FOR ENVELOPE TRACKERS” which is a continuation of U.S. patent application Ser. No. 14/242,135, filed Apr. 1, 2014, titled “APPARATUS AND METHODS FOR ENVELOPE TRACKING IN RADIO FREQUENCY SYSTEMS,” which is a continuation of U.S. patent application Ser. No. 13/452,620, filed Apr. 20, 2012, titled “APPARATUS AND METHODS FOR ENVELOPE TRACKING,” which claims the benefit of priority under 35 U.S.C. § 119(e) of U.S. Provisional Patent Application No. 61/478,769, filed Apr. 25, 2011 titled “APPARATUS AND METHODS FOR ENVELOPE TRACKING”, each of which are herein incorporated by reference in their entireties. 
    
    
     BACKGROUND 
     Field 
     Embodiments of the invention relate to electronic systems, and in particular, to radio frequency (RF) electronics. 
     Description of the Related Technology 
     Power amplifiers can be used to boost the power of a RF signal having a relatively low power. Thereafter, the boosted RF signal can be used for a variety of purposes, included driving the antenna of a transmitter. 
     Power amplifiers can be included in mobile phones to amplify a RF signal for transmission. For example, in mobile phones having a time division multiple access (TDMA) architecture, such as those found in Global System for Mobile Communications (GSM), code division multiple access (CDMA), and wideband code division multiple access (W-CDMA) systems, a power amplifier can be used for RF signal amplification. It can be important to manage the amplification of a RF signal, as a desired transmit power level can depend on how far the user is away from a basestation and/or the mobile environment. Power amplifiers can also be employed to aid in regulating the power level of the RF signal over time, so as to prevent signal interference from transmission during an assigned receive time slot. 
     The power consumption of a power amplifier and therefore efficiency can be an important consideration. One technique for reducing power consumption of a power amplifier is envelope tracking, in which the voltage level of the power supply of the power amplifier is varied or controlled in relation to the envelope of the RF signal. Thus, when the envelope of the RF signal increases, the voltage supplied to the power amplifier can be increased. Likewise, when the envelope of the RF signal decreases, the voltage supplied to the power amplifier can be decreased to reduce power consumption. 
     There is a need for improved power amplifier systems. Furthermore, there is a need for improved envelope trackers for controlling power amplifier supply voltage. 
     SUMMARY 
     In certain embodiments, the present disclosure relates to a power amplifier system including a power amplifier configured to amplify a radio frequency (RF) signal and an envelope tracker configured to generate a power amplifier supply voltage for the power amplifier using an envelope of the RF signal. The envelope tracker includes a buck converter configured to generate a buck voltage from a battery voltage and a digital-to-analog converter (DAC) module configured to adjust a magnitude of the buck voltage based on the envelope of the RF signal to generate the power amplifier supply voltage. 
     In various embodiment, the DAC module includes a push DAC and a pull DAC, the push DAC configured to increase the power amplifier supply voltage when the envelope of the RF signal increases and the pull DAC configured to decrease the power amplifier supply voltage when the envelope of the RF signal decreases. 
     In a number of embodiments, the power amplifier system further includes a digital filter configured to receive the envelope of the RF signal and the power amplifier supply voltage and to generate a filtered envelope signal by filtering the envelope of the RF signal based at least in part on the power amplifier supply voltage. 
     In accordance with several embodiments, the power amplifier system further includes a digital shaping and delay module configured to receive the filtered envelope signal and to generate a shaped envelope signal. 
     In some embodiments, the power amplifier system further includes a thermometer decoder configured to receive the shaped envelope signal and to decode the shaped envelope signal to generate a plurality of push DAC control signals and a plurality of pull DAC control signals, the plurality of push DAC control signals and the plurality of pull DAC control signals coded in a thermometer coding. 
     According to a number of embodiments, the pull DAC includes a plurality of NMOS current sources and the push DAC includes a plurality of PMOS current sources. The plurality of NMOS current sources is disposed between the power amplifier supply voltage and a power low supply voltage and the plurality of PMOS current sources is disposed between the battery voltage and the power amplifier supply voltage. The gates of the plurality of NMOS current sources and the gates of the plurality of PMOS current sources are controlled by the plurality of pull DAC control signals and the plurality of push DAC control signals, respectively. 
     In various embodiments, a number of the plurality of NMOS current sources and a number of the plurality of PMOS current sources are each greater than or equal to sixteen. 
     In some embodiments, the power amplifier system further includes a ripple control module configured to receive the filtered envelope signal and to generate a first buck control signal and a second buck control signal using the filtered envelope signal. 
     In a number of embodiments, the buck converter includes a NMOS transistor and a PMOS transistor each including a gate, a source and a drain. The gates of the NMOS and PMOS transistors are electrically connected to the first and second buck control signals, respectively, the sources of the NMOS and PMOS transistors are electrically connected to a power low supply voltage and the battery voltage, respectively, and the drains of the NMOS and PMOS transistors are electrically connected together. 
     In accordance with several embodiments, the buck converter further includes an inductor having a first end electrically connected to the supply voltage of the power amplifier and a second end electrically connected to the drains of the NMOS and PMOS transistors. 
     In various embodiments, the power amplifier system further includes a transceiver for providing the envelope of the RF signal to the envelope tracker and the RF signal to the power amplifier. 
     In some embodiments, the power amplifier includes a bipolar transistor having an emitter, a base and a collector, the base configured to receive the RF signal, the emitter electrically connected to a power low supply voltage, and the collector configured to generate an amplified version of the RF signal. 
     In certain embodiments, the present disclosure relates to a method of envelope tracking in a power amplifier system. The method includes providing a power amplifier for amplifying a radio frequency (RF) signal and providing an envelope tracker for generating a supply voltage of the power amplifier using an envelope of the RF signal, the envelope tracker including a buck converter and a digital-to-analog (DAC) module. The method further includes generating a buck voltage from a battery voltage using the buck converter and adjusting the buck voltage using the DAC module to generate the supply voltage, a voltage magnitude of the adjustment based on the envelope of the RF signal. 
     In various embodiments, the digital-to-analog converter includes a push DAC and a pull DAC. 
     In some embodiments, adjusting the buck voltage using the DAC module includes increasing the supply voltage using the push DAC when the envelope of the RF signal increases and decreasing the supply voltage using the pull DAC when the envelope of the RF signal decreases. 
     In a number of embodiments, the method further includes filtering the envelope of the RF signal using a digital filter. 
     In accordance with several embodiments, the method further includes delaying the filtered envelope signal before providing the filtered envelope signal to the DAC module. 
     In some embodiments, the method further includes delaying the filtered envelope signal before providing the filtered envelope signal to the DAC module includes determining a duration of delay based on a difference in delays between the DAC module and the buck converter. 
     In certain embodiments, the method further includes shaping the filtered envelope signal to generate a shaped envelope signal. 
     In various embodiments, the method further includes converting the shaped envelope signal to a push DAC control signal and a pull DAC control signal, the push DAC and pull DAC control signals coded in a thermometer coding. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a schematic diagram of a power amplifier module for amplifying a radio frequency (RF) signal. 
         FIG. 2  is a schematic block diagram of an example wireless device that can include one or more of the power amplifier modules of  FIG. 1 . 
         FIG. 3A  is a schematic block diagram of one example of a power amplifier system including an envelope tracking system. 
         FIG. 3B  is a schematic block diagram of another example of a power amplifier system including an envelope tracking system. 
         FIGS. 4A-4C  show three examples of a power supply voltage versus time. 
         FIG. 5  is a schematic diagram of another example of a power amplifier system including an envelope tracking system. 
         FIG. 6  is a schematic diagram of one embodiment of an envelope tracking system. 
         FIG. 7  is a schematic diagram of another embodiment of an envelope tracking system. 
         FIG. 8  is a flow chart illustrating a method for generating a power amplifier supply voltage in accordance with one embodiment. 
         FIG. 9  is a schematic diagram of one embodiment of a pull digital-to-analog converter (DAC). 
         FIG. 10  is a graph of one example of input power versus efficiency for various power amplifier supply voltages. 
         FIG. 11  is a graph of one example of an input envelope signal versus a shaped envelope signal. 
         FIG. 12  is a graph of one example of power versus frequency for an envelope tracker. 
     
    
    
     DETAILED DESCRIPTION OF EMBODIMENTS 
     The headings provided herein, if any, are for convenience only and do not necessarily affect the scope or meaning of the claimed invention. 
     Apparatus and methods for envelope tracking are disclosed herein. In certain implementations, an envelope tracker is provided for generating a supply voltage of a power amplifier based on an envelope of a RF signal amplified by the power amplifier. The envelope tracker includes a buck converter, and a push-pull digital-to-analog converter (DAC). The buck converter can generate a buck or step-down voltage based on a low frequency component of the envelope signal, while the push-pull DAC can adjust the DC voltage to generate the supply voltage based on a high frequency component of the envelope signal. The push-pull DAC can be controlled, for example, by using digital signals generated by filtering, shaping, and/or delaying the envelope signal. Employing a combination of a buck converter and a push-pull DAC can reduce design complexity and/or improve overall power efficiency of the envelope tracking system relative to a scheme employing a DC-to-DC converter and a class AB amplifier, which typically requires an analog band pass filter for noise reduction and/or an analog delay element for output alignment. 
     Overview of Power Amplifier Systems 
       FIG. 1  is a schematic diagram of a power amplifier module for amplifying a radio frequency (RF) signal. The illustrated power amplifier module (PAM)  10  can be configured to amplify a RF signal IN to generate an amplified RF signal OUT. As described herein, the power amplifier module can include one or more power amplifiers. 
       FIG. 2  is a schematic block diagram of an example wireless or mobile device  11  that can include one or more of the power amplifier modules of  FIG. 1 . The wireless device  11  can include an envelope tracker implementing one or more features of the present disclosure. 
     The example wireless device  11  depicted in  FIG. 2  can represent a multi-band and/or multi-mode device such as a multi-band/multi-mode mobile phone. By way of examples, Global System for Mobile (GSM) communication standard is a mode of digital cellular communication that is utilized in many parts of the world. GSM mode mobile phones can operate at one or more of four frequency bands: 850 MHz (approximately 824-849 MHz for Tx, 869-894 MHz for Rx), 900 MHz (approximately 880-915 MHz for Tx, 925-960 MHz for Rx), 1800 MHz (approximately 1710-1785 MHz for Tx, 1805-1880 MHz for Rx), and 1900 MHz (approximately 1850-1910 MHz for Tx, 1930-1990 MHz for Rx). Variations and/or regional/national implementations of the GSM bands are also utilized in different parts of the world. 
     Code division multiple access (CDMA) is another standard that can be implemented in mobile phone devices. In certain implementations, CDMA devices can operate in one or more of 800 MHz, 900 MHz, 1800 MHz and 1900 MHz bands, while certain W-CDMA and Long Term Evolution (LTE) devices can operate over, for example, about 22 radio frequency spectrum bands. 
     One or more features of the present disclosure can be implemented in the foregoing example modes and/or bands, and in other communication standards. For example, 3G, 4G, LTE, and Advanced LTE are non-limiting examples of such standards. 
     In certain embodiments, the wireless device  11  can include switches  12 , a transceiver component  13 , an antenna  14 , power amplifiers  17 , a control component  18 , a computer readable medium  19 , a processor  20 , a battery  21 , and an envelope tracker  30 . 
     The transceiver component  13  can generate RF signals for transmission via the antenna  14 . Furthermore, the transceiver component  13  can receive incoming RF signals from the antenna  14 . 
     It will be understood that various functionalities associated with the transmission and receiving of RF signals can be achieved by one or more components that are collectively represented in  FIG. 2  as the transceiver  13 . For example, a single component can be configured to provide both transmitting and receiving functionalities. In another example, transmitting and receiving functionalities can be provided by separate components. 
     Similarly, it will be understood that various antenna functionalities associated with the transmission and receiving of RF signals can be achieved by one or more components that are collectively represented in  FIG. 2  as the antenna  14 . For example, a single antenna can be configured to provide both transmitting and receiving functionalities. In another example, transmitting and receiving functionalities can be provided by separate antennas. In yet another example, different bands associated with the wireless device  11  can be provided with different antennas. 
     In  FIG. 2 , one or more output signals from the transceiver  13  are depicted as being provided to the antenna  14  via one or more transmission paths  15 . In the example shown, different transmission paths  15  can represent output paths associated with different bands and/or different power outputs. For instance, the two example power amplifiers  17  shown can represent amplifications associated with different power output configurations (e.g., low power output and high power output), and/or amplifications associated with different bands. Although  FIG. 2  illustrates a configuration using two transmission paths  15 , the wireless device  11  can include more or fewer transmission paths  15 . 
     In  FIG. 2 , one or more detected signals from the antenna  14  are depicted as being provided to the transceiver  13  via one or more receiving paths  16 . In the example shown, different receiving paths  16  can represent paths associated with different bands. For example, the four example paths  16  shown can represent quad-band capability that some wireless devices are provided with. Although  FIG. 2  illustrates a configuration using four receiving paths  16 , more or fewer receiving paths  16  can be employed in the wireless device  11 . 
     To facilitate switching between receive and transmit paths, the switches  12  can be configured to electrically connect the antenna  14  to a selected transmit or receive path. Thus, the switches  12  can provide a number of switching functionalities associated with an operation of the wireless device  11 . In certain embodiments, the switches  12  can include a number of switches configured to provide functionalities associated with, for example, switching between different bands, switching between different power modes, switching between transmission and receiving modes, or some combination thereof. The switches  12  can also be configured to provide additional functionality, including filtering and/or duplexing of signals. 
       FIG. 2  shows that in certain embodiments, a control component  18  can be provided for controlling various control functionalities associated with operations of the switches  12 , the power amplifiers  17 , the envelope tracker  30 , and/or other operating component(s). Non-limiting examples of the control component  18  are described herein in greater detail. 
     In certain embodiments, a processor  20  can be configured to facilitate implementation of various processes described herein. For the purpose of description, embodiments of the present disclosure may also be described with reference to flowchart illustrations and/or block diagrams of methods, apparatus (systems) and computer program products. It will be understood that each block of the flowchart illustrations and/or block diagrams, and combinations of blocks in the flowchart illustrations and/or block diagrams, may be implemented by computer program instructions. These computer program instructions may be provided to a processor of a general purpose computer, special purpose computer, or other programmable data processing apparatus to produce a machine, such that the instructions, which execute via the processor of the computer or other programmable data processing apparatus, create means for implementing the acts specified in the flowchart and/or block diagram block or blocks. 
     In certain embodiments, these computer program instructions may also be stored in a computer-readable memory  19  that can direct a computer or other programmable data processing apparatus to operate in a particular manner, such that the instructions stored in the computer-readable memory produce an article of manufacture including instruction means which implement the acts specified in the flowchart and/or block diagram block or blocks. The computer program instructions may also be loaded onto a computer or other programmable data processing apparatus to cause a series of operations to be performed on the computer or other programmable apparatus to produce a computer implemented process such that the instructions that execute on the computer or other programmable apparatus provide steps for implementing the acts specified in the flowchart and/or block diagram block or blocks. 
     The illustrated wireless device  11  also includes the envelope tracker  30 , which can be used to generate a supply voltage for one or more of the power amplifiers  17 . For example, the envelope tracker  30  can be configured to vary or control the supply voltage provided to the power amplifiers  17  based upon an envelope of the RF signal to be amplified. 
     The envelope tracker  30  can be electrically connected to the battery  21 . The battery  21  can be any suitable battery for use in the wireless device  11 , including, for example, a lithium-ion battery. As will be described in detail further below, by controlling a magnitude of the supply voltage provided to the power amplifiers, the power consumption of the battery  21  can be reduced, thereby improving performance of the wireless device  11 . The envelope signal can be provided to an envelope tracker of the envelope tracker  30  from the transceiver  13 . However, the envelope can be determined in other ways. For example, the envelope can be determined by detecting the envelope from the RF signal using any suitable envelope detector. 
       FIG. 3A  is a schematic block diagram of one example of a power amplifier system  25  including an envelope tracking system. The illustrated power amplifier system  25  includes the switches  12 , the transceiver  13 , the antenna  14 , the battery  21 , a delay element  29 , a power amplifier or PA  32 , and an envelope tracker  30 . 
     The transceiver  13  can generate a RF signal, and can provide the RF signal to the power amplifier  32 . The power amplifier  32  can amplify the RF signal and provide the amplified RF signal to an input of the switches  12 , which can be as described earlier. The switches  12  can have an output electrically connected to the antenna  14 . Although not illustrated in this figure, persons of ordinary skill in the art will appreciate that additional power amplifiers can be electrically connected to the antenna  14  through the switches  12  to aid in providing a desired number of transmit paths. 
     The transceiver  13  can provide the envelope of the RF signal to the envelope tracker  30 . In certain implementations, a delay element  29  can be included at an input of the envelope tracker  30  to compensate for a difference in delays between a path of the RF signal through the power amplifier  32  and a path of the envelope signal through the envelope tracker  30 . The envelope tracker  30  can receive a battery voltage V BATT  from the battery  21 , and can use the envelope signal to generate a power amplifier supply voltage V CC   _   PA  for the power amplifier  32  that changes in relation to the envelope signal. 
     Although the transceiver  13  is illustrated as providing the envelope signal to the envelope tracker  30 , the envelope signal can be generated in any suitable manner. For example, an envelope detector  31  can be provided and used to generate an envelope signal from the RF signal. 
       FIG. 3B  is a schematic block diagram of another example of a power amplifier system  26  including an envelope tracking system. The illustrated power amplifier system  26  includes the switches  12 , the antenna  14 , the battery  21 , a directional coupler  24 , the envelope tracker  30 , the power amplifier  32 , and a transceiver  33 . The illustrated transceiver  33  includes a baseband processor  34 , an envelope shaping block  35 , a digital-to-analog converter (DAC)  36 , an I/Q modulator  37 , a mixer  38 , and an analog-to-digital converter (ADC)  39 . 
     The baseband signal processor  34  can be used to generate an I signal and a Q signal, which can be used to represent a sinusoidal wave or signal of a desired amplitude, frequency, and phase. For example, the I signal can be used to represent an in-phase component of the sinusoidal wave and the Q signal can be used to represent a quadrature component of the sinusoidal wave, which can be an equivalent representation of the sinusoidal wave. In certain implementations, the I and Q signals can be provided to the I/Q modulator  37  in a digital format. The baseband processor  34  can be any suitable processor configured to process a baseband signal. For instance, the baseband processor  34  can include a digital signal processor, a microprocessor, a programmable core, or any combination thereof. Moreover, in some implementations, two or more baseband processors  34  can be included in the electronic system  26   
     The I/Q modulator  37  can be configured to receive the I and Q signals from the baseband processor  34  and to process the I and Q signals to generate a RF signal. For example, the I/Q modulator  37  can include DACs configured to convert the I and Q signals into an analog format, mixers for upconverting the I and Q signals to radio frequency, and a signal combiner for combining the upconverted I and Q signals into a RF signal suitable for amplification by the power amplifier  32 . In certain implementations, the I/Q modulator  37  can include one or more filters configured to filter frequency content of signals processed therein. 
     The envelope shaping block  35  can be used to convert envelope or amplitude data associated with the I and Q signals into shaped envelope data. Shaping the envelope data from the baseband processor  34  can aid in enhancing performance of the power amplifier system  26  by, for example, adjusting the envelope signal to optimize linearity of the power amplifier  32  and/or to achieve a desired gain compression of the power amplifier  32 . In certain implementations, the envelope shaping block  35  is a digital block, and the DAC  36  is used to convert the shaped envelope data into an analog envelope signal suitable for use by the envelope tracker  30 . However, in other implementations, the DAC  36  can be omitted in favor of providing the envelope tracker  30  with a digital envelope signal to aid the envelope tracker  30  in further processing of the envelope signal. 
     The envelope tracker  30  can receive the envelope signal from the transceiver  33  and a battery voltage V BATT  from the battery  21 , and can use the envelope signal to generate a power amplifier supply voltage V CC   _   PA  for the power amplifier  32  that changes in relation to the envelope. The power amplifier  32  can receive the RF signal from the I/Q modulator  37  of the transceiver  33 , and can provide an amplified RF signal to the antenna  14  through the switches  12 . 
     The directional coupler  24  can be positioned between the output of the power amplifier  32  and the input of the switches  12 , thereby allowing an output power measurement of the power amplifier  32  that does not include insertion loss of the switches  12 . The sensed output signal from the directional coupler  24  can be provided to the mixer  38 , which can multiply the sensed output signal by a reference signal of a controlled frequency so as to downshift the frequency of the sensed output signal. The downshifted signal can be provided to the ADC  39 , which can convert the downshifted signal to a digital format suitable for processing by the baseband processor  34 . By including a feedback path between the output of the power amplifier  32  and the baseband processor  34 , the baseband processor  34  can be configured to dynamically adjust the I and Q signals and/or envelope data associated with the I and Q signals to optimize the operation of the power amplifier system  26 . For example, configuring the power amplifier system  26  in this manner can aid in controlling the power added efficiency (PAE) and/or linearity of the power amplifier  32 . 
       FIGS. 4A-4C  show three examples of a power amplifier supply voltage versus time. 
     In  FIG. 4A , a graph  47  illustrates the voltage of a RF signal  41  and a power amplifier supply  43  versus time. The RF signal  41  has an envelope  42 . 
     It can be important that the voltage of the power amplifier supply  43  be greater than a voltage of the RF signal  41 . For example, providing a supply voltage to a power amplifier having a magnitude less than that of the RF signal  41  can clip the RF signal, thereby creating signal distortion and/or other problems. Thus, it is important the power amplifier supply  43  have a voltage greater than that of the envelope  42 . However, it can be desirable to reduce a difference in voltage between the power amplifier supply  43  and the envelope  42  of the RF signal  41 , as the area in the graph  47  between the power amplifier supply  43  and the envelope  42  can represent lost energy, which can reduce battery life and increase heat generated in a mobile device. 
     In  FIG. 4B , a graph  48  illustrates the voltage of a RF signal  41  and a power amplifier supply  44  versus time. In contrast to the power amplifier supply  43  of  FIG. 4A , the power amplifier supply  44  of  FIG. 4B  varies or changes in relation to the envelope  42  of the RF signal  41 . The area between the power amplifier supply  44  and the envelope  42  in  FIG. 4B  is less than the area between the power amplifier supply  43  and the envelope  42  in  FIG. 4A , and thus the graph  48  of  FIG. 4B  can be associated with a power amplifier system having greater energy efficiency. 
       FIG. 4C  is a graph  49  illustrating a power supply voltage  45  that varies in relation to the envelope  42  of the RF signal  41 . In contrast to the power supply voltage  44  of  FIG. 4B , the power supply voltage  45  of  FIG. 4C  varies in discrete voltage increments. Certain implementations described herein can be used in combination with envelope trackers that control a power supply voltage in relation to an envelope signal either continuously or in discrete increments. 
       FIG. 5  is a schematic diagram of another example of a power amplifier system  60  including an envelope tracking system. The illustrated power amplifier system  60  includes the envelope tracker  30 , the power amplifier  32 , an inductor  62 , a load capacitor  63 , an impedance matching block  64 , the switches  12 , and the antenna  14 . The illustrated envelope tracker  30  is configured to receive a battery voltage V BATT  and an envelope of the RF signal and to generate a power amplifier supply voltage V CC   _   PA  for the power amplifier  32 . 
     The illustrated power amplifier  32  includes a bipolar transistor  61  having an emitter, a base, and a collector. The emitter of the bipolar transistor  61  can be electrically connected to a power low supply voltage V 1 , which can be, for example, a ground node, and a radio frequency (RF) signal can be provided to the base of the bipolar transistor  61 . The bipolar transistor  61  can amplify the RF signal and provide the amplified RF signal at the collector. The bipolar transistor  61  can be any suitable device. In one implementation, the bipolar transistor  61  is a heterojunction bipolar transistor (HBT). 
     The power amplifier  32  can be configured to provide the amplified RF signal to the switches  12 . The impedance matching block  64  can be used to aid in terminating the electrical connection between the power amplifier  32  and the switches  12 . For example, the impedance matching block  64  can be used to increase power transfer and/or reduce reflections of the amplified RF signal generated by the power amplifier  32 . 
     The inductor  62  can be included to aid in biasing the power amplifier  32  with the power amplifier supply voltage V CC   _   PA  generated by the envelope tracker  30 . The inductor  62  can include a first end electrically connected to the envelope tracker  30 , and a second end electrically connected to the collector of the bipolar transistor  61 . The load capacitor  63  can have a first end electrically connected to the collector of the bipolar transistor  61  and a second end electrically connected to a power low supply voltage V 1 , and can represent the capacitance of the power amplifier  32  that is seen by the envelope tracker  30 . For example, the capacitor  63  can represent the parasitic capacitance of the bipolar transistor  61  and/or capacitive elements of the match block  64 . The capacitor  63  can aid in providing noise filtering of the power supply voltage V CC   _   PA  generated by the envelope tracker  30 . However, the capacitor  63  also can impact the bandwidth response of the envelope tracker  30 . 
     Although  FIG. 5  illustrates one implementation of the power amplifier  32 , skilled artisans will appreciate that the teachings described herein can be applied to a variety of power amplifier structures, including, for example, multi-stage power amplifier structures and power amplifiers employing other transistor structures. 
     Overview of Envelope Tracking Systems 
     An envelope tracker can be used to vary or control a power amplifier supply voltage to improve the efficiency of a power amplifier system. It can be important to improve the power efficiency and/or to reduce the design complexity of the envelope tracker. For example, it can be desirable to provide a power amplifier system that does not require analog filters and analog delay elements that can increase power amplifier complexity. 
     Conventional envelope tracking systems can include a DC-to-DC converter operating in parallel with a class AB amplifier. The DC-to-DC converter can have a relatively high efficiency and low bandwidth, and can be used to track a relatively low frequency component of the envelope signal. The class AB amplifier can have a lower efficiency than the DC-to-DC converter, but can also have a wider bandwidth that is suitable for tracking a relatively high frequency component of the envelope signal. However, since the class AB amplifier can have a relatively large bandwidth, the class AB amplifier can require a complex analog band pass filter for noise reduction. Furthermore, it can be difficult to align the outputs of the class AB amplifier and the DC-to-DC converter. 
     In certain implementations described herein, an envelope tracker including a buck converter and a push-pull digital-to-analog converter (DAC) is provided. The buck converter can aid in controlling the supply voltage at a relatively low frequency, while the push-pull DAC can be employed to provide relatively high frequency control of the supply voltage. The push-pull DAC can be controlled using digital signals generated by filtering, shaping, and/or delaying the envelope signal. Employing a combination of a buck converter and a push-pull DAC can reduce design complexity and/or improve overall power efficiency relative to a scheme employing a DC-to-DC converter and a class AB amplifier, which can require an analog band pass filter to reduce noise of the class AB amplifier and an analog delay block to align the output of the DC-to-DC converter and the class AB amplifier. 
       FIG. 6  is a schematic diagram of one embodiment of an envelope tracking system  70 . The envelope tracking system  70  includes the battery  21  and an envelope tracker  72 . The envelope tracker  72  is configured to receive an envelope signal and a battery voltage V BATT  and to generate a power amplifier supply voltage V CC   _   PA . 
     The envelope tracker  72  can control the amplitude of the power amplifier supply voltage V CC   _   PA  in relation to the amplitude of the envelope signal. The illustrated envelope tracker  72  includes a buck converter  73 , a control block  74 , a push DAC  78 , a pull DAC  79  and a load capacitor  77 . 
     The buck converter  73  includes first switch S 1 , a second switch S 2  and an inductor  75 . The first switch S 1  includes a first end electrically connected to the battery voltage V BATT  and a second end electrically connected to a first end of the inductor  75  and to a first end of the second switch S 2 . The second switch S 2  further includes a second end electrically connected to a power low supply voltage V 1 . The inductor  75  includes a second end electrically connected to the power amplifier supply voltage V CC   _   PA . 
     The control block  74  is configured to receive the envelope signal and to use the envelope signal to generate control signals for the buck converter  73 , the push DAC  78 , and the pull DAC  79 . For example, the control block  74  can generate a first plurality of control signals for controlling the state of the first and second switches S 1 , S 2 , and a second plurality of control signals for controlling the state of the push and pull DACs  78 ,  79 . In certain implementations, the control signals generated by the control block  74  are digital signals. Controlling both the buck converter  73  and the push and pull DACs  78 ,  79  using digital signals can aid in aligning the outputs of the push and pull DACs  78 ,  79  and the buck converter  73 , thereby reducing design complexity and/or improving the efficiency of the envelope tracker  73  relative to a scheme using a DC-to-DC converter operating in parallel with a class AB amplifier. 
     The control block  74  can receive one or more feedback signals to aid in enhancing envelope tracking control. For example, the control block  74  can receive a signal indicative of the amplitude of the power amplifier supply voltage V CC   _   PA . Additionally, to aid in controlling the first and second switches S 1 , S 2 , the control block  74  can be electrically connected to the first end of the inductor  75 . Providing feedback in this manner can help determine the direction of the current through the inductor  75 , which can aid in determining when to actuate the first and second switches S 1 , S 2 . 
     The push DAC  78  is disposed between the battery voltage V BATT  and the power amplifier supply voltage V CC   _   PA  and is controlled using the control block  74 . The pull DAC  79  is disposed between the power amplifier supply voltage V CC   _   PA  and the power low supply voltage V 1  and is controlled using the control block  74 . The control block  74  can use the push DAC  78  to increase the power amplifier supply voltage V CC   _   PA  when the envelope signal increases and can use the pull DAC  79  to decrease power amplifier supply voltage V CC   _   PA  when the envelope signal decreases. 
     As illustrated in  FIG. 6 , the load capacitor  77  can be disposed between the power amplifier supply voltage V CC   _   PA  and the power low supply voltage V 1 , and can represent the load capacitance of a variety of loads on the power amplifier supply voltage V CC   _   PA , such as a parasitic load capacitance associated with one or more power amplifiers electrically connected to the power amplifier supply voltage V CC   _   PA . The illustrated load capacitor  77  includes a first end electrically connected to the power amplifier supply voltage V CC   _   PA  and a second end electrically connected to the power low supply voltage V 1 . The load capacitor  77  can aid in reducing the noise of the power amplifier supply voltage V CC   _   PA , but can also reduce the bandwidth response of the envelope tracker  70 . In certain implementations, the load capacitor  77  is configured to have a value small enough to avoid constraining bandwidth while large enough to provide suitable noise filtering. The capacitance of the load capacitor  77  can be controlled in any suitable way, such as by the selection of the type and geometry of the devices electrically connected to the power amplifier supply voltage V CC   _   PA  and/or by controlling the geometry and/or layers used to form the power amplifier supply voltage node. In certain implementations, the load capacitor  77  has a value selected to be in the range of about 200 pF to about 4000 pF. 
       FIG. 7  is a schematic diagram of another embodiment of an envelope tracking system  80 . The envelope tracking system  80  includes a battery  21  and an envelope tracker  82 . The envelope tracker  82  is configured to receive a digital envelope signal and a battery voltage V BATT  and to generate a power amplifier supply voltage V CC   _   PA . 
     The envelope tracker  82  can change the amplitude of the power amplifier supply voltage V CC   _   PA  in relation to the amplitude of the digital envelope signal. The illustrated envelope tracker  82  includes a buck converter  83 , a push DAC  88 , a pull DAC  89 , a load capacitor  87 , a digital filter  90 , a ripple control block  91 , a digital shaping and delay block  92 , and a thermometer decoder  93 . The envelope tracker  82  can receive the digital envelope signal from any suitable source, such as a transceiver. In certain implementations, the digital envelope signal can be generated using an analog envelope signal and an analog-to-digital converter. 
     The buck converter  83  includes an NMOS transistor  82 , a PMOS transistor  81  and an inductor  85 . The PMOS transistor  81  includes a source electrically connected to the battery voltage V BATT , a gate configured to receive a first control signal from the ripple control block  91 , and a drain electrically connected to a first end of the inductor  85  and to a drain of the NMOS transistor  82 . The NMOS transistor  82  further includes a gate configured to receive a second control signal from the ripple control block  91  and a source electrically connected to the power low supply voltage V 1 . The inductor  85  includes a second end electrically connected to the power amplifier supply voltage V CC   _   PA . 
     The push DAC  88  is disposed between the battery voltage V BATT  and the power amplifier supply voltage V CC   _   PA , and includes a plurality of PMOS current cell transistors  98   a - 98   c . Each PMOS current cell transistor  98   a - 98   c  includes a source electrically connected to the battery voltage V BATT  and a drain electrically connected to the power amplifier supply voltage V CC   _   PA . The gates of the PMOS current cell transistors  98   a - 98   c  are controlled by the thermometer decoder  93 , which can selectively activate one or more of the PMOS current cell transistors  98   a - 98   c  so as to increase the power amplifier supply voltage V CC   _   PA . In certain implementations, the number of PMOS current cell transistors  98   a - 98   c  is selected to be greater than or equal to about 16. For example, the number of PMOS current cell transistors  98   a - 98   c  can be selected to be in the range of about 16 to about 128. 
     The pull DAC  89  is disposed between the power amplifier supply voltage V CC   _   PA  and the power low supply voltage V 1 , and includes a plurality of NMOS current cell transistors  99   a - 99   c . Each NMOS current cell transistor  99   a - 99   c  includes a source electrically connected to the power low supply voltage V 1  and a drain electrically connected to the power amplifier supply voltage V CC   _   PA . The gates of the NMOS current cell transistors  99   a - 99   c  are controlled by the thermometer decoder  93 , which can be used to selectively activate one or more of the NMOS current cell transistors  99   a - 99   c  so as to decrease the power amplifier supply voltage V CC   _   PA . In certain implementations, the number of NMOS current cell transistors  99   a - 99   c  is selected to be greater than or equal to about 16. For example, the number of NMOS current cell transistors  99   a - 99   c  can be selected to be in the range of about 16 to about 128. 
     As illustrated in  FIG. 7 , the load capacitor  87  can be disposed between the power amplifier supply voltage V CC   _   PA  and the power low supply voltage V 1 . For example, the illustrated load capacitor  87  includes a first end electrically connected to the power amplifier supply voltage V CC   _   PA  and a second end electrically connected to the power low supply voltage V 1 . The load capacitor  87  can aid in reducing the noise of the power amplifier supply voltage V CC   _   PA  and/or can be used to provide stability to a power amplifier that is connected to the power amplifier supply voltage V CC   _   PA . Additional details of the load capacitor  87  can be similar to those described above with respect to the load capacitor  77  of  FIG. 6 . 
     The digital filter block  90  is configured to receive the envelope signal and one or more feedback signals, and can use the feedback signals to filter the envelope signal to generate a filtered envelope signal. For example, the digital filter block  90  can be electrically connected to the power amplifier supply voltage V CC   _   PA  and/or to one or more nodes of the buck converter  83 , thereby improving the operation of the digital filter block  90 . The digital filter block  90  can employ a variety of filtering techniques, including, for example, finite impulse response techniques. As illustrated in  FIG. 7 , both the digital shaping and delay block  92  and the ripple control block  91  can be configured to receive the filtered envelope signal and to use the filtered envelope signal to control the DACs  88 ,  89  and the buck converter  83 , respectively. Configuring the envelope tracker  82  in this manner can aid in aligning the outputs of the DACs  88 ,  89  and the buck converter  83 , thereby improving the efficiency of the power amplifier system and/or reducing design complexity. 
     The ripple control block  91  can receive the filtered envelope signal from the digital filter block  90  and a feedback signal from the buck converter  83 , and can use the filtered envelope signal and the feedback signal to generate control signals for the buck converter  83 . For example, the ripple control block  91  has been configured to generate first and second switch control signals for controlling the flow of current through the NMOS transistor  81  and the PMOS transistor  82 , respectively. 
     The digital shaping and delay block  92  is configured to receive the filtered envelope signal from the digital filter  90 , and to shape and/or delay the filtered envelope signal to generate a shaped envelope signal. For example, the digital shaping and delay module  92  can delay the filtered envelope signal to align the outputs of the buck converter  83  and the push and pull DACs  88 ,  89  so as to compensate for a difference in delay between the digital envelope and the output of the buck converter  83  and the digital envelope and the output of the DACs  88 ,  89 . The digital shaping and delay block  92  can also be used to shape the envelope signal to generate a signal used to control the push and pull DACs  88 ,  89 . For example, the digital shaping and delay module  92  can include a look-up-table that maps the digital envelope signal to a DAC output level. The look-up-table can be configured based on, for example, the electrical properties of the transistors used in the push and pull DACs  88 ,  89 . 
     To aid in improving output noise, the envelope tracker  82  can include a thermometer decoder  93  disposed between the digital shaping and delay block  92  and the push and pull DACs  88 ,  89 . The thermometer decoder  93  can be used to convert the shaped envelope signal generated by the digital shaping and delay block, which can be a binary coded signal, into a thermometer coded signal. Converting the signal in this manner can aid in reducing switching noise generated by the push and pull DACs. For example, when using a thermometer decoder and a 16-bit push DAC, the thermometer decoder can control the gates of the PMOS transistors in the push DAC such that only one PMOS transistor switches when transitioning from a binary-coded shaped envelope signal value of “0000000011111111” to a binary-coded shaped envelope signal value of “0000000100000000”. 
       FIG. 8  is a flow chart illustrating a method for generating a power amplifier supply voltage in accordance with one embodiment. It will be understood that the method can include greater or fewer operations and the operations may be performed in any order, as necessary. 
     The method  100  starts at block  101 , in which a power amplifier is provided for amplifying a RF signal. For example, a power amplifier can be provided for amplifying a W-CDMA or GSM signal. 
     In an ensuing block  102 , an envelope tracker is provided for controlling the supply voltage of the power amplifier using the envelope of the RF signal. For example, the envelope tracker can be electrically connected to a battery, and can control an amplitude of a supply voltage provided to the power amplifier using an envelope received from a transmitter or other source. The envelope tracker includes a buck converter and a digital-to-analog conversion (DAC) module. The buck converter can be used to track a relatively low frequency component of the envelope to generate a buck or step-down voltage that is less than the battery voltage, while the DAC module can include a push DAC and a pull DAC for adjusting the output of the buck converter to correct for a relatively high frequency component of the envelope. In one embodiment, the corner frequency of the buck converter is less than or equal to about 200 kHz. 
     The method  100  continues at a block  102 , in which the buck converter is used to generate a buck voltage based on the envelope signal. In an ensuing block  103 , the DAC module is used to adjust the buck voltage to generate the supply voltage based on the envelope signal. Using an envelope tracker including a buck converter and a DAC module can increase the power efficiency of the system, and can avoid the need of implementing an analog band pass filter and/or analog delay block. For example, designs using a class AB amplifier and a buck converter can require the envelope signal to be processed to a format suitable for controlling the buck converter and to be filtered and translated to the class AB amplifier. Thus, a delay between the outputs of the buck converter and the class AB amplifier can occur, and techniques used to compensate for the delay can increase design complexity and/or lead to a reduction in power efficiency due to the output misalignment. 
     In certain implementations, the DAC module can include a push DAC having an array of PMOS current sources and a pull DAC having an array of NMOS current sources. The push DAC can increase the voltage of the power supply using the PMOS current sources when the envelope signal indicates that the output from the buck converter should be increased. Similarly, the pull DAC can decrease the voltage of the power supply using the NMOS current sources when the envelope signal indicates that the output from the buck converter should be decreased. 
       FIG. 9  is a schematic diagram of one embodiment of a pull DAC  120 . The pull DAC  120  includes a bias circuit  121  and a current source array  122 . The current source array  122  includes a bias input configured to receive a bias voltage V BIAS  from the bias circuit  121  and an output electrically connected to a power amplifier supply voltage V CC   _   PA . The pull DAC  120  has been annotated to include a load capacitance  123  and a load resistor  124  electrically connected in parallel between the power amplifier supply voltage V CC   _   PA  and the power low supply voltage V 1 . 
     The bias circuit  121  includes a current source  126  and a bias NMOS transistor  127 . The current source  126  includes a first end electrically connected to the power low supply voltage V 1  and a second end electrically connected to a source and a gate of the bias NMOS transistor  127 . The bias NMOS transistor  127  further includes a drain electrically connected to the battery voltage V BATT . As illustrated in  FIG. 9 , the current source  126  can be configured to generate a bias current I BIAS  and to provide the bias current I BIAS  through a channel of the bias NMOS transistor  127  so that the gate of the bias NMOS transistor  127  is biased to the bias voltage V BIAS . 
     The current source array  122  includes first to sixth switches  141 - 146  and first to sixth NMOS current source transistors  131 - 136 . The first to sixth NMOS current source transistors  131 - 136  each include a gate electrically connected to the bias voltage V BIAS  and a source electrically connected to the power amplifier supply voltage V CC   _   PA . The first or x1 NMOS current source transistor  131  further includes a drain electrically connected to a first end of the first switch  141 . The second or x2 NMOS current source transistor  132  further includes a drain electrically connected to a first end of the second switch  142 . The third or x4 NMOS current source transistor  133  further includes a drain electrically connected to a first end of the third switch  143 . The fourth or x8 NMOS current source transistor  134  further includes a drain electrically connected to a first end of the fourth switch  144 . The fifth or x16 NMOS current source transistor  135  further includes a drain electrically connected to a first end of the fifth switch  145 . The sixth or x32 NMOS current source transistor  136  further includes a drain electrically connected to a first end of the sixth switch  146 . The first to sixth switches  141 - 146  each further include a second end electrically connected to the battery voltage V BATT . 
     The current source array  122  can be configured to generate an output current I DAC  in response to a digital input signal. For example, the first to sixth switches  141 - 146  can be used to connect the battery voltage V BATT  to the drains of the first to sixth NMOS current source transistors  141 - 146 , respectively, based on the value of a six-bit digital input. Additionally, the first to sixth NMOS current source transistors  141 - 146  can have binary weighted values such that the output currents from the sources of the first to sixth NMOS current source transistors  141 - 146  sum to generate an output current I DAC  having a current magnitude that changes in relation to the digital input signal. As illustrated in  FIG. 9 , the bias voltage V BIAS  can be provided to the gates of the NMOS current source transistors  131 - 136 , which can be replicas of the bias NMOS transistor  127  such that the NMOS current source transistors  131 - 136  generate output currents that scale in relation to the bias current I BIAS . 
     The push DAC  120  has been annotated to show one example of a supply voltage waveform  125  for the power amplifier supply voltage V CC   _   PA . As shown in  FIG. 9 , the supply voltage waveform  125  changes relatively smoothly in response to changes in digital input to the push DAC  120 . Although the push DAC  120  generates an output current I DAC  that is digitized and changes in discrete increments in response to a digital input, the load capacitor  123  and the load resistor  124  can operate as a low pass filter to the power amplifier supply voltage V CC   _   PA , thereby generating a relatively smooth supply voltage waveform  125 . 
     The operation of the load capacitor  123  and the load resistor  124  as a low pass filter can reduce the impacts of quantization noise on the operation of a power amplifier electrically powered using the power amplifier supply voltage V CC   _   PA . Since the load capacitor  123  and the load resistor  124  can prevent the power amplifier supply voltage V CC   _   PA  from rapidly changing in response to changes in a digital input signal of the DAC, in certain implementations a separate explicit filter need not be included to filter the power amplifier supply voltage V CC   _   PA . 
       FIG. 10  is a graph  160  of one example of input power versus efficiency for various power amplifier supply voltages. The graph  160  includes a first to fourth plots  161 - 164  of input power versus efficiency for a first supply voltage V PA1 , a second supply voltage V PA2 , a third supply voltage V PA3 , and a fourth supply voltage V PA4 , respectively, where V PA1 &lt;V PA2 &lt;V PA3 &lt;V PA4  and the first to fourth supply voltages V PA1 -V PA4  are each fixed DC supply voltages. As shown in  FIG. 10 , efficiency peaks at different input power levels for each of the first to fourth plots  161 - 164 . The graph  160  further includes a fifth plot  165  of input power efficiency for a supply voltage that changes in relation to the envelope signal of the input. As shown in  FIG. 10 , the fifth plot  165  associated with a power supply generated by an envelope tracker exhibits high efficiency levels over a wide range of input power levels. 
       FIG. 11  is a graph  170  of one example of an input envelope signal versus a shaped envelope signal. The graph  170  includes a plot  172  of a shaped envelope signal in relation to an input envelope signal. The graph  170  further includes a line  171  associated with no envelope shaping. As shown in  FIG. 11 , the plot  172  is associated with an envelope signal that has been shaped to have a larger amplitude relative to the line  171  for relatively small input envelope values. Shaping the envelope signal in this manner can help optimize linearity of a power amplifier system over a wide range of signal power levels. 
       FIG. 12  is a graph  180  of one example of power versus frequency for an envelope tracker described herein. The graph  180  includes a first plot  181  of envelope tracker output power versus frequency. The graph  180  further includes a second plot  182  of buck converter output power versus frequency and a third plot  183  of DAC output power versus frequency. As shown in the second and third plots  182 ,  183 , the buck converter can provide more output power than the DAC at low envelope signal frequencies while the DAC can provide more output power than the buck converter at high envelope frequencies. By configuring the buck converter to track low frequency components of the envelope signal, such as frequency components less than about 200 kHz, and by configuring the DAC to track high frequency components of the envelope signal, such as frequency components greater than about 200 kHz, the overall power efficiency of the envelope tracker can be increased. 
     Applications 
     Some of the embodiments described above have provided examples in connection with mobile phones. However, the principles and advantages of the embodiments can be used for any other systems or apparatus that have needs for power amplifier systems. 
     Such power amplifier systems can be implemented in various electronic devices. Examples of the electronic devices can include, but are not limited to, consumer electronic products, parts of the consumer electronic products, electronic test equipment, etc. Examples of the electronic devices can also include, but are not limited to, memory chips, memory modules, circuits of optical networks or other communication networks, and disk driver circuits. The consumer electronic products can include, but are not limited to, a mobile phone, a telephone, a television, a computer monitor, a computer, a hand-held computer, a personal digital assistant (PDA), a microwave, a refrigerator, an automobile, a stereo system, a cassette recorder or player, a DVD player, a CD player, a VCR, an MP3 player, a radio, a camcorder, a camera, a digital camera, a portable memory chip, a washer, a dryer, a washer/dryer, a copier, a facsimile machine, a scanner, a multi functional peripheral device, a wrist watch, a clock, etc. Further, the electronic devices can include unfinished products. 
     Conclusion 
     Unless the context clearly requires otherwise, throughout the description and the claims, the words “comprise,” “comprising,” and the like are to be construed in an inclusive sense, as opposed to an exclusive or exhaustive sense; that is to say, in the sense of “including, but not limited to.” The word “coupled”, as generally used herein, refers to two or more elements that may be either directly connected, or connected by way of one or more intermediate elements. Additionally, the word “connected”, as generally used herein, refers to two or more elements that may be either directly connected, or connected by way of one or more intermediate elements. Additionally, the words “herein,” “above,” “below,” and words of similar import, when used in this application, shall refer to this application as a whole and not to any particular portions of this application. Where the context permits, words in the above Detailed Description using the singular or plural number may also include the plural or singular number respectively. The word “or” in reference to a list of two or more items, that word covers all of the following interpretations of the word: any of the items in the list, all of the items in the list, and any combination of the items in the list. 
     Moreover, conditional language used herein, such as, among others, “can,” “could,” “might,” “can,” “e.g.,” “for example,” “such as” and the like, unless specifically stated otherwise, or otherwise understood within the context as used, is generally intended to convey that certain embodiments include, while other embodiments do not include, certain features, elements and/or states. Thus, such conditional language is not generally intended to imply that features, elements and/or states are in any way required for one or more embodiments or that one or more embodiments necessarily include logic for deciding, with or without author input or prompting, whether these features, elements and/or states are included or are to be performed in any particular embodiment. 
     The above detailed description of embodiments of the invention is not intended to be exhaustive or to limit the invention to the precise form disclosed above. While specific embodiments of, and examples for, the invention are described above for illustrative purposes, various equivalent modifications are possible within the scope of the invention, as those skilled in the relevant art will recognize. For example, while processes or blocks are presented in a given order, alternative embodiments may perform routines having steps, or employ systems having blocks, in a different order, and some processes or blocks may be deleted, moved, added, subdivided, combined, and/or modified. Each of these processes or blocks may be implemented in a variety of different ways. Also, while processes or blocks are at times shown as being performed in series, these processes or blocks may instead be performed in parallel, or may be performed at different times. 
     The teachings of the invention provided herein can be applied to other systems, not necessarily the system described above. The elements and acts of the various embodiments described above can be combined to provide further embodiments. 
     While certain embodiments of the inventions have been described, these embodiments have been presented by way of example only, and are not intended to limit the scope of the disclosure. Indeed, the novel methods and systems described herein may be embodied in a variety of other forms; furthermore, various omissions, substitutions and changes in the form of the methods and systems described herein may be made without departing from the spirit of the disclosure. The accompanying claims and their equivalents are intended to cover such forms or modifications as would fall within the scope and spirit of the disclosure.