Patent Publication Number: US-2013234795-A1

Title: Free-fly class d power amplifier

Description:
TECHNICAL FIELD 
     The invention relates generally to a class D amplifier and, more particularly, to a free-fly class D power amplifier (PA). 
     BACKGROUND 
     Turning to  FIG. 1 , an example of a conventional class D PA  100  that is capable of amplitude modulation can be seen. This PA  100  generally comprises drivers  102 - 1  to  102 -N (which are typically inverters), capacitors CI 1  to CIN, and a matching network  104 . In operation, a predetermined number of capacitors (i.e., capacitor CI 1  to CIn) are switched, while the remainder (i.e., capacitors CI(n+1) to CIN) are left “off.” This generates an effective capacitance CIEFF that is the sum of the switched capacitor (i.e., CIEFF=CI 1 + . . . +CIn) and an effective supply voltage VEFF (i.e., VEFF=VDD*(n/N)). A functionally equivalent circuit having an inverter  106  that represents the drivers  102 - 1  to  102 -N and that includes the effective capacitance CIEFF and effective supply voltage VEFF can be seen in  FIG. 2 . This allows an output radio frequency (RF) signal RFOUT to be generated from an input RF signal RFIN (which can be amplitude modulated) so as to be applied to the load RL. 
     A problem with this configuration, however, is that the switching losses is a function of the effective capacitance CIEFF and frequency. These losses can be so significant so as to be prohibitive for many applications, and the efficiency can be very low. In particular, the efficiency of PA  100  may only reach about 54%. Therefore, there is a need for an improved class D PA. 
     Some other conventional circuits are: Yoo et al., “A Switched-Capacitor Power Amplifier for EER/Polar Transmitters,”  IEEE Intl, Solid-State Circuits Conference Digest of Technical Papers (ISSCC)  2011, Feb. 20-24, 2011, pp. 428-430; U.S. Pat. No. 6,882,829; U.S. Pat. No. 7,509,102; U.S. Pat. No. 7,733,187; and U.S. Pat. No. 7,831,227. 
     SUMMARY 
     An embodiment of the present invention, accordingly, provides an apparatus. The apparatus comprises a first driver having a first input, a first output, and a first parasitic capacitance, wherein the first input is configured to receive complementary first and second radio frequency (RF) signals, and wherein there is a first set of free-fly intervals between consecutive pulses from the first and second RF signals; a second driver having a second input, a second output, and a second parasitic capacitance, wherein the second input is configured to receive complementary third and fourth RF signals, and wherein there is a second set of free-fly intervals between consecutive pulses from the third and fourth RF signals; a switch network that is coupled to the first and second outputs and that is configured to couple the first and second outputs together when the second driver is enabled; a first output capacitor that is coupled to the first output; a second output capacitor that is coupled to the second output; and a matching network that is coupled to the first and second output capacitors. 
     In accordance with an embodiment of the present invention, the switch network further comprises a switch that is coupled between the first and second outputs. 
     In accordance with an embodiment of the present invention, the apparatus further comprises: a first enable circuit that is coupled to the first driver and that receives the first and second RF signals; a second enable circuit that is coupled to the first driver and that receives the third and fourth RF signals; and a controller that is coupled to the first and second enable circuits so as to provide first and second enable signals and that is coupled to control the switch. 
     In accordance with an embodiment of the present invention, the first driver further comprises: a PMOS transistor that is coupled to the first enable circuit at its gate and the first output capacitor at its drain; and an NMOS transistor that is coupled to the first enable circuit at its gate and the first output capacitor at its drain. 
     In accordance with an embodiment of the present invention, the first enable circuit further comprises: a first logic circuit that is coupled to the gate of the PMOS transistor and the controller; and a second logic circuit that is coupled to the gate of the NMOS transistor and the controller. 
     In accordance with an embodiment of the present invention, the first and second logic circuits further comprise AND gates. 
     In accordance with an embodiment of the present invention, the first and third RF signals are substantially the same signal, and wherein the second and fourth RF signals are substantially the same signal. 
     In accordance with an embodiment of the present invention, a method is provided. The method comprises asserting a first enable signal so as to enable a first driver, wherein the first driver has a first output and a first parasitic capacitance; asserting a second enable signal so as to enable a second driver, wherein the second driver has a second output and a second parasitic capacitance; coupling the first and second outputs together when the second driver is enabled; applying pulses from complementary first and second RF signals to the first driver, wherein there is a first set of free-fly intervals between consecutive pulses from the first and second RF signals; and applying pulses from complementary third and fourth RF signals to the second driver, wherein there is a second set of free-fly interval between consecutive pulses from the third and fourth RF signals. 
     In accordance with an embodiment of the present invention, the step of asserting the first enable signal further comprises: providing the first enable signal to first and second logic circuits; receiving, by the first logic circuit the first RF signal; and receiving, by the second logic circuit the second RF signal. 
     In accordance with an embodiment of the present invention, the step of asserting the second enable signal further comprises: providing the second enable signal to third and fourth logic circuits; receiving, by the third logic circuit the third RF signal; and receiving, by the fourth logic circuit the fourth RF signal. 
     In accordance with an embodiment of the present invention, the first, second, third, and fourth logic circuits further comprise AND gates. 
     In accordance with an embodiment of the present invention, an apparatus is provided. The apparatus comprises a controller that is configured to select an output power level by asserting a set of enable signals from a plurality of enable signals; a plurality of enable circuits, wherein each enable circuit is coupled to the controller so as to receive at least one of the enable signals, and wherein each enable circuit is configured to receive complementary first and second RF signals, and wherein there is a free-fly interval between consecutive pulses from the first and second RF signals for each set; a plurality of drivers, wherein each driver has an input, an output, and a parasitic capacitance, wherein the input of each driver is coupled to at least one of the enable circuits; a switch network that is coupled to the output of each driver and that is coupled to the controller, wherein the couples the outputs of the drivers together based at least in part on the set of enable signals that are asserted to select the output power level; a plurality of output capacitors, wherein each output capacitor is coupled to the output of at least one of the drivers; and a matching network that is coupled to each output capacitor. 
     In accordance with an embodiment of the present invention, the plurality of drivers are arranged in a sequence, and wherein the switch network further comprises a plurality of switches, wherein each switch is coupled between outputs of consecutive drivers in the sequence. 
     In accordance with an embodiment of the present invention, each enable circuit further comprises: a first logic circuit that is coupled to the controller so as to receive at least one of the enable signals and that receives the first RF signal; and a second logic circuit that is coupled to the controller so as to receive at least one of the enable signals and that receives the second RF signal. 
     In accordance with an embodiment of the present invention, each driver further comprises: a PMOS transistor that is coupled at its gate to the first logic circuit of its corresponding enable circuit and that is coupled at its drain to its corresponding output capacitor; and an NMOS transistor that is coupled at its gate to the second logic gate of its corresponding enable circuit and that is coupled at its drain to its corresponding output capacitor. 
     In accordance with an embodiment of the present invention, the first and second logic circuits further comprise AND gates. 
     In accordance with an embodiment of the present invention, each switch further comprises a transmission gate. 
     In accordance with an embodiment of the present invention, each switch further comprises a microelectromechanical systems (MEMS) switch, a relay, or CMOS switch. 
     The foregoing has outlined rather broadly the features and technical advantages of the present invention in order that the detailed description of the invention that follows may be better understood. Additional features and advantages of the invention will be described hereinafter which form the subject of the claims of the invention. It should be appreciated by those skilled in the art that the conception and the specific embodiment disclosed may be readily utilized as a basis for modifying or designing other structures for carrying out the same purposes of the present invention. It should also be realized by those skilled in the art that such equivalent constructions do not depart from the spirit and scope of the invention as set forth in the appended claims. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       For a more complete understanding of the present invention, and the advantages thereof, reference is now made to the following descriptions taken in conjunction with the accompanying drawings, in which: 
         FIG. 1  is a diagram of an example of a conventional class D PA that is capable of amplitude modulation; 
         FIG. 2  is a diagram of a functionally equivalent circuit to the PA of  FIG. 1 ; 
         FIG. 3  is a diagram of an example of a PA in accordance with the present invention; 
         FIG. 4  is a diagram of an example of the enable circuit and driver for the PA of  FIG. 3 ; 
         FIG. 5  is a diagram of an example of the switching network of  FIG. 3 ; 
         FIG. 6  is a diagram of an example of a functionally equivalent circuit to the PA of  FIG. 3 ; 
         FIGS. 7-10  are diagrams depicting the drain efficiency for an example of the PA of  FIG. 3 ; and 
         FIG. 11  is a diagram comparing the efficiencies for examples of the PA of  FIGS. 1 and 3 . 
     
    
    
     DETAILED DESCRIPTION 
     Refer now to the drawings wherein depicted elements are, for the sake of clarity, not necessarily shown to scale and wherein like or similar elements are designated by the same reference numeral through the several views. 
     Turning to  FIGS. 3-5 , an example of a class D PA  200  in accordance with an embodiment of the present invention can be seen. As shown, PA  200  is generally comprised of enable circuits  202 - 1  to  202 -R, controller  204 , drivers  206 - 1  to  206 -R, a switch network  208 , output capacitors CO- 1  to CO-N (which can, for example, each have approximately the same capacitance), and a matching network  104 . The drivers  206 - 1  to  206 -R are generally comprised of transistors Q 1  and Q 2  (which are typically PMOS and NMOS transistors, respectively), which, respectively, have parasitic capacitances CP 1  and CP 2  and which, respectively, receive RF input signals RFINU and RFIND via the logic circuits  302  and  304  (i.e., AND gates or other functionally equivalent logic circuits) of enable circuits  202 - 1  to  202 -R. The switch network or bridge switch  208  generally comprises switches S- 2  to S-R (which can, for example, be and are shown as transmission gates) that are each coupled between consecutive or adjacent outputs of drivers  206 - 1  to  206 -R and that are controlled by the signal CNTL[2:R] from controller  204 . Also, the matching network  104  may include inductor, capacitors, and resistors. Microelectromechanical system (MEMS) switches, relays, or other CMOS switches may also be employed as switches S- 2  to S-R. 
     In operation, an RF output signal RFOUT is generated and applied to load RL. Typically, the PA  200  has a several output power levels, which are selected by the controller through the activation and deactivation of drivers  202 - 1  to  202 -R with enable signal EN[1:R]. Each enable circuit  202 - 1  to  202 -R typically receives the input RF signals RFINU and RFIND at its input, and, based on the desired output level, a corresponding number of enable circuit  202 - 1  to  202 -R are activated, allowing RF signals RFINU and RFIND to be passed along to the corresponding drivers  206 - 1  to  206 -R. These signals RFINU and RFIND are generally complementary pulse width modulated (PWM) input signals that are able to activate transistors Q 1  and Q 2  within drivers  206 - 1  to  206 -R, but for PA  200 , these signals are not “adjacent” to one another, meaning that these signals are truly complementary from a timing perspective. Between consecutive pulses of the signals RFINU and RFIND, there is a free-fly or dead time interval, meaning that there is an interval between consecutive activations of transistors Q 1  and Q 2 . Because one of the parasitic capacitances CP 1  and CP 2  is charged as a result of its transistor Q 1  or Q 2  being “on,” the parasitic capacitor CP 1  or CP 2  creates distortion when it is discharged during this free-fly interval, which has the effect of reducing the interval. This results in an improvement inefficiency by itself 
     To better improve efficiency beyond use of the free-fly intervals alone, the parasitic capacitance of the PA  200  can be tuned. Typically, this can be accomplished by activating switches S- 2  to S-R in accordance with the desired power level. For example, for a power level of 2, the enable signals EN[ 1 ] and EN[ 2 ] would be asserted so as to activate enable circuits  202 - 1  and  202 - 1 , and the control signal CNTL[ 2 ] would be asserted to activate or close switch S- 2 . As another example, for a power level of 4, the enable signals EN[ 1 ] to EN[ 4 ] would be asserted so as to activate enable circuits  202 - 1  to  202 - 4 , and the control signal CNTL[ 2 ] to CNTL[ 4 ] would be asserted to activate or close switch S- 2 , S- 3 , and S- 4 . When these switches S- 2  to S-R in switch network  208  are activated in accordance with the desired power level, the parasitic capacitance increases, allowing a desirable (i.e., theoretically optimal) voltage to develop. This, therefore, provides a significant improvement in drain efficiency, especially at power outputs levels lower than a maximum or upper limit. 
     As an example, in  FIG. 6 , there is an example of a functionally equivalent circuit of PA  200 . For this circuit, transistors Q 3  and Q 4  represent drivers  206 - 1  to  206 -n, which have been activated or enabled corresponding to a desired power level (denoted “n”). As shown, capacitors CO- 1  to CO-n are coupled in parallel with one another, providing a total output capacitance of N*CO. Additionally, because drivers  206 -(n+1) to  206 -R are inactivated, capacitors CO-(n+1) to CO-R are coupled between matching network  104  and ground. The total parasitic capacitance for power level n is also n 2 *CP (as provided through switch network  208 ) so as to provide the proper RF matching. 
     To illustrate, the improvement in efficiency, it can be assumed (as an example) that PA  200  employs four drivers (i.e.,  206 - 1  to  206 - 4 ), four enable circuits (i.e.,  202 - 1  to  202 - 4 ), and three switches (i.e., S- 2  to S 4 ) so as to have four output levels. In this example, for the lowest output level (i.e., level 1), enable circuit  202 - 1  is activated, so as to provide an output power of about 8 mW. The drain voltage of driver  206 - 1  for level 1 can be seen in  FIG. 7  with a drain efficiency of about 50%. For level 2 (where enable circuits  202 - 1  and  202 - 2  and switch S- 2  are activated), the output power is about 35 mW, and the drain efficiency (which can be seen in  FIG. 8 ) is about 78%. Similarly, for power levels 3 and 4, the drain efficiency is about 86% and 91% (respectively and which is shown in  FIGS. 9 and 10 ) for output power levels of about 89 mW and about 170 mW (respectively). This improvement in efficiency is dramatically higher than a comparative 4-level PA  100 , which can be seen in  FIG. 11 . 
     Having thus described the present invention by reference to certain of its preferred embodiments, it is noted that the embodiments disclosed are illustrative rather than limiting in nature and that a wide range of variations, modifications, changes, and substitutions are contemplated in the foregoing disclosure and, in some instances, some features of the present invention may be employed without a corresponding use of the other features. Accordingly, it is appropriate that the appended claims be construed broadly and in a manner consistent with the scope of the invention.