GB2484475A

GB2484475A - A power supply modulator for an RF amplifier, using a current-output class G amplifier

Info

Publication number: GB2484475A
Application number: GB201017116A
Authority: GB
Inventors: Gavin Watkins
Original assignee: Toshiba Research Europe Ltd
Current assignee: Toshiba Europe Ltd
Priority date: 2010-10-11
Filing date: 2010-10-11
Publication date: 2012-04-18
Also published as: GB201017116D0

Abstract

The low frequency portion of an envelope signal is amplified by a switching power supply 100, while the high frequency portion of the envelope signal is amplified by an amplifier 90 operating in class G or class H. The output of the amplifier 90 comprises a set of current mirrors (figure 2), one being selected at any time. The current mirror outputs provide thermal stability. The use of a class G/H amplifier provides greater efficiency. The amplified envelope signal may be applied to an RF amplifier in an LTE or WiMAX base station or in a DVB transmitter.

Description

Envelope Modulator and Method of Modplating a Signal Envelope

FIELD

Embodiments described herein relate generally to power efficient envelope modulators. Embodiments described herein specifically relate to envelope modulators employing class G or class H amplifiers.

BACKGROUND

Envelope modulators often use a linear class AB or a class B amplifier to amplify high frequency AC signal components. Envelope modulators that use such an amplifier to amplify the entire bandwidth of a signal are inherently inefficient. Another type of envelope modulator splits the frequencies of the signals to be operated upon and applies only a higher signal frequency component to the class AB or class B amplifier, thereby increasing the modulator's efficiency to some degree.

BRIEF DESCR1PTION OF THE DRAWINGS Embodiments of the present invention will now be described by way of example only and with reference to the accompanying drawings, in which: Figure 1 shows a split frequency envelope modulator according to an embodiment; Figure 2 shows a symmetrical shunt class G amplifier according to an embodiment; Figure 3 shows an asymmetric class G amplifier according to another embodiment; Figure 4 shows a split frequency envelope modulator according to another embodiment; and Figure 5 shows a DC coupled envelope modulator according to an embodiment; and Figure 6 shows the class G amplifier of Figure 2 and further including a Zener diode biasing arrangement.

DETAILED DESCRIPTION

According to an embodiment there is provided a class G or class H amplifier comprising a controller arranged to enable supply of output power from one of a plurality of different power sources at any one time and a current limiting means for limiting an output current of the amplifier.

When referring to class C amplifiers herein reference is made to an amplifier that is provided with a plurality of supply voltage levels. The amplifier controller of this amplifier is arranged to sense the input voltage that is to be amplified and to select, from among the available supply voltages the one supply voltage that reflects a desired amplified version of the input signal most closely. The controller is arranged to connect the voltage source having the selected voltage level to an output stage. An amplified output is generated using the selected supply voltage.

When referring to class H amplifiers herein reference is made to an amplifier that comprises a controller arranged to sense the input voltage that is to be amplified and to select, from among the available supply voltages the one supply voltage that reflects a desired amplified versipn of the input signal most closely. In contrast to class C amplifiers, which use voltage supplies that provide a constant supply voltage, the class H amplifier relies upon one or more high voltage supplies in which the supply voltage is linearly varied according to requirements imposed by the input signal. It may be the controller itself that linearly varies the supply voltage of the high voltage supplies.

The current limiting means may comprise a current mirror or a plurality of current mirrors arranged in parallel/in a shunt configuration. The controller may be arranged to drive/enable and disable two or more output current mirrors. The current mirrors may be arranged in parallel, with each current mirror individually connected to a respective output of the controller.

The current mirror can comprise two transistors connected to each other by their bases or gates, wherein one transistor is diode connected and wherein the output signal of the controller is applied to the connected bases/gates of the transistors. The emitters or sources of the transistors are connected to a power supply rail. This connection may be made via respective transistors. The magnitude of the resistance, together with the difference in potential between the output signal provided by the controller and the potential provide by the power supply, determines the current flowing through the current mirror.

The power sources can comprise a plurality of voltage supply rails. The amplifier may be arranged to amplify positive and negative signal excursions. A number of positive and/or negative power sources may be included in or provided to the amplifier. The number of positive and negative voltage levels provided may be the same. This is useful for amplifying signals that have positive and negative voltage excursions of equal, similar or comparable magnitude. Alternatively the number of positive and negative voltage levels used by the amplifier may be different. It may, for example, not be necessary to provide a number voltage supplies for amplifying negative voltage excursions if it is known that the magnitudes of these negative voltage excursions is small when compared to that of the positive voltage excursions, and vice versa. One voltage supply rails may, for example be provided for one polarity while several supply rails are provided for the opposite polarity.

According to another embodiment there is provided an envelope modulator comprising an amplifier as described above. This amplifier may be arranged to amplify a high frequency component of an envelope signal that is to be modulated. The envelope modulator may further comprise a further amplifier for amplifying the low frequency component of the envelope signal. The class G or class H amplifier and the further amplifier may be provided in separate, parallel signal paths.

The envelope modulator may comprise a frequency separator arranged to separate a high frequency component of an envelope signal to be modulated and a low frequency component of the envelope signal and to provide the high frequency component to a first output and the low frequency component to a second output. The class G or class H ampJifier may be connected to the first output, while the further amplifier may be connected to the second output. The frequency separator may comprise a high pass filter and a low pass filter provided in parallel with a common input. In this case the first output is the output of the high pass filter and the second output is the output of the low pass filter.

The further amplifier may be a DC/DC converter, for example a switched mode power supply.

The amplified signals provided by the two amplifiers may be combined using a combining network. The combining network may comprise high and low pass filters that are provided in parallel with each other with a common output and that receive their respective inputs from the class G or class H amplifier and from the further amplifier respectively.

The further amplifier may comprise a feedback line connected to a common output of the envelope modulator or to a point on an output side of a circuit that combines the outputs of the class G or class H amplifier with that of the further amplifier, if different. If the feedback for the further amplifier is taken from the common output of the envelope modulator a low pass output filter of the further amplifier can be combined with the components of a low pass part of a circuit arranged to combine the amplified signals of the two amplifiers. In this way the overall number of circuit components required can be reduced, thus reducing the amount of required circuit area.

In another embodiment there is provided an envelope modulator comprising an input for receiving an envelope signal that comprises a high frequency component and a low frequency component and a class C or class H amplifier that is connected to receive the high and low frequency components of the envelope signal and to output an amplified envelope signal. The envelope modulator further comprises a further amplifier that is arranged to amplify a low frequency component of the amplified envelope signal to create an amplified low frequency signal and to combine the amplified low frequency signal with the amplified envelope signal to create a combined signal. The amplifier, be it a class G or a class H amplifier, comprises a feedback arranged to feed at least part of the combined signal back to an input of the class C or class H amplifier.

The further amplifier may comprise a current sensor arranged to sense an output current of the class C or class H amplifier or part of the output current.

A low pass filter may be provided to limit the sensed signal provided to the further amplifier to the frequencies of the low frequency component.

The feedback may be arranged to subtract the combined signal from the envelope signal at an input of the class G or class H amplifier.

S The above discussed high and low frequency components may include a frequency band with a high centre frequency and a frequency band with a centre frequency lower than the high centre frequency. The two frequency bands are not or not fully overlapping. A highest frequency of the high frequency band is higher than a highest frequency of the low frequency band. A lowest frequency of the low frequency band is lower than a lowest frequency of the high frequency band. The lower frequency band may comprise DC. The cut off frequencies of any of the above mentioned high and low pass filters are respectively so that they reliably separate these two frequency bands.

According to another embodiment there is provided a RF amplifier comprising an envelope modulator as described above. According to another embodiment there is provided a base station or a transmitter comprising such a RF amplifier. The base station may be operated according to an OFDM standard, such as the LTE or WIMAX standards. The transmitter may be operating according to the DVB standard.

According to another embodiment there is provided a method of amplifying a signal. The method comprises providing the signal to a class G or a class H amplifier and limiting an output current of the amplifier.

According to another embodiment there is provided an envelope modulation method. The method comprises separating an envelope signal into a high frequency component! and a low frequency component and separately amplifying the high and low frequency components. The high frequency component may be amplified using the above described method.

The method may further comprise combining the separately amplified signals to generate a combined signal and providing a feedback based on the combined signal to an amplifier used for amplifying the low frequency component.

According to another embodiment there is provided a method of modifying an envelope signal that comprises a high frequency component and a low frequency component. The method comprises providing the high and low frequency components to a class G or a class H amplifier and generating an amplified envelope signal using the amplifier. The output signal of the amplifier is sensed and a low frequency component of the sensed output signal is amplified using a further amplifier. The amplified envelope signal is combined with the amplified low frequency component to generate a combined signal and a feedback to an input of the class G or class H amplifier is provided based on at least part of the combined signal.

Figure 1 illustrates an amplifier arrangement 10 comprising an envelope modulator according to an embodiment. The amplifier arrangement 10 comprises and RF input 20 to which a RF signal that is to be amplified by the amplifier 30 is applied. The amplifier arrangement 10 also comprises an output 40 to which a load can be connected.

An envelope detector 50 is connected to the input 20. The envelope detector 50 provides a signal indicative of an instantaneous magnitude of the envelope of the input RF signal to an input node 60 of the envelope modulator.

The envelope modulator of the embodiment comprises two signal paths, one high frequency signal path 70 and a hw frequency signal path 80. Both these signal paths share a common input in node 60.

The high frequency signal path comprises a RC high pass input filter.

The low frequency path comprises a RC low pass input filter. The two input filters together form the depicted splitting network that separates high frequency components of the envelope signal from low frequency components of the envelope signal in accordance with the respective filter frequency characteristics of the respective input filters. Although RC filtering networks are shown in the embodiment of Figure 1, it will be appreciated that other high and/or low pass filter architectures may be used instead of the ones shown. lt will be appreciated that the pass frequency bandwidth of the high and low pass filters can be chosen by the person skilled in the art to meet the requirements imposed upon the envelope modulator by the nature of the RF signal that is to be amplified.

The high frequency signal path of the embodiment uses a class C amplifier to amplify the signal passed by the high frequency input filter of this signal path and uses a capacitor to feed the amplified signal to the voltage supply input of the RF amplifier 30. The use of the Class G amplifier improves the efficiency of the envelope modulator, when compared to known envelope modulators that use class AS amplifiers for amplifying a high frequency envelope signal component.

The low frequency signal path of the embodiment uses a switched mode power supply (SMPS) 100 to amplify the low frequency components of the envelope signal. A low pass filter is provided in series with the controller of the SMPS 100 to suppress switching induced noise. The SMPS 100 controller takes its feedback from a node 110 on the output side of the low pass filter to enable the controller to control the filtered output. The amplified signal provided by the SMPS 100 is fed to the voltage supply input of the RF amplifier 30 via an inductor. This inductor forms a combining network together with the capacitor provided at the output of the high frequency current path. It will be appreciated that this combining network may comprise high and low pass filters in the high and low frequency paths respectively that differ from the high and low pass filters shown in Figure 1.

Figure 2 shows a class C amplifier 200 suitable for use with the envelope modulator 10 shown in Figure 1. The amplifier 200 comprises an input 210, an output 220, a supply input/rail 230 for the supply of a positive high supply voltage, a supply input/rail 240 for the supply of a positive low supply voltage, a supply input/rail 250 for the supply of a negative high supply voltage, a supply input/rail 260 for the supply of a negative low supply voltage and a class C amplifier controller 270. The supply inputs/rails 230 to 260 provide the plurality of supply voltage levels used by the class C amplifier 200. It will be appreciated that the values of the voltages on the voltage supplies can be chosen to facilitate a truthful representation of the high frequency component of the envelope signal as long as the positive high supply voltage has a larger magnitude than the positive low supply voltage and as long as the negative high supply voltage has a larger magnitude than the negative low supply voltage.

The magnitude of the high positive voltage can correspond to the magnitudes of the high negative voltage. The magnitude of the low positive voltage can equally correspond to the magnitudes of the lovi negative voltage. It will, however, be appreciated that a correspondence in the magnitudes of the high and low positive and negative voltages is not essential and that the voltage magnitudes can be chosen such that they allow the amplifier to operate in a manner that most adequately amplifies the signals expected to be presented to the envelope modulator.

The class G controller 270 is of known type and in this embodiment comprises four output signal lines 280. The number of output lines 280 of the controller 270 corresponds to the number of available voltage inputs/rails. Each of the output signal lines can carry a signal for activating switching circuitry that connects one of the supply voltages provided on inputs/rails 230, 240, 250 and 260 to the output 220. The controller 270 operates in a known manner and provides only one output signal on the output lines 280 at any one time.

Each of the output lines 280 is connected to a current mirror. Each of the current mirrors comprises a first diode connected transistor 290/310/330/350, with the base connected to the collector. The output lines 280 of the controller 270 is connected to the base of the first transistor. Also provided are second transistors 300/320/340/360. The bases of the second transistors 300/320/340/360 are connected to the bases of the corresponding ones of the first transistors 290/310/330/350. Also provided are resistors 370 to 440 connected to emitters of the first and second transistors 290 to 360.

The magnitude of the current flowing through the first transistors 290/310/330/350 is determined by the difference in the voltage supplied by the relevant one of the voltage inputs/supply rails 230 to 260 and the output voltage of the controller 270 as well as the value of the relevant resistor 370/390/410/430. The current flowing through the first transistors 290/310/330/350 is mirrored into the second transistors 300/320/340/360 through the connection of the bases of the first 290/310/330/350 and second transistors 300/320/340/360, in a 1:1 relationship if the two transistors within the current mirror are the same. The so created mirror current is supplied to the output 220, via the output capacitor 450. It will be appreciated that the output capacitor 450 may be the same capacitor as the capacitor in the high frequency half of the combining network shown in Figure 1.

The use of the current mirrors shown in Figure 2 limits the output current available from the class G amplifier to a value determined in the above manner.

Any lowering of an impedance of a component or circuit in the path of the output current, including in the load attached to the amplifier 200, cannot therefore cause the consumption of a current that is larger than the predetermined maximum, therefore avoiding the problem of thermal runaway. The embodiment shown in Figure 2 thus avoids the thermal runaway disadvantage known class G or class H amplifiers using a common emitter configuration suffer from.

Figure 3 shows another class G amplifier 500 that is suitable for use in an envelope modulator described herein. As can be seen from Figure 3, the amplifier 500 differs from the amplifier 200 shown in Figure 2 in that only a single negative voltage input/supply rail 510 is provided, instead of the high and low negative voltage supply rails 250 and 260 shown in Figure 2. The amplifier 500 may be of use in for amplifying signals with negative voltage excursions that have a smaller dynamic range than that of the positive voltage excursions.

As can be seen from Figure 3, the positive voltage supply part of the amplifier 500 corresponds to the positively voltage supply part of the amplifier shown in Figure 2. Like components have consequently been identified by like reference numerals. The controller 520 only comprises three output lines 530, in correspondence with the number of input voltage supply rails. The structure and operation of the negative output current mirror shown in Figure 3 corresponds to that of either of the individual ones of the negative output current mirrors of the amplifier 200 shown in Figure 2.

Figure 4 illustrates another amplifier 600 with an envelope modulator.

The amplifier/envelope modulator shown in Figure 4 is similar to that shown in Figure 1 and like components have been given like reference numerals in both these figures. The low frequency path 610 of the envelope modulator shown in Figure 4, however, differs from the low frequency path 80 of the envelope modulator shown in Figure 1 in that the SMPS 620 of Figure 4 receives its feedback through a feedback line 630 connected on the output side of the inductor/low pass filter 640, rather than to the input side of the corresponding inductor/low pass filter in Figure 1. The low pass filtering function provided by the output network of the SMPS 100 shown in Figure us in the Figure 4 embodiment performed by the low pass filter 640 that also forms part of the combining network. Connecting the feedback line 640 in this manner accordingly allows eliminating the output network of the SMPS, thereby reducing the circuit area required by the lower frequency branch 610 of the envelope modulator of the amplifier 600.

Figure 5 shows another embodiment of an amplifier 700 comprising an envelope modulator. Some of the components of this amplifier 700 correspond to components of the amplifiers shown in Figures 1 and 4 and are identified by like reference numerals. As can be seen from Figure 5, in contrast to the envelope modulators shown in Figures 1 and 4, the entire bandwidth of the envelope signal is applied to the input of the class 0 amplifier 90. The class 0 amplifier 90 employed in the embodiment shown in Figure 5 has a bandwidth that is sufficient to amplify the entire bandwidth of the envelope signal (this can also be the case for the class G amplifiers used in the envelope modulators shown in Figures 1 and 4 but is less important in these cases), so that the output signal provided by the amplifier 90 provides a low frequency or DC output as well as an AC output, both reflecting the low frequency/DC and the AC components of the input envelope signal. The voltage output by the class 0 amplifier 90 of the envelope modulator is directly applied to the RF amplifier 30.

The envelope modulator also comprises a current sensor 710 that detects the current output by the class 0 amplifier 90 and provides it to a low pass filter 720. The low pass filter has a bandwidth that allows sensed DC and low frequency AC current signals to pass to the switched mode power supply 730 for amplification therein. Higher frequency sensed AC current components in contrast are blocked by the low pass filter 720. The pass band of the low pass filter 720 may correspond to the pass band of the low pass filter of the splitting networks used in the Figure 1 and/or 4 embodiments.

The switched mode power supply 720 amplifies the received signal and provides a resulting low frequency current to the RF amplifier 30 via the combining/output inductor 730. It will be appreciated that the addition of these low frequency currents creates a rise in the voltage across the power supply input of the RF power amplifier 30. As the full bandwidth voltage drop across the RE amplifier 30 is defined by the amplified output of the class G amplifier 90, the presence of this voltage rise enables the class G amplifier, via its feedback, to predominantly amplify the lower power high frequency components of the envelope signal. The switched mode power supply 720 amplifies the higher power low frequency components of the envelope signal.

It will be appreciated from the above that the envelope modulator of the amplifier 700 shown in Figure 5 provides a frequency splitting operation, albeit in a manner different from that of the envelope modulators of the amplifiers shown in Figures 1 and 4.

Referring now to Figure 6, this figure shows the amplifier illustrated in Figure 2 in combination with a Zener diode biasing network that comprises four Zener diodes 810 to 840 provided in series with two resistors 850 and 860 between the positive and negative high supply voltage rails respectively. This biasing network can be employed because the current mirror output stages are inheritably thermally stable.

The class G driver stage 270 is arranged to use the biased voltages created by the Zener diodes 810 and 820 respectively for driving the positive low stage (comprising the transistors 310 and 320 and the resistors 390 and 400 and connected to the positive low supply voltage rail 240) and the negative low stage (comprising the transistors 350 and 360 and the resistors 430 and 440 and connected to the negative low supply voltage rail 260). The positive and negative low stages are hereby biased to operate in class AB.

The class C driver stage 270 is further arranged to use the further biased voltages created by the combination of Zener diodes 810 with 830 and the combination of Zener diodes 820 with 840 respectively for driving the positive high stage (comprising the transistors 290 and 300 and the resistors 370 and 380 and connected to the positive high supply voftage rail 230) and the negative high stage (comprising the transistors 330 and 340 and the resistors 410 and 420 and connected to the negative high supply voltage rail 250). The positive and negative high supply stages are hereby biaed to operate in class C. The class G amplifier 800 shown in Figure 6 also comprises diodes 870 and 880 respectively provided in parallel with the resistors 850 and 860. The biasing voltage applied to the input signal by the series of two Zener diodes, 810 and 830 or 820 and 840, can, if the voltage excursion of the input signal is large, provide a negative biasing voltage across transistors in the driver stage 270. The diodes 870 and 880 protect the driver stage from damage due to such negative bias.

It will be appreciated from the above that the envelope modulators shown in Figures 1, 4 and 5 use the class G or class H amplifier to amplify a high frequency component of the signal. The use of class 0 or H amplifiers for this purpose renders this amplification highly efficient and allows to maintain bandwidth and linearity. In the presence of signals with a high peak-to-average- 16 power-ratio class B and class AS amplifiers would spend most of their time operating in a low efficiency region of their transfer characteristic. Class 0 effectively defines two high efficiency operating regions, which are specified depending on the signal statistics.

The use of a separate low frequency amplifier in the split frequency.

designs shown in Figures 1, 4 an 5 increases the efficiency of the envelope modulator further, as through this use the amplification of the higher power low frequency components of the envelope signal is separated form the amplification of the lower power higher frequency components. Because of the high efficiency of the amplifiers/envelope modulators described herein these 26 device require less cooling than known amplifiers/envelope modulators. The amplifiers shown in Figures 1 to 5 can moreover be implemented using low cost components and are consequently less costly to manufacture than some known alternatives.

Experimental prototypes of both non-class 0 and class 0 envelope modulators have shown thai when amplifying the envelope of an OFUM signal a large power saving can be achieved. A non-class 0 envelope modulator has been shown to achieve an overall efficiency of 54% when reproducing the OFDM envelope signal across a resistive load. A class G based can achieve 73% efficiency. The class G based envelope modulator consumes 26% less power than the non-class G. It will be appreciated that, although the above description made with reference to the drawings focuses on the use of class C amplifiers, class H amplifiers may be used in other embodiments.

It will be appreciated that, although in the figures bipolar transistors are depicted, field effect transistors may equally be used.

While certain embodiments have been described, the embodiments have been presented by way of example only, an area not intended to limit the scope of the inventions. Indeed, the novel methods, apparatus 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 inventions. The accompanying claims and their equivalents are intended to cover such forms or modifications as would fall within the scope and spirit of the inventions.

Claims

CLAiMS: 1. A class 0 or class H amplifier comprising a controller arranged to enable supply of output power from one of a plurality of different power sources at any one time and a current limiting means arranged to limit an output current of the amplifier.
2. An amplifier according to claim 1, wherein the current limiting means comprises a plurality of current mirrors arranged in parallel.
3. An amplifier according to Claim I or 2, wherein the plurality of power sources comprises a first number of power sources of one polarity and a second, lower number of power sources of a polarity opposite to the one polarity.
4. An amplifier according to Claims 1, 2 or 3, further comprising an input Zener diode biasing network.
5. An envelope modulator comprising an amplifier according to any of Claims I to 4.
6. An envelope modulator according to Claim 5, wherein the amplifier is arranged to amplify a high frequency component of an envelope signal that is to be modulated, the envelope modulator further comprising a further amplifier for amplifying the low frequency component of the envelope signal, wherein the class 0 or class H amplifier and the further amplifier are provided in separate signal paths.
7. An envelope modulator according to Claim 6, wherein the further amplifier comprises a feedback line connected to a common output of the envelope modulator.
8. An envelope modulator comprising: an input for receiving an envelope signal comprising a high frequency component and a low frequency component; -a class G or class H amplifier connected to receive the high and low frequency components of the envelope signal and to output an amplified envelope signal; a further amplifier arranged to amplify a low frequency component of the amplified envelope signal to create an amplified low frequency signal and to combine the amplified low frequency signal with the amplified envelope signal to create a combined signal; wherein the class G or class H amplifier comprises a feedback arranged to feed at least part of the combined signal back to an input of the class G or class H amplifier.
9. An envelope modulator according to Claim 8, wherein the feedback is Is arranged to subtract the feedback signal from the envelope signal at an input of the class G or class H amplifier.
10. An envelope modulator according to any of Claims 8 or 9, wherein the class C or class H amplifier is in accordance with any of Claims 1 to 4.
11 A RF amplifier comprising an envelope modulator according to any of Claims 5 to 10.
12. A base station or a transmitter comprising a RF amplifier according to Claim 11.
13. A method of amplifying a signal comprising providing the signal to a class C or a class H amplifier and limiting an output current of the amplifier.
14. An envelope modulation method comprising separating an envelope signal into a high frequency component and a low frequency component and separately amplifying the high and low frequency components, wherein the high frequency component is amplified using a method according to claim 13.
15. A method according to Claim 14, further comprising combining the separately amplified signals to generate a cOmbined signal and providing a feedback based on the combined signal to an amplifier used for amplifying the low frequency component.
16. A method of modifying an envelope signal comprising a high frequency component and a low frequency component, the method comprising: providing the high and low frequency components to a class C or a class H amplifier and generating an amplified envelope signal using the amplifier; sensing an output signal of the amplifier and amplifying a low frequency component of the sensed output signal using a further amplifier; combining the amplified envelope signal with the amplified low frequency component to generate a combined signal; and providing a feedback to an input of the class G or class H amplifier based on at least pad of the combined signal.
17. An envelope modulator as hereinbefore described with reference to Figures 1,4 or 5.
18. A class C amplifier as hereinbefore described with reference to Figures 2, 3or6.