Method and device for controlling a power amplifier

Abstract

Method for controlling a power amplifier (101) for operation in a time-slot data transmission system with: Providing a bias voltage to set an operating point to the power amplifier (101), wherein the bias voltage has a respective bias voltage value, Providing a supply voltage to the power amplifier (101), wherein a supply voltage value is set such that a predetermined output power value of the power amplifier (101) is achieved, Changing the preload value to a new preload value, Repeat the previous steps for at least one subsequent time slot until a criterion is met. Setting the bias value of the following time slot to the bias value of the previous time slot, and wherein the criterion is met if the supply voltage value exceeds a given threshold.

Description

[0001] The present invention relates to a method and a device for controlling a power amplifier for operation in a time-slot data transmission system.

[0002] A power amplifier is used particularly in data transmission systems, such as mobile communication systems. For mobile communication systems, a number of standardized transmission systems have been defined in recent years. These systems define, in particular, the type of modulation used for the information to be transmitted and the method of transmission between a base station and a mobile device.

[0003] In a transmission system that uses constant amplitude envelope modulation, the output power is partly determined by the gain of the power amplifier. Constant amplitude envelope modulation is used, for example, in a transmission system according to the GSM (Global System Mobile Communication) standard. This standard specifies that the information to be transmitted is applied to a carrier signal using GMSK (Gaussian Minimum Shift Keying) modulation.

[0004] In a communication system, bidirectional communication is desirable. For this purpose, some communication systems, such as DECT (Digital Enhanced Cordless Communication), use time division duplex (TDD). In TDD, the transmission is divided into a sequence of time slots. During one time slot, transmission occurs in one direction, for example, from a mobile device to a base station. In another time slot, transmission occurs from the base station to a mobile device.

[0005] Dividing communication into time slots allows, in another form, multiple access, enabling several participants in a system to communicate with each other or with a shared transmission system. In GSM, this is defined as Time Division Multiple Access (TDMA), where different mobile devices in a cell communicate with the base station at different times.

[0006] Due to the possibilities of time duplex and time multiple access, these methods are also used in other communication systems, such as wired or optical communication systems. Technological developments in recent decades have enabled manufacturers of mobile devices to produce increasingly smaller devices. Since the size of these devices is determined, among other factors, by the size of a power reserve, i.e., a battery or accumulator, it is desirable to reduce the power consumption of the mobile device. Furthermore, lower power consumption increases the talk time or operating time between recharging of the mobile device's power reserve. A significant portion of the power consumption in the mobile device is caused by the power amplifier, which provides a substantial portion of the output power of the transmission signal.It is therefore desirable to enable the most energy-efficient operation of the power amplifier possible.

[0007] On the other hand, the output power of the power amplifier must be high enough for a base station to reliably receive a signal from the mobile device. The base station often specifies the output power at which the mobile device should transmit its signal.

[0008] In a mobile device, the output signal from the power amplifier is fed into a radio communication channel via an antenna. This output signal is a high-frequency signal with frequencies between 600 MHz and over 2.5 GHz. Therefore, impedance matching at the antenna plays a crucial role. If the antenna impedance deviates from a certain value, parts of the output signal will return to the power amplifier via the connection between the amplifier and the antenna due to reflections. With an antenna impedance mismatch, the transmitted power from the antenna is significantly lower than the output power provided by the power amplifier. This results in low energy efficiency for the power amplifier. An antenna impedance mismatch can occur randomly during the use of the mobile device.For example, it is enough for a user to touch the antenna.

[0009] Without a built-in safety margin when setting the output power, the mobile device cannot behave according to the standard's specifications under all expected conditions. Non-standard behavior is undesirable for network operators and is therefore avoided by mobile device manufacturers. Instead, they accept increased power consumption and thus a shorter operating time for the mobile device.

[0010] EP 000 002 634 916 A1 concerns an improved Doherty amplifier.

[0011] EP 000 001 912 328 A1 relates to a high-efficiency amplifier used for broadcasting and communication.

[0012] US 020 150 263 678 A1 relates to a power amplifier and a power amplification method for amplifying an RF (radio frequency) signal containing signals from a variety of frequency bands used in wireless communication.

[0013] US 2006 / 0192616A1 discloses a technique for improving the start of a power control loop at the beginning of transmission in an electronic part for high-frequency power amplification (RF power module) with a high-frequency power amplifier circuit. In particular, the invention relates to the above technique, which is effectively applied to the RF power module for use in, for example, mobile phones for GSM networks.

[0014] US 6 914 487 B1 refers generally to power amplifiers and specifically to a method and system for power management in a high-frequency power amplifier using adaptive envelope tracking.

[0015] The present invention addresses the problem of providing a method and device for controlling a power amplifier that is suitable for operation in a time-slot transmission system, in which a certain power can be provided with the least possible energy expenditure.

[0016] The problem underlying the invention is solved by a method, a device, and a transmitter having the features of the respective independent claims 1, 10 and 11.

[0017] One embodiment of the inventive method for controlling a power amplifier for operation in a time-slot data transmission system comprises the features of claim 1.

[0018] One embodiment of the device according to the invention for controlling a power amplifier for operation in a time-slot data transmission system comprises the features of claim 10.

[0019] A key aspect of the invention is that the bias voltage for setting the operating point of the power amplifier is adjusted over a period of successive time slots. This is achieved by a predetermined, successive change in the power amplifier's bias voltage from one time slot to the next. The bias voltage determines the amplifier's saturation power. During each time slot, the supply voltage control signal adjusts the power amplifier's supply voltage to achieve the desired output power. Thus, the power amplifier's operating point is first set, and then the necessary output power is provided. This dynamically determines the operating point that corresponds to the smallest possible safety margin. The power amplifier always operates close to saturation as a switching amplifier, thereby maximizing its power efficiency.

[0020] The supply voltage control signal is often also referred to as VRAMP. This signal controls the power supply of the power amplifier. For this purpose, the supply voltage control signal can, for example, be fed to a regulator that derives a supply voltage from an input supply voltage, such as a battery voltage. It is also conceivable that the supply voltage control signal could directly correspond to a supply voltage.

[0021] Advantageous embodiments of the invention are specified in the dependent claims.

[0022] In one embodiment of the inventive method, the criterion is met when the ratio of a change in the supply voltage value to a change in a measured output power exceeds a given threshold. As the power amplifier approaches saturation, the change in the actual output power relative to the change in the supply voltage value becomes progressively smaller. The inverse of this ratio becomes progressively larger. Thus, the dynamic adjustment can be terminated when the ratio of a change in the supply voltage value to a change in a measured output power exceeds a given threshold.

[0023] In an alternative embodiment of the method according to the invention, the criterion is met when the supply voltage exceeds a given threshold. This threshold can, for example, be defined by the supply voltage point at which the output power saturates for a minimum adjustable preload. Similarly, the point could be chosen for an average or mean adjustable preload. It is also conceivable that the threshold is chosen based on statistical data.

[0024] In one embodiment of the method according to the invention, a maximum bias value is provided for a first time slot of a sequence of time slots. This ensures that the method quickly converges to the optimal bias value. If a high bias value is initially set, the desired power can always be achieved. Subsequently, the bias value is reduced to lower the current consumption.

[0025] In a further embodiment of the method according to the invention, the supply voltage is set depending on a predetermined output power of the power amplifier. The output power can be specified by a communication partner, e.g., a base station, in a radio system. Thus, the power amplifier can be set according to the existing conditions in the transmission channel.

[0026] In a further development of the inventive method, the supply voltage is set as a function of a determined output power of the power amplifier. The power amplifier is thus controlled via a control loop, which is also referred to as a closed loop.

[0027] In a further development of the method according to the invention, the preload value is changed by reducing it by a given amount. This provides a simple method for optimizing the operating point.

[0028] In one embodiment of the inventive method, the change in the preload value can also be achieved by reducing the preload value by a constant amount for all time slots.

[0029] In a further embodiment of the method according to the invention, the supply voltage value is stored for the duration of at least one subsequent time slot. Similarly, in an additional or alternative embodiment, the bias voltage value can be stored for the duration of at least one subsequent time slot.

[0030] The invention is explained in more detail below using several exemplary embodiments with reference to the drawing.

[0031] This shows Fig. 1 a schematic representation of an embodiment of the device according to the invention; Fig. 2 a schematic representation of a power amplifier for use in an embodiment of the device according to the invention; Fig. 3 a schematic representation of the dependence of the output power on the supply voltage at different bias values; Fig. 4 a schematic representation of a flowchart of an embodiment of the method according to the invention, and Fig. 5 a schematic representation of the temporal course of various quantities during an execution of an embodiment of the method according to the invention.

[0032] Fig. Figure 1 shows a schematic representation of an embodiment of the device according to the invention. A device 100 for controlling a power amplifier 101 is shown, which can be, for example, a transceiver or transmitter module in the form of an integrated semiconductor device. The device 100 is connected to the power amplifier 101 via a signal line 102 to supply the power amplifier 101 with a signal to be amplified. A signal amplified by the power amplifier is provided at an output 103. Furthermore, the device 100 is connected to the power amplifier 101 via a bias line 104, through which a bias voltage is supplied to the power amplifier 101. The bias voltage sets an operating point for the power amplifier.

[0033] The device 100 is additionally connected to the power amplifier 101 via a supply voltage control line 105. A supply voltage control signal is provided to the power amplifier 101 via the supply voltage control line 105, which regulates the output power of the power amplifier 101.

[0034] A power detector is coupled to output 103, comprising a coupling element 106 and a detector diode 107. The coupling element 106 and the detector diode 107 are connected to the device 100 via a detector line 108.

[0035] The device comprises a first device 109 for providing a bias value; this can be, for example, a DSP or an arithmetic logic unit. The first device 109 is connected to a memory 110 in which the bias value is stored. The first device 109 can be configured to store the bias value at the beginning of a time slot in the memory 110. The memory 110 can be designed as volatile memory, such as register memory or ROM memory, or as non-volatile memory, such as EEPROM or FLASH memory.

[0036] Memory 110 is connected to a second device 111 for providing the supply voltage. The second device 111 supplies the supply voltage to the power amplifier 101 via the bias line 104. The second device can, for example, be a digital-to-analog converter that converts the bias value stored in memory 110 into an (analog) bias voltage. Furthermore, the second device 111 can also include an amplifier.

[0037] The device 100 further comprises a third device 112. This device is configured to provide a supply voltage, the supply voltage being set such that a predetermined output power value of the power amplifier is achieved. The third device 112 can include a DSP, a digital-to-analog converter, and an amplifier. To provide the predetermined output power, the third device 112 is connected to the first device 109 so that the bias voltage can be applied to it. The third device 112 is connected to the power detector via the detector line 108, so that the actual output power value can be applied to the third device 112. Accordingly, a closed-loop control system can be configured to set the output power.

[0038] Fig. Figure 2 shows a schematic representation of a power amplifier for use in an embodiment of the device according to the invention. The power amplifier has an input 200, which is connected via a capacitor 201 to a base terminal of an amplifier transistor 202. A bias input 203 is also coupled to the base terminal. The power transistor is configured as an emitter (or common emitter) circuit. An emitter terminal of the power transistor is connected to a ground terminal, or a reference potential terminal. A collector terminal of the power transistor is coupled to a voltage input 204 via a regulator 205. The collector terminal is also connected to a power amplifier output 206.

[0039] The power amplifier amplifies a signal provided at input 200. DC components of the provided signal are suppressed by capacitor 201. The operating point of the power transistor is set by a bias voltage provided at bias input 202. The gain of the power transistor 202 is controlled by the supply voltage derived from regulator 205. The supply voltage is derived by regulator 205 from an input voltage provided at supply voltage input 204, depending on the supply voltage control signal.

[0040] The regulator 205 has a control input 207. The supply voltage control signal is fed to the control input 207. In the illustrated embodiment, the regulator 205 is represented as a linear regulator in the form of a MOS transistor. The control input 207 corresponds to a gate terminal of the MOS transistor. In the illustrated embodiment, the supply voltage thus corresponds to a collector voltage applied to the amplifier transistor 202. The voltage input 204 can be connected to a constant voltage supply, such as a battery. The output power of the power amplifier is thus regulated via the bias voltage and the supply voltage control signal.

[0041] Fig. Figure 3 shows a schematic representation of the dependence of the output power on the supply voltage at different bias voltage values. The diagram depicts the output power of a power amplifier as a function of a supply voltage plotted on an abscissa 301, on an ordinate 300. A total of six different cases are shown.

[0042] The first curve, 303, shows the output power versus the supply voltage for a maximum bias voltage. This assumes no impedance mismatch at an antenna connected to an output of the power amplifier. A typical antenna impedance would be, for example, 50 ohms.

[0043] A second curve, 304 (shown as a dashed line), also shows the output power curve for a maximum bias voltage. However, there is an antenna impedance mismatch. This mismatch causes the outgoing wave of the power amplifier's output signal to reflect off the antenna impedance, thus reducing the radiated output power. The voltage standing wave ratio (VSWR) can be used as a measure of this mismatch. The VSWR is calculated as the ratio of the maximum to the minimum voltage. The maximum voltage is the sum of the outgoing and reflected voltages, while the minimum voltage is the difference between the two. In the example shown, a VSWR of 4:1 is assumed.

[0044] A third curve 304 shows the output power curve for a medium bias voltage. This curve assumes no impedance mismatch at an antenna coupled to an output of the power amplifier. A fourth curve 305 (shown as a dashed line) shows the output power curve for a medium bias voltage with an antenna impedance mismatch at a VSWR of 4:1.

[0045] A fifth curve, 306, shows the output power curve for a minimum bias voltage. This curve assumes no impedance mismatch at an antenna connected to the power amplifier's output. A sixth curve, 307 (shown as a dashed line), shows the output power curve for a medium bias voltage with an antenna impedance mismatch at a VSWR of 4:1.

[0046] The diagram shows that the output power of the power amplifier saturates at high supply voltages. The saturation power is determined by the bias voltage. The higher the bias voltage, the higher the saturation power. However, the saturation power is also influenced by the VSWR. The higher the VSWR, and thus the impedance mismatch, the lower the saturation power.

[0047] In an unfavorable case, the following can happen due to a mismatch. A specific output power 308 is required, but a short-term mismatch in the antenna impedance occurs, for example, because a user accidentally touches the antenna. In order to achieve the desired output power 108, the bias voltage must be selected such that a corresponding saturation power is reached. In the example shown, a maximum bias voltage would therefore have to be selected so that the power amplifier achieves the desired output power 308 by adjusting the supply voltage along a curve on the second curve 303. Without a mismatch, however, a medium bias voltage would suffice to achieve the desired output power 308 via the curve on the third curve 104. However, selecting a higher bias voltage induces a higher leakage current, which is already evident from... Fig. 2 is described in more detail.

[0048] Fig. Figure 4 shows a schematic representation of a flowchart of an embodiment of the method according to the invention. In the method for controlling a power amplifier for operation in a time-slot data transmission system, a bias voltage is applied to the power amplifier in a first step 401. The bias voltage has a specific bias value. The bias voltage is used to set an operating point of the power amplifier.

[0049] Accordingly, a saturation power, and thus a maximum possible output power of the power amplifier, is set. At the beginning of the process, the bias voltage can be set to a maximum bias value of the power amplifier. This allows the power amplifier to be set to its maximum output power.

[0050] The output power of the power amplifier is determined by a supply voltage. In a second step, a supply voltage control signal is provided to the power amplifier. This control signal is used to adjust the supply voltage to achieve a predetermined output power value. The bias voltage can be determined by a predetermined output power, or it can be determined by comparing a predetermined output power with an actual, measured output power.

[0051] In a third step (403), the bias value is changed to a new bias value. This is done, for example, by reducing the bias value by another value, such as by subtracting this value from the bias value. The reduction can be by a constant amount for all time slots, for example, by one LSB before the bias value is converted into the bias by a DAC. It is also conceivable that the value is initially chosen to be larger and then decreased with each time slot. The value can change by the same percentage from time slot to time slot, for example, by 10% each time, so that there would be a logarithmic relationship between the value and the indices of the time slots.

[0052] When the preload value is changed, the supply voltage value and the preload value can also be saved. The preload value is then changed to a new preload value.

[0053] In a fourth step (404), it is checked whether a predefined criterion is met. This can be done by verifying whether the ratio of a change in the bias value during the time slot to a change in a measured output power exceeds a given threshold. When the power amplifier approaches saturation during supply voltage regulation, the slope of the derivative of the output power with respect to the supply voltage decreases.

[0054] A detector that measures output power is itself unaffected by saturation. Accordingly, the derivative of the detector voltage over the supply voltage is also smaller. This change can be measured, for example, by determining how many bit changes in the supply voltage value are required to change one LSB bit at the output of an ABC amplifier to which the detector voltage is applied.

[0055] Another criterion that can be used is to check whether the supply voltage exceeds a given threshold. The efficient operating points of the power amplifier lie within a relatively limited range of the supply voltage. The optimal operating point depends on an absolute value of the supply voltage during the time slot.

[0056] Steps 401 to 404 are repeated until the specified criterion is met. Once the specified criterion is met, the bias value of the next time slot is set to the bias value of the following time slot. This value can be retained for all subsequent time slots. The process can be restarted as soon as a new requirement for the output power arises.

[0057] Fig. Figure 5 shows a schematic representation of the time course of various quantities during an execution of an embodiment of the method according to the invention.

[0058] The abscissa 500 represents time. The first ordinate 501 represents the bias voltage of a power amplifier. The second ordinate 502 represents a supply voltage control signal of the power amplifier. The third ordinate 503 represents the output power of the power amplifier. The fourth ordinate 504 represents the detector voltage of a power detector connected to the output of the power amplifier. The fifth ordinate 505 represents the ratio of a change in the supply voltage to a change in the detector voltage.

[0059] In the Fig.Figure 5 shows four time slots. During the first time slot, 506, a maximum bias voltage is applied. To achieve the desired output power of the power amplifier, a supply voltage is regulated. The supply voltage control signal is relatively low because a significant portion of the output power is achieved via the high bias value. This provides the power amplifier with a large safety margin. The ratio of a change in the supply voltage to a change in the detector voltage is relatively small.

[0060] During a second time slot 507, the bias voltage is set to a constant value lower than the bias voltage of the first time slot 506. Accordingly, the supply voltage control signal is regulated to a higher value compared to the first time slot 506 in order to achieve the desired output power. The ratio of a change in the supply voltage to a change in the detector voltage also increases compared to the first time slot 506. This ratio is not greater than a threshold value 510 set for fulfilling the criterion. The safety margin is smaller than in the previous first time slot 506. During a third time slot 508, the bias voltage is set to a further constant value lower than the bias voltage of the first time slot 506. It is also lower than in the second time slot 507.Accordingly, the supply voltage control signal is regulated to a higher value compared to the second time slot 507 in order to achieve the desired output power. The ratio of a change in the supply voltage to a change in the detector voltage also increases compared to the second time slot 507. This ratio, however, remains no greater than the threshold value 510 set for fulfilling the criterion. The safety margin is smaller than in the previous time slots 506 and 507.

[0061] During the third time slot 509, the bias voltage is further reduced, reaching a higher value than in the preceding time slots. The ratio of a change in the supply voltage to a change in the detector voltage also increases further. It is now so large that it exceeds the threshold value 510. The optimal operating point was therefore exceeded during the third time slot 509. The power amplifier is operating at or near its saturation point.

[0062] Accordingly, the bias for the fourth time slot 509 is reset to the bias value of the second time slot 507. The other values ​​adjust themselves accordingly to the value of the second time slot 507.

[0063] Thus, the power amplifier is iteratively adjusted to an optimal, i.e., energy-efficient, operating point. This occurs during transmission and while maintaining the desired output power. The process starts with a suboptimal bias setting and initially accepts an excessively high safety margin, resulting in unnecessary current losses. This avoids a situation where the desired output power is not achieved, thereby violating a system requirement.

Claims

[1] Method for controlling a power amplifier (101) for operation in a time slot data transmission system, comprising: Providing a bias voltage for setting an operating point to the power amplifier (101), the bias voltage having a respective bias value, Providing a supply voltage of the power amplifier (101), wherein a supply voltage value is set such that a predetermined output power value of the power amplifier (101) is achieved, Changing the preload value to a new preload value, Repeating the previous steps for at least one subsequent time slot until a criterion is met, Setting the bias value of the following time slot to the bias value of the previous time slot, and wherein the criterion is met when the supply voltage value exceeds a given threshold. [2] Method according to claim 1, wherein the criterion is met when the ratio of a change in the supply voltage value to a change in a determined output power exceeds a given threshold value. [3] A method according to any one of the preceding claims, wherein a maximum bias value is provided for a first time slot of a sequence of time slots. [4] Method according to one of the preceding claims, wherein the supply voltage is set as a function of an assigned output power of the power amplifier (101). [5] Method according to one of the preceding claims, wherein the supply voltage is set as a function of a determined output power of the power amplifier. [6] Method according to one of the preceding claims, wherein the change in the bias value is effected by a reduction of the bias value by a given amount. [7] Method according to one of the preceding claims, wherein the change in the bias value is effected by a reduction of the bias value by a value which is constant for all time slots. [8] Method according to one of the preceding claims, comprising: Storing the supply voltage value for the duration of at least one subsequent time slot. [9] Method according to one of the preceding claims, comprising: Storing the bias value for the duration of at least one subsequent time slot. [10] Device (100) for controlling a power amplifier (101) for operation in a time slot data transmission system, comprising: means for storing a bias value of a time slot; means for providing a bias voltage to the power amplifier (101) for setting an operating point of the power amplifier (101), the bias voltage having the bias value, a means for providing a supply voltage control signal, wherein a supply voltage value is set such that a predetermined output power value of the power amplifier (101) is achieved, means for changing the bias value to a new bias value for a subsequent time slot, wherein the power amplifier (101) comprises an amplifier transistor (202) having a base terminal, wherein a bias input is coupled to the base terminal, wherein an operating point of the amplifier transistor (202) is set by the bias voltage provided at the bias input, and wherein a control loop is used to repeat the changing of the bias voltage value for at least one subsequent time slot until a criterion is met, and wherein the criterion is met when the supply voltage value exceeds a given threshold value. [11] Transmitter for operation in a time slot data transmission system comprising: a device according to claim 10. [12] Transmitter according to claim 11 comprising: a power amplifier connected to the device. [13] Transmitter according to claim 12 comprising: a power detector coupled to the power amplifier for determining an output power of the power amplifier and connected to the device.

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