Automatic Vcc control for optimum power amplifier efficiency

[0002] The present invention relates to controlling a power amplifier by manipulation of the V_CCinput supply voltage and bias set point, and particularly through manipulation of the V_CCinput supply voltage and bias set point based on an output power from the power amplifier.

[0003] Wireless communication systems enjoy significant popularity, finding widespread use in both developed and developing regions. Indeed, the very popularity of wireless communication systems spurs their development and advancement, driving system designers and service providers to devise ways of supporting more users within a finite radio frequency spectrum. Existing and pending wireless communications standards typically rely on frequency, code, or time-division multiplexing techniques that allow multiple users of mobile terminals (such as cellular phones) to share the same frequencies within a given service area. Commonly, such access schemes benefit from each mobile terminal controlling its transmitted signal power to help minimize its interference with other active devices in a given wireless service area.

[0004] This power control approach presents mobile terminal designers with significant challenges. For example, power control techniques typically require transmit signal amplifier circuits that provide a range of transmit signal gain. This allows the mobile terminal to transmit with a desired signal power based on adjusting the gain of one or more such transmit signal amplifier circuits. For example, as the mobile terminal moves closer to a supporting base station, the wireless communications systems may instruct the mobile terminal, via control signaling, to reduce its transmit power. Essentially, in such wireless communications systems, active mobile terminals are controlled such that they transmit with the minimum necessary signal power at all times.

[0005] During this power control activity, the power amplifier may operate at less than full output power to provide the desired continuous transmit signal gain. Unfortunately, when operating at less than full output power, many power amplifiers are not efficient and power is wasted. Because mobile terminals are typically battery-operated, power consumption is of great concern for these devices. Any technique that could extend battery life to provide longer talk time is to be appreciated.

[0006] In the past, Doherty amplifiers, load switches and segmented output stages have been used to address efficiency concerns. To date, only the Doherty configuration has provided automatic efficiency compensation as the power is backed off. The other approaches require the intervention of an external controller. These approaches leave something to be desired in terms of simplicity, cost, and results.

[0007] The present invention takes a signal proportional to the RF output power of a power amplifier and controls a DC-DC converter that supplies DC power to a power amplifier, thereby increasing the efficiency of the power amplifier. Specifically, a signal proportional to the output of the power amplifier is used to control the DC-DC converter, thereby controlling the DC power supplied to the power amplifier. A second signal proportional to the output of the power amplifier may control a bias circuit, thereby changing the bias point of the power amplifier.

[0008] In a first embodiment, the signal proportional to the output of the power amplifier is created by rectifying a sample of the output signal from the power amplifier. In a second embodiment, the signal proportional to the output of the power amplifier is created by using a power sensed output and a power sensed reference output available on some power amplifier circuits.

[0009] The signal proportional to the output of the power amplifier may be used not only to control the DC-DC converter, but also may control a bias control circuit associated with the power amplifier. In one embodiment, a control processing circuit makes the bias control signal a function of the control signal for the DC-DC converter, and the two control signals cooperate to change the bias provided to the power amplifier concurrently with the power control signal to control the power amplifier in a coordinated fashion to increase efficiency.

[0010] Those skilled in the art will appreciate the scope of the present invention and realize additional aspects thereof after reading the following detailed description of the preferred embodiments in association with the accompanying drawing figures.

[0011] The accompanying drawing figures incorporated in and forming a part of this specification illustrate several aspects of the invention, and together with the description serve to explain the principles of the invention.

[0012] FIG. 1 illustrates a first embodiment of the present invention wherein power is sensed from the power amplifier directly;

[0013] FIG. 2 illustrates a second embodiment of the present invention wherein power is sensed from the power amplifier directly;

[0014] FIG. 3 illustrates a schematic diagram of power amplifier circuitry well-suited for use with the present invention; and

[0015] FIG. 4 illustrates a third embodiment of the present invention wherein the power is sensed from the power amplifier indirectly.

[0016] The embodiments set forth below represent the necessary information to enable those skilled in the art to practice the invention and illustrate the best mode of practicing the invention. Upon reading the following description in light of the accompanying drawing figures, those skilled in the art will understand the concepts of the invention and will recognize applications of these concepts not particularly addressed herein. It should be understood that these concepts and applications fall within the scope of the disclosure and the accompanying claims.

[0017] The present invention is well-suited for use in a mobile terminal such as a cellular phone, and particularly within a transmission chain within a mobile terminal. However, it will be readily apparent to those of ordinary skill in the art that other power amplifiers may also benefit from the teachings herein and the embodiments illustrated are exemplary and not limiting.

[0018] FIG. 1 illustrates schematically a first embodiment of the present invention. Specifically, an amplifier circuit 10 comprises a power amplifier 12 having an RF input 14, an RF output 16, a voltage supply input 18, and a bias input 20. RF input 14 receives a radio frequency signal (RF_IN) to be amplified. The RF output 16 sends an amplified version of the radio frequency signal (RF_OUT) to a matching circuit 22 prior to transmission through an antenna 24 as is well understood.

[0019] The voltage supply input 18 receives a supply voltage V_CCfrom a DC-DC converter 26. In an exemplary embodiment, the DC-DC converter 26 comprises a buck converter, which is sometimes known as a step-down converter. Alternatively, a boost-type DC-DC converter may be used, which is sometimes known as a step-up converter. As still another alternative, a buck-boost converter may be used, which can function as both a step-up and a step-down converter. The DC-DC converter 26 draws upon a battery (not shown) or other power source and converts the battery voltage to the supply voltage V_CCfor use by the power amplifier 12.

[0020] The bias input 20 receives a bias set point signal from a bias circuit 28. In some embodiments, the bias circuit 28 may be incorporated into the same integrated circuit as the amplifying elements. This is illustrated by the broken line extending around the bias circuit 28. The bias circuit 28 preferably outputs a bias set point signal that is a linear function of an input bias voltage V_REG.

[0021] In the prior art, the power amplifier 12 operates at a constant supply voltage V_CCand operates inefficiently at low power outputs as comparatively large amounts of power are provided by supply voltage V_CCand small RF_OUTsignals are generated. Likewise, at low power outputs, a constant bias set point results in inefficiency as comparatively large amounts of power are provided by the bias set point and small RF_OUTsignals are generated.

[0022] In contrast, the present invention provides a feedback element which controls the supply voltage V_CCand the bias set point as a function of a signal reflective of the RF_OUTsignal. In one embodiment, the RF_OUTsignal is split prior to reaching the matching circuit 22 and some portion of the RF_OUTsignal is coupled to a control processing circuit 30. The remainder of the RF_OUTsignal is sent to the matching circuit 22 as previously described.

[0023] The control processing circuit 30 may comprise Schottky or regular diodes that are used to rectify the RF_OUTsignal and provide a signal reflective of the output power. In the embodiment illustrated, two signals reflective of the output power are generated by the control processing circuit 30. The first signal is directed to DC-DC converter 26, and the second signal (input bias voltage V_REG) is directed to the bias circuit 28.

[0024] The signal sent to the DC-DC converter 26 changes the conversion function of the DC-DC converter 26 and thus changes the supply voltage V_CC. When low RF_OUTsignals are received by the control processing circuit, the first signal generated by the control processing circuit 30 causes the DC-DC converter 26 to generate a lower supply voltage V_CC. This creates greater efficiencies as less power is consumed by the power amplifier.

[0025] Concurrently with the control of the DC-DC converter 26, the second signal (input bias voltage V_REG) sent to the bias circuit 28 changes the bias set point signal directed to the bias input 20. At low RF_OUTsignals, the bias set point is cut back and battery drain is reduced. Because of the linear relation between the input bias voltage V_REGand the bias set point, appropriate circuitry may be used to scale the input bias voltage V_REGas desired to achieve the best efficiency.

[0026] In one embodiment, the control processing circuit 30 makes the bias control signal a function of the control signal for the DC-DC converter 26, and the two control signals cooperate to change the bias provided to the power amplifier 12 concurrently with the supply voltage V_CCso as to control the power amplifier 12 in a coordinated fashion.

[0027] A closely related embodiment is illustrated in FIG. 2, wherein the elements are substantially the same, but the RF_OUTsignal is split after the matching circuit 22. A capacitor 32 may be used to isolate the split path. As yet another alternative, not illustrated, the RF_OUTsignal is split within the matching circuit 22.

[0028] Both the embodiments of FIG. 1 and FIG. 2 are defined herein to be direct sensing of the output power because they directly sample the RF_OUTsignal and derive therefrom—with control processing circuit 30—a signal reflective of the RF_OUTsignal. By controlling the bias set point and the supply voltage V_CCsupplied to the power amplifier 12, the power amplifier 12 may more readily be controlled, and more specifically may be backed off its full power operation with greater efficiency.

[0029] While the direct sensing techniques of FIGS. 1 and 2 are adequate, they do cause some loss in the transmitted signal. Thus, a technique by which the RF_OUTsignal may be sensed indirectly would prove advantageous, so as to avoid such losses. The embodiment of FIGS. 3 and 4 illustrates one such indirect approach.

[0030] The assignor of the present invention also owns U.S. patent application Ser. No. 09/952,524, filed Sep. 14, 2001, which is herein incorporated by reference. The '524 application outlines a power amplifier that comprises an array of transistor cells. Another possible arrangement for the power amplifier is illustrated in U.S. Pat. No. 6,265,943, commonly owned, which is also hereby incorporated by reference. While fully explained in the incorporated '524 application and '943 patent, a review of the '524 application power amplifier 40 is illustrated in FIG. 3. This power amplifier 40 is well-suited for use with the present invention.

[0031] As illustrated in FIG. 3, the power amplifier 40 comprises an array of transistors Q1 to QN-2. Transistors QN-1 and QN are identical to transistors Q1 to QN-2. Transistors Q1 through QN-2 are used in the power amplifier 40 to amplify the RF_INsignal. A single transistor, QN-1, also receives the RF_INsignal and is used to provide a scaled version of the RF_OUTsignal as further explained below.

[0032] In this configuration, each transistor Q1 through QN-1 receives an identical bias set point from the bias circuit 28 through resistors RB1 through RBN-1, and also receives the RF_INsignal (via RF input pin 42) through capacitors C1 through CN-1. Notably, the collectors of transistors Q1 through QN-2 are coupled together to provide a common output signal RF_OUTat RF output pin 44, which is passed to the matching circuit 22 and the load of antenna 24.

[0033] The collector of transistor QN-1 is coupled to a power sensed output pin 50, and is not joined to the other collectors of transistors Q1 through QN-2. The signals at the collectors of transistors Q1 through QN-2 are combined to form an output signal A times that of the scaled output signal provided by transistor QN-1. A in this case equals N-2, derived from the fact that there are N-2 transistors in the array.

[0034] The transistor QN receives the bias set point from the bias circuit 28 in the same manner as the other transistors Q1 through QN-1, but does not receive the RF_INsignal. Thus, the transistor QN only amplifies the bias set point and provides it at power sensed reference output 52.

[0035] The transistors Q1 through QN are preferably heterojunction bipolar transistors (HBTs) formed on a single semiconductor and equally sized to form a transistor array. However, the concepts of this chip are independent of technology (Si, GaAs, SiGe, etc.) as well as device type (BJT, FET, MESFET, HBT, etc.).

[0036] Further information pertaining to the transistor array illustrated in FIG. 3 may be found in commonly owned U.S. Pat. Nos. 5,608,353, and 5,629,648, which are incorporated herein by reference in their entirety. Exemplary bias circuits 28 capable of being used in association with the present invention are described in further detail in commonly owned U.S. patent application Ser. No. 09/467,415, entitled BIAS NETWORK FOR HIGH EFFICIENCY RF LINEAR POWER AMPLIFIER, filed Dec. 20, 1999, the disclosure of which is incorporated herein by reference in its entirety. Upon understanding the present invention, those skilled in the art will be able to construct any number of bias networks that are compatible with the present invention.

[0037] The present invention is particularly well-suited for use with the power amplifier 40 of FIG. 3, as illustrated in FIG. 4. As discussed above, the power amplifier 40 comprises an RF input pin 42, an RF output pin 44, a voltage supply input 46, a bias input pin 48, a power sensed output pin 50 and a power sensed reference output pin 52. The bias circuit 28 may be integrated into the power amplifier 40, and still preferably exhibits a linear response to the input bias voltage V_REG.

[0038] The RF input pin 42 receives the RF_INsignal and passes it to the transistor cell array as described above. The RF output pin 44 takes the combined outputs of the collectors of the transistors in the transistor cell array and produces the RF_OUTsignal.

[0039] The output of the power sensed reference output pin 52 drives a current mirror 58 comprised of transistors 60 and 62 (separated on the schematic). Specifically, a current I₁is produced and mirrored into the transistor 62 as I₄. Thus, I₄=I₁. Alternatively, the currents may be scales by the relative sizes of the transistors 60, 62 such that I₄×I₁. At the same time, the output of the power sensed output pin 50 drives a current mirror 64 comprised of transistors 66, 68. Specifically, a current I₂is produced and mirrored into the transistor 68 as I₃. Thus, I₃=I₂.

[0040] I₃flows into a current mirror 70 comprised of transistors 72, 74. This arrangement causes a copy of the amplified bias current to enter node A and a copy of the power-sense current (bias plus RF) to leave node A. This creates an analog subtraction function in node A and creates current I₅equal to the difference of the output of the power sensed output pin 50 and the power sensed reference output pin 52. In effect, I₅is a signal proportional to the RF_OUTsignal, albeit in a current form rather than a voltage. Current I₅is supplied by the op-amp 82. The op-amp 82 provides this current by adjusting its output voltage V_PSuntil the proper current flows through the feedback resistor R₁, which, in an exemplary embodiment, may be 15 kΩ. The 1 nF feedback capacitor in parallel with R₁is to provide low-pass filtering. Output voltage V_PSis, as a function of its being adjusted to provide the proper current I₅, proportional to the RF_OUTsignal.

[0041] The voltage supply input 46 receives a supply voltage (V_CC) from the DC-DC converter 54. The DC-DC converter 54 is, in an exemplary embodiment, a buck converter, and converts a battery voltage (V) to the supply voltage (V_CC) based on the input signal at VSET input 56.

[0042] As in the previous embodiments, the DC-DC converter 54 may alternatively be a buck-boost converter or a boost converter as needed or desired. In all cases, the DC-DC converter 54 creates a supply voltage V_CCas a known function of the signal at VSET input 56. Thus, as output voltage V_PSchanges reflective of changes in the RF_OUTsignal, the signal applied to the VSET input 56 changes and supply voltage V_CCchanges. For example, as the RF_OUTsignal decreases, output voltage V_PSdecreases and supply voltage V_CCapplied to the power amplifier 40 decreases, thus improving efficiency and decreasing the drain on the battery.

[0043] Concurrently with the provision of output voltage V_PSto the VSET input 56, a current I₆is induced in a current mirror 76 comprised of transistors 78; 80. A 120 kΩ resistor is positioned across the transistor 80. The current I₆mirrors across the transistors 78, 80 and is presented along with a signal from band gap reference 84 to an op-amp 86.

[0044] The band gap reference 84 may produce a voltage that is a function of the difference in current between two or more diodes. This acts as a floor voltage. In an exemplary embodiment, this is 1.07 volts.

[0045] The op-amp 86 compares the voltage from its inverting input to ground (the voltage across the 120 kΩ resistor and the 32 kΩ resistor) with the band-gap voltage and adjusts the input bias voltage V_REGuntil they match. Thus, the input bias voltage V_REGis a function of output voltage V_PS.

[0046] When the RF_OUTsignal is high, output voltage V_PSdrives the current mirror 76 hard enough to short the 120 kΩ resistor. The op-amp 86 measures the voltage across the 32 kΩ resistor and creates an input bias voltage V_REGthat, in turn, is provided to the bias input pin 48. When, however, the RF_OUTsignal is low or zero, the current mirror 76 is off, and the op-amp 86 generates a much lower input bias voltage V_REGbecause of the large resistance (120 kΩ+32 kΩ) between the inverting input and ground. Between this minimum point and maximum point, input bias voltage V_REGwill scale according to the RF_OUTsignal and thus scale the bias set point of the bias circuit 28.

[0047] Note that a threshold voltage is also presented to the op-amp 82 through the voltage divider formed by R₂and R₃. In an exemplary embodiment, R₂=R₃₂=11 kΩ, thus making the threshold voltage half the band gap value.

[0048] For the purposes of this embodiment, the control processing circuit 30 is somewhat diffuse, and comprises the elements downstream of pins 50, 52 and upstream of the DC-DC converter 54 and the bias input pin 48.

[0049] The end result of this arrangement is that as the power in the RF_OUTsignal is backed off, and the control processing circuit 30 provides signals proportional or reflective of the RF_OUTsignal to the bias circuit 28 and the DC-DC converter 26 so as to improve the efficiency of the power amplifier 40. Specifically, if the RF_OUTsignal is small, the voltage supplied to the voltage supply input pin 46 is small and the bias set point of the transistor cell array within the power amplifier 40 is small. Thus, as the supply voltage for the power amplifier is decreased, the bias for the amplifier is decreased in a manner enduring ensuring efficient operation. This results in greater efficiency. Further, this arrangement supports the required linearity over the entire power range needed for normal operation.

[0050] Those skilled in the art will recognize improvements and modifications to the preferred embodiments of the present invention. All such improvements and modifications are considered within the scope of the concepts disclosed herein and the claims that follow.