United States Patent
`[19]
`5,929,702
`[11] Patent Number:
`[45 J Date of Patent: Jul. 27, 1999
`Myers et al.
`
`
`
`[1800592970211
`
`[54] METHOD AND APPARATUS FOR HIGH
`EFFICIENCY HIGH DYNAMIC RANGE
`POWER AMPLIFICATION
`
`[75]
`
`Inventors: Ronald Gene Myers, Scottsdale;
`Kenneth Vern Buer, Gilbert, both of
`Ariz.; Frederick H. Raab, Burlington,
`Vt.
`
`[73] Assignee: Motorola, Inc., Schaumburg, Ill.
`
`[21] Appl. No.: 08/980,435
`
`[22]
`
`Filed:
`
`Nov. 28, 1997
`
`Int. Cl.6 ....................................................... H03G 3/20
`[51]
`[52] U.S. Cl.
`............................. 330/136; 330/10; 330/297
`[58] Field of Search .............................. 330/10, 136, 202,
`330/297; 332/149, 155
`
`[56]
`
`References Cited
`U.S. PATENT DOCUMENTS
`
`8/1980 Carver ..................................... 330/297
`4,218,660
`11/1983 Zeis ...............
`330/297 X
`4,417,358
`
`5/1989 Hudspeth et al.
`..... 330/10
`4,831,334
`
`..... 455/91
`..
`5,251,330 10/1993 Chiba et a1.
`
`7/1998 Simopoulos ..
`5,777,519
`330/297
`5,825,248 10/1998 Ozawa ................................ 330/297 X
`OTHER PUBLICATIONS
`
`entitled “HighiEfficiency Singleisideband
`An article
`HF/VHF Transmitter Based Upon Envelope Elimination
`And Restoration ” by RH. Raab and D]. Rupp, Green
`
`Mountain Radio Research Company, USA from IEEE
`CP392, UK pp. 21—25, Jul. 4—7, 1994.
`
`An article entitled “Class—S High—Efficiency Amplitude
`Modulator” by Dr. Frederick H. Raab and DJ. Rupp, Green
`Mountain Radio Research from GMRR TP93712RF Design,
`vol. 17, No. 5, pp. 70—74, May 1994.
`
`An article entitled “Single—Sideband Transmission By
`Envelope Elimination and Restoration” by Leonard R. Kahn
`from Proceedings Of The IRE, Jul. 1952.
`
`Primary Examiner—Robert Pascal
`Assistant Examiner—Ifllanh Van Nguyen
`Attorney, Agent, or Firm—Dana B. LeMoine
`
`[57]
`
`ABSTRACT
`
`A method and apparatus for efficient power amplification of
`a high dynamic range signal includes an envelope detector
`(220), a multi-range modulator (270), and a power amplifier
`(260). The multi-range modulator (270) efliciently amplifies
`the envelope of the input signal by selecting a power source
`as a function of the amplitude of the input signal. Multi-
`range modulator (200) produces a pulsewidth modulated
`signal with a duty cycle and an amplitude. When the
`amplitude of the input signal rises above a reference, the
`duty cycle and the amplitude are modified so as to keep the
`multi-range modulator in an operating region of high elli-
`ciency.
`
`12 Claims, 3 Drawing Sheets
`
`ENVELOPE
`DETECTOR
`
`220 RFIN
`
`POWER
`DIVIDER
`
`
`
`MULTI—RANGE
`MODULATOR
`
`270
`
`240
`
`
`
`LIMITER
`
`RFOUT
`
`260
`
`Vdd2
`
`
`
`LOW
`PASS
`FILTER
`
`ENVELOPE
`INPUT
`
`INTEL 1212
`
`INTEL 1212
`
`

`

`US. Patent
`
`Jul. 27, 1999
`
`Sheet 1 0f3
`
`5,929,702
`
`ENVELOPE
`DETECTOR
`
`220
`
`
`
`210
`
`RFIN
`
`
`
`POWER
`DIVIDER
`
`MULTI—RANGE
`MODULATOR
`
` 240
`
`270
`
`
`
`
`
`LIMITER
`
`R%UT
`
`260
`
`274
`
`278
`
`Vdd2
`
`ENVELOPE
`INPUT
`
`VREF
`
`ENVELOPE
`
`FIG. 2
`
`
`
`
`310
`
`Vdd1
`
`320
`
`PWM } 325
`
`315
`
`340
`
`LOW
`
`PASS
`FILTER
`
`éfigg
`
`'
`
`290
`
`

`

`US. Patent
`
`Jul. 27, 1999
`
`Sheet 2 0f3
`
`5,929,702
`
`430
`r—H
`
`420
`r—&-\
`
`Vdd2
`
`Vdd1
`
`410"
`
`FIG. 4
`
`220
`
`270
`
`IFIN
`
`POWER
`DIVIDER
`
`ENVELOPE
`DETECTOR
`
`MULTI—RANGE
`MODULATOR
`
`
`
`
`
`
`
`
`RFOUT
`
`L0
`
`FIG. 6
`
`AMPLIFIER
`
`SIGNAL
`INPUT
`
`
`
`
`
`COMMUNICATION
`DEVICE
`
`600
`
`

`

`US. Patent
`
`Jul. 27, 1999
`
`Sheet 3 0f3
`
`5,929,702
`
`START
`
`
`
`
`
`
`COMPARE AN INPUT SIGNAL TO A REFERENCE
`
`710
`
`SELECT AT LEAST ONE OF A PLURALITY OF
`POWER SOURCES FOR USE IN AN AMPLIFIER
`
`
`AMPLIFY THE INPUT SIGNAL WITH THE
`AMPLIFIER USING THE POWER SOURCE(S) SELECTED
`
`
`
`
`730
`
`M
`
`
`
`FIG- 7
`
`
`
`
`
`FIG 8
`
`
`
`COMPARE AN INPUT
`SIGNAL TO A REFERENCE
`
`INPUT > REFERENCE
`
`
`
`
`
`
`
`
`SELECT A SECOND POWER
`SOURCE FOR USE IN A
`PULSEWIDTH MODULATOR
`
`WHERE THE SECOND POWER
`SOURCE HAS A VOLTAGE
`GREATER THAN THE FIRST
`
`POWER SOURCE
`
`
`SELECT A FIRST POWER
`SOURCE FOR USE IN A
`PULSEWIDTH MODULATOR
`
`
`
`CREATE A PULSEWIDTH
`MODULATED SIGNAL USING
`
`
`THE SELECTED POWER SOURCE
`850
`
`
`
`FILTER THE
`PULSEWIDTH
`
`
`MODULATED SIGNAL
`850
`
`
`
`

`

`5,929,702
`
`1
`METHOD AND APPARATUS FOR HIGH
`EFFICIENCY HIGH DYNAMIC RANGE
`POWER AMPLIFICATION
`FIELD OF THE INVENTION
`
`This invention relates in general to power amplifiers and,
`in particular, to high efficiency, high dynamic range power
`amplifiers.
`BACKGROUND OF THE INVENTION
`
`10
`
`Various apparatus are available for amplifying signals. In
`amplifier applications that involve the amplification and
`transmission of modulated signals, a premium is placed on
`amplifier efficiency. In addition, because many applications
`require a high dynamic range, a premium is placed on the
`ability to efficiently create a high fidelity reproduction of a
`signal with a widely varying amplitude.
`Communications devices, which often transmit high
`dynamic range signals, are an example of an application
`where these qualities are in demand. High dynamic range ,
`allows the communications devices to communicate more
`
`15
`
`reliably over a variety of distances, and high efficiency
`allows the devices to operate longer on a single battery.
`One method of achieving increased efficiency is to use
`envelope elimination and restoration (_EER)-type amplifiers.
`EER is a technique through which highly efficient but
`nonlinear radio frequency (RF) power amplifiers can be
`combined with other, highly efficient amplifiers to produce
`a high efficiency linear amplifier system. The signal to be
`amplified is split into two paths: an amplitude path, and a
`phase path. The detected envelope is amplified efficiently in
`the amplitude path by a class S or other highly efficient
`power amplifier which operates on the bandwidth of the RF
`envelope rather than the RF bandwidth. The phase modu-
`lated carrier in the phase path is then amplitude modulated
`by the amplified envelope signal, creating an amplified
`replica of the input signal.
`In EER-type amplifiers the dynamic range is limited in
`part by the dynamic range of the class S modulator used to
`amplify the envelope. The class S modulator is a pulsewidth
`modulated system, so the maximum signal level that can be
`output is limited by the maximum duty cycle of the pulse—
`width modulator. In addition, the dynamic range is limited
`by the minimum duty cycle. It would be desirable to have a
`method and apparatus which increases the dynamic range
`beyond that possible with a class S modulator.
`In addition, the efficiency of class S modulators decreases
`for small signal levels. In class S modulators, a drive power
`is necessary to operate power devices that create an ampli-
`fied pulsewidth modulated signal, and when the duty cycle
`is small, the drive power is a large percentage of the total
`power consumed. It would be desirable to have a method
`and apparatus for increasing the efficiency of a pulsewidth
`modulated system for small signal levels.
`Accordingly, a need exists for a power amplifier that
`efficiently amplifies a RF signal exhibiting a high dynamic
`range.
`
`BRIEF DESCRIPTION OF THE DRAWINGS
`
`FIG. 1 shows a diagram of an amplifier in accordance
`with a preferred embodiment of the present invention;
`FIG. 2 shows a diagram of a multi-range modulator in
`accordance with a preferred embodiment of the present
`invention;
`FIG. 3 shows a diagram of a multi-range modulator in
`accordance with an alternate embodiment of the present
`invention;
`
`40
`
`45
`
`60
`
`65
`
`2
`FIG. 4 shows a switching waveform in accordance with a
`preferred embodiment of the present invention;
`FIG. 5 shows an amplifier in accordance with an alternate
`embodiment of the present invention;
`FIG. 6 shows a diagram of a communications device in
`accordance with a preferred embodiment of the present
`invention;
`FIG. 7 shows a flow chart for a method of amplifying a
`signal in accordance with a preferred embodiment of the
`present invention; and
`FIG. 8 shows a flow chart for a method of amplifying a
`signal in accordance with a preferred embodiment of the
`present invention.
`DETAILED DESCRIPTION OF THE DRAWINGS
`
`FIG. 1 shows a diagram of an amplifier in accordance
`with a preferred embodiment of the present
`invention.
`EER-type amplifier 10 includes power divider 210, envelope
`detector 220, multi-range modulator 270, time delay element
`230, limiter 240, and power amplifier 260. EER-typc ampli-
`fier 10 receives an RF input into power divider 210. Power
`divider 210 splits the RF input signal into an amplitude path
`which feeds envelope detector 220, and a phase path which
`feeds time delay element 230.
`The phase path of EER-type amplifier 10 includes time
`delay element 230, limiter 240 and power amplifier 260.
`Time delay element 230, which produces a delay equal to
`that introduced by multi-range modulator 270 in the ampli-
`tude path, receives an output from power divider 210.
`Limiter 240 receives the time delayed signal output from
`time delay element 230, and amplitude limits the signal.
`Limiter 240 can be omitted, or it can perform soft limiting,
`but limiter 240 preferably performs hard limiting so that the
`output of limiter 240 includes phase information with little
`or no amplitude information. After limiting, with the ampli—
`tude information removed, the resultant signal is the phase
`modulated carrier. The phase modulated carrier output from
`limiter 240 is input to power amplifier 260. Power amplifier
`260 is any amplifier stage capable of being modulated, and
`it is preferably a field eifect transistor (FET) amplifier. The
`drain of the FET is conventionally connected to a DC power
`source; however, as will be discussed below, in a preferred
`embodiment exemplified herein,
`the drain of the FET is
`driven with a signal, resulting in an amplitude modulated
`output signal.
`In a preferred embodiment, time delay element 230 is
`used in the phase path because it is desirable to recombine
`the signals from the amplitude path and the phase path after
`each has been subjected to substantially equal delays. The
`absolute delay of time delay element 230 is such that the
`total delay in the phase path is substantially equal to the total
`delay in the amplitude path. Time delay element 230 is
`shown as the first element in the phase path; however, the
`actual placement of time delay element 230 within the phase
`path is not a limitation of the present invention. Because the
`function of time delay element 230 is to balance the delays
`in the phase path and the amplitude path, the actual position
`of time delay element 230 in the phase path is not important.
`Alternate embodiments of the present invention substan-
`tially match the delay in the two paths using circuit arrange-
`ments other than the one using time delay element 230
`alone, In a first alternate embodiment, multiple delay lines,
`one in each of the phase path and amplitude path are used.
`In this case, the absolute delay of any one delay line assumes
`less importance, and the differential delay between the two
`delay lines is used to match the delays in the two paths. In
`
`

`

`5,929,702
`
`3
`another alternate embodiment, a differential delay line, such
`as a surface acoustic wave (SAW) delay line, with one input
`and multiple outputs is used as a combination of power
`divider 210 and time delay element 230. In this alternate
`embodiment, as in the first alternate embodiment, the dif-
`ferential delay is used to match the delay in the two paths.
`The amplitude path of EER-type amplifier 10 includes
`envelope detector 220 and multi-range modulator 270.
`Envelope detector 220 detects the envelope of the RF input
`signal and outputs an envelope signal which represents the
`amplitude information included in the original RF input
`signal. Envelope detector 220 is preferably a diode detector;
`however, other types of detectors, such as a synchronous
`detector based upon a double balanced mixer, could be used.
`Multi-range modulator 270 amplifies the envelope signal
`output from envelope detector 220 and drives the drain bias
`of power amplifier 260. Multi-range modulator 270 ampli-
`fies the envelope signal to a level commensurate with the
`desired output. The output of multi-range modulator 270 is
`the power supply for power amplifier 260, and the resultant
`remodulation of the phase modulated carrier restores the
`envelope, producing an amplified replica of the input signal.
`Multi-range modulator 270 is an amplifier that uses at
`least one of multiple power sources. The choice as to which
`power source to use is made as a function of the amplitude
`of the envelope signal. By selecting the appropriate power
`source as a function of input signal
`level, multi-range
`modulator 270 operates with a higher average efficiency.
`The EER-type amplifier of FIG. 1 varies the drain bias of
`power amplifier 260 in such a way as to maintain operation
`near saturation and therefore in a region of high efliciency.
`Because the highly efficient power amplifier 260 consumes
`the majority of the power consumed in EER-type amplifier
`10,
`the entire circuit is considerably more efficient
`than
`conventional amplifiers.
`FIG. 2 shows a diagram of a multi-range modulator in
`accordance with a preferred embodiment of the present
`invention. Multi-range modulator 200 includes pulsewidth
`modulator (PWM) 272, reference comparator 274, logic
`gates 278 and 280, driver 276, switching transistors 282,
`284, and 286, and low pass filter 290. An envelope signal is
`input to multi-range modulator 200 at PWM 272 and refer-
`ence comparator 274. Reference comparator 274 can be one
`of many devices capable of comparing two signals, but is
`preferably an operational amplifier.
`The operation of multi-range modulator 200 differs based
`on whether the amplitude of the envelope signal is below or
`above the reference voltage shown as Vref in FIG. 2. When
`the amplitude of the envelope signal is below Vref, multi-
`range modulator 200 operates in a first mode. When the
`amplitude of the envelope signal is above Vref, multi-range
`modulator 200 operates in a second mode. Multi-range
`modulator 200 changes from the first mode of operation to
`the second as the amplitude of the envelope signal increases,
`and changes from the second mode of operation to the first
`as the amplitude of the envelope signal decreases. The first
`mode of operation of multi-range modulator 200 will now be
`described.
`
`When the envelope signal is below the reference voltage,
`the output of reference comparator 274 is low. With the
`output of reference comparator 274 in a low state, logic gate
`278 is disabled and logic gate 280 is enabled. PWM 272
`outputs a pulsewidth modulated waveform which has a duty
`cycle proportional to the amplitude of the envelope signal.
`Driver 276 accepts the pulsewidth modulated signal from
`PWM 272, and drives switching transistor 282 and logic
`
`10
`
`15
`
`40
`
`45
`
`60
`
`65
`
`4
`gates 278 and 280. In the first mode of operation, logic gate
`278 is disabled because the output of reference comparator
`274 is low. As a result, switching transistor 286 does not
`come on. Instead, switching transistors 284 and 282 alter-
`nately turn on as a function of the duty cycle of the
`pulsewidth modulated signal.
`When the pulsewidth modulated signal is high, switching
`transistor 284 is on, thereby presenting a voltage substan-
`tially equal to Vdd1 at low pass filter 290. Conversely, when
`the pulsewidth modulated signal is low, switching transistor
`284 is off and switching transistor 282 is on. This discharges
`the node at the input to low pass filter 290, thereby bringing
`it close to ground potential. Low pass filter 290 filters the
`resulting amplified pulsewidth modulated signal, suppress—
`ing the switching frequency of the pulsewidth modulator,
`and producing an amplified replica of the envelope signal.
`The operation of multi—range modulator 200 described thus
`far is that of a class S modulator with a power source of
`Vddl. This mode of operation will be maintained so long as
`the envelope signal has an amplitude less than Vref.
`When the amplitude of the envelope signal increases
`beyond Vref, multi-range modulator 200 transitions from the
`first mode of operation to the second mode of operation. The
`output of reference comparator 274 changes state and
`becomes high. The high output of reference comparator 274
`disables logic gate 280 and enables logic gate 278. With the
`output of reference comparator 274 high, switching transis-
`tors 286 and 282 alternately turn on as a function of the duty
`cycle of the pulsewidth modulated signal. When the pulse-
`width modulated signal is high, switching transistor 286
`turns on and supplies a voltage substantially equal to Vdd2
`to the input of low pass filter 290. This is in contrast to the
`first mode of operation where switching transistor 284 came
`on and supplied a voltage substantially equal to Vddl to the
`input of low pass filter 290. Vddl and Vdd2 can have any
`relationship, but for exemplary purposes, Vdd2 is assumed
`to be twice the voltage of Vdd1. Vref is preferably deter-
`mined so that the operation of multi-range modulator 200
`will switch from Vddl to Vdd2 as the pulsewidth modulated
`signal approaches a duty cycle of one hundred percent. The
`output of reference comparator 274 is input to PWM 272 so
`that PWM 272 receives an indication when operation
`switches from Vddl to Vdd2.
`
`Switching transistor 286 has a higher current handling
`requirement than switching transistor 284 because switching
`transistor 286 is connected to a larger power supply, namely
`Vdd2.
`In general,
`transistors that have a higher current
`handling capability have larger physical dimensions, and the
`larger physical dimensions, in turn, cause increased capaci-
`tance at the driving input. The increased capacitance is seen
`by the driving device, which causes more power to be
`dissipated as the transistor is turned on off. For example,
`gate 278 has to deliver more current to turn on transistor 286
`than gate 280 has to deliver to turn on transistor 284. By
`switching operation from switching transistor 286 to switch-
`ing transistor 284 for small signal levels, drive power is
`saved and overall efficiency improves.
`Multi-range modulator 200 includes a single switching
`transistor coupled to ground for discharging the input to low
`pass filter 290. Switching transistor 282 is selected to handle
`the largest foreseeable current, and so is sized in accordance
`with the current handling requirements of switching tran-
`sistor 286. In an alternate embodiment, multiple switching
`transistors couple the input of low pass filter 290 to ground,
`and the transistors are sized to handle the different amounts
`of current as required by the available supply voltages. In
`operation, the additional switching transistors are selected
`
`

`

`5,929,702
`
`10
`
`15
`
`5
`by the use of gates analogous to gates 278 and 280, and so
`the drive power necessary to operate the transistors is
`reduced based on input signal level. In the alternate embodi-
`ment just described, the efficiency is further improved at low
`signal levels.
`Multi-range modulator 200 has been described with two
`voltages, Vddl and Vdd2, and a single reference comparator
`274 and an associated reference voltage. This yields two
`modes of operation and two corresponding operating ranges
`for multi-range modulator 200, namely those set by Vddl
`and Vdd2.
`In an alternate embodiment, more than two
`ranges are contemplated. Multi-range modulator 200 can be
`scaled to use any number of supply voltage values beyond
`those of Vddl and Vdd2 as shown in FIG. 2. In support of
`the additional ranges, multiple reference comparators and
`logic gates are contemplated.
`Multi-range modulator 200 has a delay associated with its
`operation as a whole. When multi-range modulator 200 is
`used in an EER-type amplifier to amplify the envelope of a
`signal, the delay of multi—range modulator 200 is compen— .
`sated for by time delay means in the EER-type amplifier. For
`example, in the EER-type amplifier embodiment of FIG. 1,
`the delay of multi—range modulator 200 is compensated for
`by time delay element 230.
`FIG. 3 shows a diagram of a multi-range modulator in ,
`accordance with a preferred embodiment of the present
`invention. Multi-range modulator 300 includes reference
`comparator 310, switch 315, pulsewidth modulators (PWM)
`320 and 340, drivers 325 and 345, switching transistors 330,
`335, 350, and 355, and low pass filter 290. An envelope
`signal is input to multi-range modulator 300 at switch 315
`and reference comparator 310. Reference comparator 310
`can be one of many devices capable of comparing two
`signals, but
`is preferably an operational amplifier. The
`operation of multi-range modulator 300 differs based on
`whether the amplitude of the envelope signal is below or
`above the reference voltage shown as Vref in FIG. 3. When
`the amplitude of the envelope signal is below Vref, multi-
`range modulator 300 operates in a first mode. When the
`amplitude of the envelope signal is above Vref, multi-range
`modulator 300 operates in a second mode. Multi-range
`modulator 300 changes from the first mode of operation to
`the second as the amplitude of the envelope signal increases,
`and changes from the second mode of operation to the first
`as the amplitude of the envelope signal decreases. The first
`mode of operation of multi-range modulator 300 will now be
`described.
`
`40
`
`45
`
`When the envelope signal is below the reference voltage,
`the output of reference comparator 310 is low. With the
`output of reference comparator 310 in a low state, switch
`315 routes the envelope signal to PWM 320. PWM 320
`outputs a pulsewidth modulated waveform which has a duty
`cycle proportional to the amplitude of the envelope signal.
`Driver 325 accepts the pulsewidth modulated signal from
`PWM 320, and drives switching transistors 330 and 335.
`Switching transistors 330 and 335 alternately turn on as a
`function of the duty cycle of the pulsewidth modulated
`signal. When the pulsewidth [modulated signal
`is high,
`switching transistor 330 is on, thereby presenting a voltage
`substantially equal
`to Vddl at
`low pass filter 290.
`Conversely, when the pulsewidth modulated signal is low,
`switching transistor 330 is off and switching transistor 335
`is on. This discharges the node at the input to low pass filter
`290, thereby bringing it close to ground potential. Low pass
`filter 290 filters the resulting amplified pulsewidth modu-
`lated signal, suppressing the switching frequency of the
`pulsewidth modulator, and producing an amplified replica of
`
`60
`
`65
`
`6
`the envelope signal. The operation of multi-range modulator
`300 described thus far is that of a class S modulator with a
`power source of Vddl. This mode of operation will be
`maintained so long as the envelope signal has an amplitude
`less than Vref.
`
`When the amplitude of the envelope signal increases
`beyond Vref, multi-range modulator 300 transitions from the
`first mode of operation to the second mode of operation. The
`output of reference comparator 310 changes state and
`becomes high. The high output of reference comparator 310
`causes switch 315 to route the envelope signal to PWM 340
`instead of PWM 320. PWM 340 outputs a pulsewidth
`modulated waveform which has a duty cycle proportional to
`the amplitude of the envelope signal, where the constant of
`proportionality is smaller than that of PWM 320. The
`constants of proportionality for PWM 320 and PWM 340 are
`related to the ratio of the supply voltages, Vddl and Vdd2,
`as is explained in further detail below.
`Driver 345 accepts the pulsewidth modulated signal from
`PWM 340, and drives switching transistors 350 and 355.
`Switching transistors 350 and 355 alternately turn on as a
`function of the duty cycle of the pulsewidth modulated
`signal. When the pulsewidth modulated signal
`is high,
`switching transistor 350 is on, thereby presenting a voltage
`substantially equal
`to Vdd2 at
`low pass filter 290.
`Conversely, when the pulsewidth modulated signal is low,
`switching transistor 350 is off and switching transistor 355
`is on. This discharges the node at the input to low pass filter
`290, thereby bringing it close to ground potential. Low pass
`filter 290 filters the resulting amplified pulsewidth modu-
`lated signal, suppressing the switching frequency of the
`pulsewidth modulator, and producing an amplified replica of
`the envelope signal.
`The second mode of operation just described is in contrast
`to the first mode of operation where switching transistor 330
`came on and supplied a voltage substantially equal to Vddl
`to the input of low pass filter 290. Vddl and Vdd2 can have
`any relationship, but for exemplary purposes, Vdd2 is
`assumed to be twice the voltage of Vddl. Vref is preferably
`determined so that the operation multi-range modulator 300
`will switch from Vddl to Vdd2 as the pulsewidth modulated
`signal approaches a duty cycle of one hundred percent. The
`switch from the first mode to the second mode is described
`more fully with reference to the next figure.
`Switch 315 functions to select a pulsewidth modulator for
`operation based on the amplitude of the envelope signal. In
`an alternate embodiment,
`the function of switch 315 is
`embedded in each pulsewidth modulator.
`In this
`embodiment, the envelope signal is routed to both pulse-
`width modulators. The output of reference comparator 310
`is also routed to both pulsewidth modulators. The output of
`reference comparator 310 selects which pulsewidth modu-
`lator is used. When it is low, PWM 320 is on, and when it
`is high, PWM 340 is on. Other methods of selecting a
`pulsewidth modulator for operation are contemplated,
`including each pulsewidth modulator receiving a separate
`reference voltage for internal comparison purposes.
`Multi-range modulator 300 has been described with two
`voltages, Vddl and Vdd2, and a single reference comparator
`310 and an associated reference voltage. This yields two
`modes of operation and two corresponding operating ranges
`for multi-range modulator 300, namely those set by Vddl
`and Vdd2.
`In an alternate embodiment, more than two
`ranges are contemplated. Multi-range modulator 300 can be
`scaled to use any number of supply voltage values beyond
`those of Vddl and Vdd2 as shown in FIG. 3. In support of
`
`

`

`5,929,702
`
`10
`
`15
`
`7
`the additional ranges, multiple reference comparators, pulse-
`width modulators with different constants of proportionality,
`drivers, and switching transistors are contemplated.
`Multi-range modulators 200 (FIG. 2) and 300 have been
`described as two separate embodiments; however, one
`skilled in the art will appreciate that multiple alternate
`embodiments are derived when combining the specific
`embodiments described. For example, a single pulsewidth
`modulator can be used in multi-range modulator 300 along
`with logic gates to select which pair of switching transistors
`to drive.
`
`Multi-range modulator 300 has two separate pairs of
`switching transistors, one pair for each supply, whereas
`multi-range modulator 200 (FIG. 2) has separate switching
`transistors coupled to each supply, but shares a common
`switching transistor coupled to ground. As previously stated,
`multi-range modulator 200 (FIG. 2) is efficient at low signal
`levels in part because of the advantageous sizing of the
`switching transistors. Multi-range modulator 300, by not
`sharing a switching transistor coupled to ground, and by
`having two separate pairs of switching transistors, further
`increases efficiency. In multi-range modulator 300 both
`transistors of a given pair are sized such that they require the
`minimum amount of current to turn on.
`
`FIG. 4 shows a switching waveform in accordance with a
`preferred embodiment of the present
`invention. FIG. 4
`shows switching waveform 410 which includes regions 420,
`430 and 440. Switching waveform 410 is a pulsewidth
`modulated signal produced in response to an envelope signal
`which is increasing in amplitude. Switching waveform 410
`represents an exemplary signal at the input to low pass filter
`290 (FIGS. 2 and 3). The remaining discussion of FIG. 4
`references multi-range modulator 200 (FIG. 2), but it will be
`appreciated that the discussion of waveform 410 is also
`applicable to other multi-range modulator embodiments,
`including that of multi—range modulator 300 as shown in
`FIG. 3.
`
`In switching waveform 410, as the duty cycle increases to
`near one hundred percent, the finite rise and fall times of the
`waveform begin to overlap as shown in regions 420 and 430.
`In region 420 the rise and fall times of the waveforms do not
`yet overlap, but in region 430 the duty cycle has increased
`to the point where overlap is beginning. If the duty cycle
`were allowed to increase beyond this point, then the rise and
`fall times would increasingly overlap causing distortion at
`the output. This causes nonlinearities at high signal output
`levels, and reduces the useable dynamic range of the modu-
`lator.
`
`40
`
`45
`
`As the duty cycle approaches one hundred percent, as
`shown by region 430, it is desirable to increase the ampli-
`tude of the pulsewidth modulated waveform so that the duty
`cycle can be reduced. In a preferred embodiment, as the duty
`cycle approaches one hundred percent, the amplitude of the
`envelope signal
`increases beyond that of Vref, causing
`reference comparator 274 (FIG. 1) to change state. When
`this occurs, multi-range modulator 200 (FIG. 2) switches
`from the first range to the second range. This is shown in
`FIG. 4 at region 430. In the second range, the input to the
`low pass filter switches between ground and Vdd2 instead of
`between ground and Vddl. In addition, PWM 272 (FIG. 2)
`cuts the duty cycle in half. The duty cycle is cut in half
`because in the exemplary multi-range modulator set forth in
`FIG. 2, Vdd2 is twice Vdd1.
`In the exemplary embodiment of FIG. 2, when the input
`signal is below the reference, a pulsewidth modulated signal
`is produced that has an amplitude Vdd1 and a duty cycle kA,
`
`60
`
`65
`
`8
`where k is a proportionality constant and Ais the amplitude
`of the input signal. When the input signal is above the
`reference, a pulsewidth modulated signal is produced that
`has an amplitude Vdd2 and a duty cycle jA, where j
`is a
`constant of proportionality less than k. More simply stated,
`when the amplitude of the input signal increases beyond a
`reference, the duty cycle of the pulsewidth modulated signal
`decreases and the amplitude of the pulsewidth modulated
`signal increases.
`In a switching modulator exemplified by multi-range
`modulator 200 (FIG. 2), efficiency is generally greater for
`larger duty cycles. It is desirable,
`therefore,
`to maintain
`operation with as high a duty cycle as possible. By adding
`additional ranges, switching waveforms can be maintained
`at consistently large duty cycles. Additional ranges are
`added by adding additional
`reference comparators,
`transistors, and logic gates. Operating voltages can be cho-
`sen so that when a reference voltage is surpassed, the duty
`cycle will decrease not by a factor of two as shown in
`switching waveform 410, but by some lesser amount. This
`provides the dual advantage of increasing the dynamic range
`and increasing the efficiency.
`FIG. 5 shows an amplifier in accordance with an alternate
`embodiment of the present invention. In FIG. 5 an interme-
`diate frequency (IF) signal is shown as the input signal to
`EER-type amplifier 20. The IF signal is input into power
`divider 210. Power divider 210 functions to split the input
`signal
`into the amplitude path and the phase path. The
`amplitude path feeds envelope detector 220, and the phase
`path feeds time delay element 230.
`The amplitude path of EER-type amplifier 20 includes
`envelope detector 220 and multi-range modulator 270.
`These elements correspond to the elements of FIG. 1 which
`have like names and like reference numbers. In addition,
`multi-range modulator 270 corresponds to multi-range
`modulator 270 of FIG. 1, embodiments of which were
`discussed in detail previously in connection with FIGS. 2—4.
`The phase path of EER-type amplifier 20 includes time
`delay element 230, limiter 240, frequency converter 250 and
`power amplifier 260. Time delay element 230, limiter 240
`and power amplifier 260 correspond to the elements shown
`in FIG. 1 with like names and like reference numbers. In
`contrast to the embodiment shown in FIG. 1, the alternate
`embodiment of FIG. 5 includes frequency converter 250 in
`the phase path. Frequency converter 250 receives the signal
`in the phase path and also receives a local oscillator (LO)
`signal. Frequency converter 250 converts the frequency of
`the carrier signal to its final RF frequency using circuits well
`known in the art, such as a mixer. The resulting signal is then
`used to drive power amplifier 260 which operates at the final
`RF frequency.
`Because of the operation of frequency converter 250, the
`amplifier of FIG. 5 takes in a signal at a frequency different
`from the final RF frequency. FIG. 5 shows an IF signal input
`to EER-type amplifier 20. The IF input signal can be above
`or below the resultant RF frequency. In addition, one skilled
`in the art will understand that a baseband signal could also
`be us