GB2329087A

GB2329087A - Improving the efficiency of a transmitter amplifier by controlling an amplifier voltage using a voltage conditioner such as a switched mode converter

Info

Publication number: GB2329087A
Application number: GB9818429A
Authority: GB
Inventors: Gustavo D Leizerovich; Dennis G Anson
Original assignee: Motorola Inc
Current assignee: Motorola Solutions Inc
Priority date: 1997-09-08
Filing date: 1998-08-25
Publication date: 1999-03-10
Also published as: GB9818429D0; FR2768277A1; DE19840092A1; CN1215260A; BR9803545A

Abstract

A radio (300) has a transmitter (302) comprising a power amplifier (308), a voltage conditioner (318), and a controller (313). The power amplifier operates at one of a plurality of output power levels. When the output power level is reduced from a maximum setting to a lower setting, the controller adjusts a control signal level provided to the voltage conditioner. The change in control signal level selects a lower output voltage from the voltage conditioner, which is supplied to the power amplifier. By selectively adjusting the voltage level supplied to the power amplifier, the power added efficiency is maintained at an optimum level. The voltage conditioner may be a switched mode converter of buck, boost or buck/boost type.

Description

1 2329087 TRANSMITTER HAVING AN IMPROVED POWER ADDED EFFICIENCY AND RADIO

USING SAME

TECHNICAL FIELD

The invention relates in general transmitters, and more particularly to transmitters which operate at one of a plurality of output power levels.

BACKGROUND

A current emphasis in the design of communications equipment is towards smaller subscriber hand held devices. This emphasis is readily apparent when one compares cellular telephones currently marketed with those sold only five years earlier. One of the largest components of any hand held portable communications device is the battery. With the emphasis on size reduction, it has been necessary to reduce the size of any associated battery or battery pack. However, in reducing battery size, it is not acceptable to reduce the usage time of the device, and since, generally, a smaller battery provides less energy storage, either a better battery must be developed, or the device must become more efficient at using the energy stored in the battery. Both strategies are being pursued by manufacturers, however, battery technology is very mature, and therefore manufacturers have looked more towards increasing the efficiency of the device electronics.

In hand held portable communications devices, one of the highest power consuming devices is the transmitter. This is why, for example, a cellular telephone may operate for days on one battery charge in a standby mode, but the "talk time" is measured in minutes or hours. This is because when the device is transmitting it uses much more power than it does in a standby or idle mode. This is particularly true in a device using an amplitude modulation (AM) scheme, including digital modulation 2 schemes such as quadrature amplitude modulation (QAM). This is because in an AM system the power over time is not constant, and the efficiency of a typical power amplifier changes depending on the output power level. As an illustration of a typical AM system, shown in FIG. 1 is a chart 100 graphing power amplifier output power 102 vs. time 104 in a typical time division multiple access (TDMA) AM system. The curve 106 is arbitrary, depending on the signal being transmitted. The curve starts and ends at no power as the signal is only transmitted during a particular time slot, in accordance with a TDMA format. Within the TDMA time slot, there is a peak power level, indicated by line 108, and an average power level, indicated by line 110. Typically the power amplifier is designed such that the peak power has some margin from saturation point (Psat), indicated by line 112. The Psat point is defined as the point on a graph of output power vs. Input power where the output power gain drops a specified amount, such as 3 decibels (dB) for example, from linear operation. In other words, as input power is increased, output power increases linearly by some constant gain factor. As the output power reaches the Psat point, the power gain begins to fall below linear, and when the difference between actual output power and the linearly expected value is, for example, 3dB, the Psat point has been reached. From FIG. 1, it can be seen that in an AM system the power amplifier average output power must be set so that the output power peaks do not exceed the Psat line, or the power amplifier will be operating in saturation, resulting in significant output distortion.

However, the most efficient operating point for a typical linear power amplifier, such as a class AB power amplifier, is close to the Psat point. Referring now to FIG. 2, there is shown a chart 200 graphing the power added efficiency (PAE) 202 vs. output power 204 in dB below the Psat point. The PAE is the efficiency of converting the DC power supplied to the power amplifier into signal power. Since, as illustrated in FIG. 1, an AM signal power vs. time varies, the nominal power output level of the power amplifier must be set low enough to allow for peak levels of output 3 power. For example, consider a typical class AB power amplifier having a peak to average power ratio of 5.6dB. To allow for peak levels in an AM system, and a margin, the nominal operating power level is set for -7.5dB from the Psat point, as indicated by line 206. Since the efficiency decreases as the power level is set lower, for a typical class AB power amplifier, the efficiency at the -7.5dB point is about 24%, as indicated by line 208. In a frequency modulated WM) system the output power will be constant, and thus the power ampIffier in a FM system can operate with the nominal output power set much closer to the Psat point.

Further worsening the efficiency is the fact that mobile communications units are often designed to adjust the output power level, depending on location to the receiving equipment. This is done allow frequency re-use in nearby operating regions, and to avoid adjacent channel interference. Thus, there is a need for a means by which the PAE is improved in a power amplifier which operates at discrete output power levels.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a chart graphing output power of a power amplifier vs.

time for an AM signal output; FIG. 2 shows a chart graphing the power added efficiency vs. output power compared to the Psat point of a typical power amplifier; FIG. 3 shows a block diagram of a radio incorporating a transmitter in accordance with the invention; and FIG. 4 shows a block diagram of a switched mode power converter in accordance with the invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

While the specification concludes with claims defining the features of the invention that are regarded as novel, it is believed that the invention will be better understood from a consideration of the following 4 description in conjunction with the drawing figures, in which like reference numerals are carded forward.

The invention solves the problem of degraded efficiency when transmitting at a lower power output level by taking advantage of the relationship between the Psat point and the voltage level supplied to the power amplifier. This relationship is expressed in the following equation:

Psat = Wdd - Vgat)2 2R1 where: Vdd is the operating voltage supplied to the power amplifier; Vsat is the saturation voltage of the power amplifier output transistor; and RI is the load resistance presented to the output of the power amplifier.

To achieve an improvement in efficiency when the output power is reduced to a "cutback" level, it would be desirable to adjust the Psat point of the power amplifier. In other words, when the output power level is reduced, the Psat point is likewise reduced to maintain the highest efficiency of operation while still providing enough margin to avoid saturating the power amplifier output transistor. For example, if the output power level is reduced by MB, then reducing the Psat point by MB substantially maintains the same efficiency as at the higher output power level.

From the equation given above, one can see that there are essentially three parameters that determine the Psat level. Therefore, if the Psat level is to be adjusted, it must be done by adjusting at least one of these three parameters. Of these parameters, the saturation voltage Vsat of the output transistor is least changeable. This value is determined by the output transistor characteristics, and is substantially constant over the output power range. The load resistance RI can be changed during operation, but to a very limited degree. This value is determined by the output matching network, and thus, the resistance is dictated by the performance of those circuits. Finally, the supply voltage Vdd must be analyzed. Typically, as mentioned above, the power source for a portable communications device is a battery or battery pack. The voltage sourced by the battery is either regulated to a prescribed level, or is supplied directly to the power amplifier. In the latter case some simple and well known bias circuitry is necessary to ensure proper biasing of the power amplifier as the battery voltage changes. Thus, if a means of selectively adjusting the voltage supplied to the power amplifier were employed, the desired improvement in efficiency will be obtained.

Referring now to FIG. 3, where there is shown a block diagram of a radio 300 incorporating a transmitter 302 in accordance with the invention.

The radio is connected to a voltage source 304, such as a battery or battery pack, and has an antenna 306 for transmitting and receiving radio frequency (RF) signals. The main component of the transmitter is the power amplifier 308. In some transmitter, such as in QAM transmitters, a linearization circuit 309 is used to provide further linearization. In the preferred embodiment, the linearization circuit provides cartesian feedback, although it is contemplated that any known linearization circuit technique may be employed, such as, for example, feed-forward, adaptive pre-distortion, or polar feedback, to name a few techniques. For cartesian feedback, a signal on line 311 is fed back to the signal source 310, which comprises a modulator.

The power amplifier receives an input signal from a signal source 310, which comprises circuitry for modulating a carrier wave with a message signal. The message signal is the information that is to be transmitted to a receiving party, and may be voice or data, for example.

Thus, the input signal is already an RF signal and the power amplifier simply increases the power of the input signal to produce an output signal 6 at the output 312 of the power amplifier. In order to accommodate transmitting in cutback modes, the power amplifier is operable at a plurality of output levels, the particular level being selectable by a controller 313. The output signal is filtered by an output match/harmonic filter network 314 to remove undesirable frequency content of the output signal, and to provide an impedance match for the antenna. Depending on the type of radio under consideration, an RF switch 316 may be employed to switch the antenna connection between a transmit mode and a receive mode of the radio.

To adjust the voltage level supplied to the power amplifier a voltage conditioner 318 is provided disposed between the voltage source 304 and the power amplifier. The voltage conditioner adjusts the raw voltage provided by the voltage source 304 to one of a plurality of discrete output voltage levels on line 320, which is used to provide voltage to the power amplifier 308. The voltage conditioner is responsive to a control signal provided by the controller 313. The control signal causes the voltage conditioner to selectively operate the power amplifier at one of the plurality of discrete output voltage levels of the voltage conditioner, which is the supply voltage for the power amplifier. Thus, the controller 313 determines at what supply voltage level the power amplifier is to operate at, then provides the voltage conditioner an appropriate control signal such that the Psat point is adjusted to maintain the highest efficiency as dictated by the graph of FIG. 2.

To illustrate how the invention functions, consider the following example. Assume that a mobile radio unit incorporating the invention is traveling and enters a region serviced by a particular base station. Upon entering the region, the mobile radio unit is transmitting at the maximum allowable power. However, as time passes, the mobile radio unit approaches the base station antenna. At some point as the mobile radio unit nears the antenna, the mobile radio unit reduces its output power. According to the graph in FIG. 2, without an adjustment of the voltage 7 supplied to the power amplifier of the mobile radio unit, the PAE of the mobile radio unit is degraded. However, the controller 313 causes the voltage conditioner to operate at a lower output voltage level such that the efficiency of the power amplifier is optimized.

In the preferred embodiment, the voltage conditioner is a switched mode power converter. A great many variety of switched mode power converter circuits exist, and are well known in the art. Generally, there are threecategories of switched mode power converters: buck, boost, and buck/boost. A buck converter converts a source voltage level to a lower voltage level. Boost converters convert the source voltage to a voltage higher than the source voltage level, and buck/boost perform both up and down conversion of the source voltage level. All three of these types may be employed according to the invention, and it is an engineering choice of which a particular application may require.

However, it should be noted that the trend in portable electronics, and portable radio devices in particular, is towards lower operating voltages. In some cases, it is a goal to provide a radio unit, such as a two way radio or a cellular telephone, that operates from a single battery cell, which can be as low as 1.2 volts nominal. While an operating voltage this low will allow for a lower power consumption, it presents significant problems in the design of a power amplifier for such a device. Thus, in such a case, a boost mode converter would be appropriate.

Referring now to FIG. 4, there is shown a block diagram of a switched mode power converter 400 in accordance with the invention. The converter diagram shown here is a generalized boost type converter which converts a source voltage level, provided by a voltage source 304, to a higher potential. In general, the converter comprises an inductor 402, a switch ^ a blocking element 406, an output filter capacitor 408 and a control circuit 410. The converter provides power to a load 412, which could be a power amplifier. The control circuit samples the output voltage across the filter capacitor, and compares it with a reference or control 8 voltage on line 414. The control circuit provides a pulse width modulated (PWM) signal to the switch, and adjusts the duty cycle of the PWM signal such that the output voltage corresponds to the reference voltage. Typically the reference voltage is at a much lower level than the desired output voltage, so common practice is to divide the output voltage down through a simple resistor network. A higher voltage level is provided at the output 416 by closing the switch, therefore momentarily connecting inductor to the reference line 418, thereby charging the magnetic core of the inductor and applying the source voltage level across the inductor. When the switch is subsequently opened, the voltage across the inductor reverses polarity, thus adding to the voltage source level, the combined voltage level is applied to the filter capacitor and load. The blocking element prevents the charge delivered to the output from returning, while the filter capacitor filters the switched voltage.

In practicing the invention, using the generalized power converter of FIG. 4 as the voltage conditioner of FIG. 3, line 414 is the line by which the controller 313 sends a control signal to the voltage conditioner. Thus. by adjusting the reference voltage supplied to the converter control circuit, the output level is selectively adjustable. Although the details of implementing the converter circuit are a matter of engineering choice, efforts have been undertaken to produce highly efficient converters. One such converter design is described in the article entitled "Chip Set With Power MOSFET Increases Cell-Phone Talk Tirne" by F. Goodenough, published in Electronic Design magazine, August 19, 1996, pp. 69, 70, and 77. The article discusses and teaches the design of a boost or step-up converter for use in powering a power amplifier of a cellular telephone. The converter discussed in the article achieves a high efficiency, making it a practical choice for use in a battery powered communications device. Similarly efficient designs for step-down or buck converters, and combination buck/boost converters are known in the art.

9 The use of a power converter has an additional benefit in certain communications devices. Another trend in portable communications devices is the use of digital communications protocols and time division/ multiple access (TDNIA) formats. In such systems the portable unit transmits information in periodic bursts. In a conventional system these bursts create peaks in the current drawn from the voltage supply. In the case of battery powered devices, high current levels cause the battery voltage to drop significantly, and may prematurely drop the battery voltage below an operating threshold of the device. This is because of the internal resistance of the battery or battery pack. To relieve this problem, manufacturers have begun incorporating large value capacitors into the battery pack. Since the capacitor has a very low resistance, it can deliver brief bursts of charge, thus maintaining the voltage supplied to the radio. Since the current burst is primarily drawn by the power amplifier, in a transmitter in accordance with the invention, the filter capacitor 408 can provide the necessary charge reserve.

Thus, according to the invention, the power added efficiency of a power amplifier having several average output power operating levels can be increased when the power amplifier is operating at power output levels below its maximum level. The gain in efficiency is obtained by providing a voltage conditioner having a plurality of output voltage levels for powering the power amplifier, and selectively controlling the voltage conditioner so that the power amplifier is operated at an optimum supply voltage level. Although the invention has been described from the perspective of an AM system, it is contemplated that an improvement can be obtained in FM transmitters as well. If an FM transmitter operates at s everal different output power levels, then it will experience a similar degradation in PAE when upon setting the output power level to a cutback level. Thus, by applying the principles of the present invention, an increase in PAE can be realized. FM transmitters typically use non-linear power amplifiers, so it is possible that both linear and non-linear amplifiers can benefit from the invention.

While the preferred embodiments of the invention have been illustrated and described, it will be clear that the invention is not so limited. Numerous modifications, changes, variations, substitutions and equivalents will occur to those skilled in the art without departing from the spirit and scope of the present invention as defined by the appended claims.

Claims

What is claimed is:

11 CLAIMS 1. A transmitter, comprising:

a power amplifier operable at a plurality of output power levels; a voltage conditioner for providing a voltage level to the power amplifier, the voltage level adjustable to one of a plurality of output levels, the voltage conditioner responsive to a control signal; and a controller for providing the control signal to the voltage conditioner to selectively operate the power amplifier at one of the plurality of output levels of the voltage conditioner.
2. A transmitter as defined in claim 1, wherein the voltage conditioner operates from a power source, the power source is a battery pack having a nominal voltage output level.
3. A transmitter as defined in claim 2, wherein the nominal voltage output level of the battery pack is lower than a maximum output level of the voltage conditioner.
4. A transmitter as defined in claim 1, wherein the power amplifier is a linear amplifier.
5. A transmitter as defined in claim 1, further comprising a 25linearization block connected between an output and an input of the power amplifier for providing feedback to the power amplifier.

12
6. A radio device, comprising:

a voltage source; a power amplifier having a plurality of output power levels, an output and an input; a voltage conditioner for providing a voltage level to the power amplifier, the voltage level adjustable to one of a plurality of output levels, the voltage conditioner responsive to a control signal for selecting one of the plurality of output levels, and connected to the voltage source; and a controller for providing the control signal to the voltage conditioner to selectively operate the power amplifier at one of the plurality of output levels of the voltage conditioner.
7. A radio device as defined in claim 6, wherein the voltage conditioner is a switched mode power converter.
8. A radio device as defined in claim 6, wherein the voltage source is battery pack having a nominal voltage output level.
9. A radio device as defined in claim 8, wherein the nominal voltage output level of the battery pack is lower than a maximum output level of the voltage conditioner.
10. A radio device as defined in claim 6, wherein the power amplifier is a linear amplifier.