I 1111111111111111 11111 1111111111 11111 1111111111 1111111111111111 IIII IIII IIII
`US008947164B2
`
`c12) United States Patent
`Eplett
`
`(IO) Patent No.:
`(45) Date of Patent:
`
`US 8,947,164 B2
`Feb.3,2015
`
`(54)
`
`INTEGRATED TECHNIQUE FOR ENHANCED
`POWER AMPLIFIER FORWARD POWER
`DETECTION
`
`(56)
`
`References Cited
`
`U.S. PATENT DOCUMENTS
`
`(71)
`
`Applicant: Microsemi Corporation, Aliso Viejo,
`CA (US)
`
`(72)
`
`Inventor: Brian Eplett, Lilburn, GA (US)
`
`(73) Assignee: Microsemi Corporation, Aliso Viejo,
`CA (US)
`
`( *) Notice:
`
`Subject to any disclaimer, the term ofthis
`patent is extended or adjusted under 35
`U.S.C. 154(b) by 92 days.
`
`(21) Appl. No.: 13/894,221
`
`(22) Filed:
`
`May 14, 2013
`
`(65)
`
`Prior Publication Data
`
`US 2013/0307624 Al
`
`Nov. 21, 2013
`
`Related U.S. Application Data
`
`(60)
`
`Provisional application No. 61/648,721, filed on May
`18, 2012.
`
`(51)
`
`(52)
`
`(58)
`
`(2006.01)
`(2006.01)
`(2006.01)
`(2006.01)
`
`Int. Cl.
`H03G3/10
`H03G3/00
`H03F 1156
`H03F 3/19
`U.S. Cl.
`CPC ................. H03G 3/004 (2013.01); H03F 1156
`(2013.01); H03F 3/19 (2013.01)
`USPC ........................................... 330/278; 330/129
`Field of Classification Search
`USPC .................................. 330/278, 279, 129, 136
`See application file for complete search history.
`
`5,148,117 A
`5,977,831 A *
`7,777,566 Bl*
`2006/0222104 Al
`2011/0148519 Al*
`
`9/1992 Talwar
`11/1999 Davis et al .................... 330/279
`8/2010 Drogi et al. ................... 330/136
`10/2006 Braithwaite
`6/2011 Drogi et al. ................... 330/129
`
`FOREIGN PATENT DOCUMENTS
`
`EP
`
`0803974 Al
`
`10/1997
`
`OTHER PUBLICATIONS
`
`European Patent Office, ISA/EP, International Search Report/Writ(cid:173)
`ten Opinion, PCT/US2013/041202, Rijswijk, The Netherlands, Nov.
`29, 2013, 7 pages.
`
`* cited by examiner
`
`Primary Examiner - Henry Choe
`(74) Attorney, Agent, or Firm -Marger Johnson &
`McCollom, PC
`
`ABSTRACT
`(57)
`A power amplifier has power detection capabilities that
`include a radio frequency (RF) power amplifier that has a gain
`stage that includes a gain stage input, a gain stage output, and
`a feedback loop coupled between an input and an output of the
`power amplifier. A detection circuit has a first detection cir(cid:173)
`cuit input electrically coupled to the gain stage input and has
`a detection circuit output. An amplitude control circuit and a
`phase control circuit are electrically coupled together in
`series between the gain stage output and a second detection
`circuit input. The amplitude control circuit and the phase
`control circuit produce a signal that is received by the second
`detection circuit input so that the detection circuit can detect
`a signal at the detection circuit output that is proportional to a
`the forward power output of the power amplifier and is insen(cid:173)
`sitive to power amplifier output load mismatch.
`
`17 Claims, 10 Drawing Sheets
`
`604
`
`CHIP
`
`6 1 0 - - -~
`602-------.
`
`Detection
`
`614
`
`60
`
`8
`Power Amplifier 60'-
`- - - -~
`Feedback
`
`GAIN=-XV/V
`
`606
`
`Impedance
`Transformation
`
`Mismatch
`
`APPLE ET AL. EXHIBIT 1001
`
`1
`
`

`

`U.S. Patent
`U.S. Patent
`
`Feb.3,2015
`Feb. 3, 2015
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`2
`cuit input that is electrically coupled to the gain stage input
`and a detection circuit output. An amplitude control circuit
`and a phase control circuit are electrically coupled together in
`series between the gain stage output and a second detection
`5 circuit input. The amplitude control circuit and the phase
`control circuit produce a signal that is received by the second
`detection circuit input so that the detection circuit can detect
`a signal at the output of the detection circuit that has a power
`proportional to a forward power output of the power ampli-
`lO fier.
`A method of detecting forward power in a detection circuit
`that is coupled to a power amplifier is also disclosed. A first
`amplitude control circuit is coupled in series to an input of a
`gain stage of a power amplifier to produce a corrected input
`15 signal. A second amplitude control circuit and a phase control
`circuit are coupled in series to an output of the gain stage of
`the power amplifier to produce a corrected output signal. The
`corrected input signal and the corrected output signal are
`sUlllilled to produce a summed node signal that is propor-
`20 tional to the forward power output of the power amplifier. The
`sUlllilled node signal is applied to the detection circuit to
`detect the forward power output of the power amplifier.
`The foregoing and other objects, features and advantages
`of the invention will become more readily apparent from the
`25 following detailed description of embodiments of the inven(cid:173)
`tion which proceeds with reference to the accompanying
`drawings.
`
`BRIEF DESCRIPTION OF THE DRAWINGS
`
`FIG. 1 is a prior art power detection circuit for use with a
`power amplifier under matched power conditions.
`FIG. 2 is the prior art power detection circuit shown in FIG.
`1 under mismatched power conditions.
`FIG. 3 is a prior art integrated forward power detection
`circuit with an external directional coupler under mismatched
`conditions.
`FIG. 4 is a prior art integrated forward power detection
`circuit with a detection circuit that detects voltage at the
`output of the power amplifier under mismatched conditions.
`FIG. 5 is a prior art integrated forward power detection
`circuit with a detection circuit that detects voltage on the input
`of the final gain stage of the power amplifier under mis(cid:173)
`matched conditions.
`FIG. 6 is an integrated forward power detector in accor(cid:173)
`dance with aspects of the disclosure.
`FIG. 7 is another embodiment of an integrated forward
`power detector in accordance with aspects of the disclosure.
`FIG. 8 is an example s=ing detection circuit electrically
`coupled to a first amplitude and phase control circuit and a
`second amplitude and phase control circuit.
`FIG. 9 is an example resistor and capacitor tuning circuit
`for an amplitude and phase control circuit combination.
`FIG. 10 is a graphical representation ofVWSR insensitive
`55 forward power detection improvement when the power detec(cid:173)
`tion of the power amplifier is performed by the disclosed
`power detectors.
`
`Many power amplifiers are used in environments in which
`the amount of power of the transmitted signal must be within
`a specified range. For example, federal agencies like the Fed(cid:173)
`eral Communications Commission (FCC) restrict the amount
`of power permissible in a signal transmitted in wireless LAN
`communications. Power detection in circuits that include
`power amplifiers in such controlled environments is critical to
`ensuring that the power of the transmitted signals are com(cid:173)
`pliant with FCC regulations.
`Accurate power detection in power amplifiers can be cha!- 30
`lenging, especially when the load condition on the output of
`the power amplifier changes over time, such as when the user
`of a cell phone moves from outside of a building to inside of
`a building. The performance of the power amplifier changes
`with the new loading condition, and this performance change 35
`must be reliably detected. Existing solutions for detecting
`power in power amplifiers can rely on printed circuit board
`(PCB) level directional couplers that are large and costly.
`Other existing solutions rely on power detection at the output
`of the power amplifier, which produces a significant amount 40
`of variation for detecting the forward power. Still other exist(cid:173)
`ing solutions rely on power detection at the input of the final
`gain stage of the power amplifier, but such an arrangement
`suffers from a dependent relationship between the power
`amplifier design and the detector and requires the power 45
`amplifier design to consider the design parameters of the
`detector, which limits the power amplifier capabilities.
`Therefore, there is a need for improvements to power
`detection in power amplifiers that can be integrated in mono(cid:173)
`lithic solutions, such as a standard CMOS/BiCMOS or GaAs 50
`process, that can be independent of power amplifier design
`parameters and provide a degree of freedom from the perfor(cid:173)
`mance of the power amplifier without sacrificing the ability to
`accurately detect power in the output signal of the power
`amplifier.
`
`US 8,947,164 B2
`
`1
`INTEGRATED TECHNIQUE FOR ENHANCED
`POWER AMPLIFIER FORWARD POWER
`DETECTION
`
`CROSS REFERENCE TO RELATED
`APPLICATIONS
`
`This application claims the benefit of U.S. provisional
`patent application Ser. No. 61/648,721, filed May 18, 2012,
`incorporated by reference herein.
`
`FIELD OF THE INVENTION
`
`This disclosure relates to enhancing power amplifier per(cid:173)
`formance and more specifically to improving the forward
`power detection under variable loading conditions caused by
`the environment.
`
`BACKGROUND OF THE INVENTION
`
`SUMMARY OF THE INVENTION
`
`An object of this invention is to provide methods and
`device structures suitable for improving the forward power 60
`detection of a power amplifier.
`An exemplary power amplifier has power detection capa(cid:173)
`bilities. Such devices and methods can include a radio fre(cid:173)
`quency (RF) power amplifier that has a gain stage that
`includes a gain stage input, a gain stage output, and a feed-
`back loop coupled between an input and an output of the
`power amplifier. A detection circuit has a first detection cir-
`
`65
`
`DETAILED DESCRIPTION
`
`In the drawings, which are not necessarily to scale, like or
`corresponding elements of the disclosed systems and meth(cid:173)
`ods are denoted by the same reference numerals.
`To detect power in power amplifiers, such as those in the
`LX5586 and LX5588 Integrated Front End Modules manu(cid:173)
`factured by Microsemi Corporation®, the disclosed circuits
`and methods provide for an integrated power detection solu-
`
`12
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`US 8,947,164 B2
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`3
`tion that provides design freedom from the power amplifier.
`The disclosed power amplifiers with integrated power detec(cid:173)
`tors minimize power loss, have a flat frequency response,
`improved directivity, can be integrated alongside and inde(cid:173)
`pendently from the power amplifier, and are physically small
`to conserve die area. All of these features of the disclosed
`power amplifiers with integrated power detectors improve the
`size and cost of power detection for power amplifiers. The
`circuits and methods of the disclosure sample the input and
`output of the final stage of a power amplifier to accurately
`detect power in the signal transmitted from the power ampli(cid:173)
`fier.
`FIGS. 1-5 show prior art solutions to detecting power in
`power amplifiers. FIGS. 1 and 2 show a prior art power
`amplifier 100 with a directional coupler 102 that detects the
`power of the signal being transmitted from the power ampli(cid:173)
`fier 100 through the antenna 104 under matched and mis(cid:173)
`matched signal conditions, respectively. In the prior art power
`detector shown in FIGS. 1 and 2, the directional coupler 102
`and antenna 104 are located off of the integrated circuit die or
`chip 106. Under the matched conditions shown in FIG. 1, the
`voltage standing wave ratio (VSWR) is zero. Under the mis(cid:173)
`matched conditions shown in FIG. 2, the VSWR is greater
`than zero. Calculating the VSWR is a ratio of the maximum
`and minimum radio frequency (RF) voltage amplitude on the
`transmission path. The variation in voltage amplitude is
`caused by non-zero reflected power (P reJ· This reverse power
`is caused by load mismatch on the output of the power ampli(cid:173)
`fier, which reflects the forward power back toward the power
`amplifier.
`A matched signal is a signal that has forward power, P fad,
`108 that is equal to the power delivered to the load (i.e., the
`antenna that transmits the signal from the power amplifier).
`Under these conditions, the reflected power, P rev, 110 is zero.
`Forward power, P fwd' 108 is the power of the signal that is
`being transmitted from the power amplifier 100 through the
`antenna 104. Reflected power, Prev, 110 is the power that is
`being reflected due to load match from the antenna 104 back
`toward the amplifier. Reflected power, Prev, 110 is generated
`when a signal is returned by the antenna 104, which often
`occurs when signals are transmitted in areas in which signals
`are likely to reflect off of an object, such as a metal box,
`building, vehicle, or the like.
`FIG. 2 shows the prior art power detection solution in
`which the forward power, P fad, 108 is no longer equal to the
`power delivered to the load. Some amount of the power is
`reflected, depending on the degree of mismatch. Under these
`conditions, Prev longer zero. Such mismatched conditions
`cause voltage amplitude variation in the transmission path
`and makes the forward power, P fad, 108 no longer propor(cid:173)
`tional to the voltage amplitude of the power signal output
`from the power amplifier. This variation prevents accurate
`power detection in a voltage detection-based solution. Varia(cid:173)
`tion in the phase and magnitude of the mismatch makes
`accurately detecting the forward power, P fad, 108 difficult
`since integrated detection schemes are most easily realized in
`the voltage domain.
`In the presence of mismatch, the voltage signal at any given
`location in the system varies in phase as well as amplitude.
`The prior art detectors shown in FIGS. 1 and 2 rely on only 60
`sampling the output voltage, which makes this solution inher(cid:173)
`ently inaccurate because the forward power, P fad, 108 is no
`longer proportional to the voltage amplitude of the power of
`the output signal from the power amplifier.
`FIG. 3 shows a prior art power amplifier that implements an 65
`external on-chip directional coupler 302 that detects forward
`power, P fad, 304 output by the power amplifier 300 all on the
`
`4
`same chip 306. The amplitude and frequency response of the
`signal detected by the directional coupler 302 is a direct
`function of the size of the on-chip directional coupler 302. As
`the size of the directional coupler 302 grows, the loss of the
`5 structure increases and the area cost of the on-chip solution
`increases. The behavior of the directional coupler 302 is
`dictated by the wavelength of the signal, the directional cou(cid:173)
`pler 302 can experience a significant trade-off between fre(cid:173)
`quency response and size, which translates into loss. Further,
`10 this prior art on-chip directional coupler 302, is intimately
`tied to the behavior of the power amplifier 300, which com(cid:173)
`plicates the design of the power amplifier 300 and compro(cid:173)
`mises power amplifier performance for detector or directional
`15 coupler performance.
`FIG. 4 shows another prior art power amplifier 400 with a
`power detector 402 in which a directional coupler 402 is
`located off the chip 404 on which the power amplifier 400 is
`located. The power from the power amplifier 400 in the
`20 example shown in FIG. 4 is detected at the output of the power
`amplifier 400. As described above, the reflected power, Prev'
`408 interacts with the forward power, P fad, 410 such that the
`voltage amplitude at a given point between the power ampli(cid:173)
`fier 400 output and the mismatch causes the voltage ampli-
`25 tude to vary. Detection of the voltage on the power amplifier
`400 output produces a significant amount of variation for a
`constant forward power, Pfwd 410.
`FIG. 5 shows yet another prior art power amplifier 500 with
`a power detector 502 located on the same chip 504. The power
`30 detector 502 detects power, on-chip, at the input ofa final gain
`stage 506 of the power amplifier 500 without need for a
`directional coupler, as shown in the prior art examples
`described above in reference to FIGS. 1-4. The input of the
`final gain stage 506 of the power amplifier 500 is less sensitive
`35 to mismatched signals because transistors in the final stage of
`the power amplifier have a negative voltage gain and a finite
`reverse isolation so the impact ofVSWR due to mismatched
`signals can be diminished. However, a detector design that
`detects power at the input of the final gain stage of the power
`40 amplifier, such as the detector 502 shown in FIG. 5, suffers
`from a dependence upon the power amplifier design, a
`reduced ability to control the phase and amplitude mismatch
`that may be seen at the input of the final gain stage of the
`power amplifier, and a reduced ability to detect signals with
`45 relatively low power values.
`Turning now to FIGS. 6 and 7, two power amplifiers with
`power detectors are disclosed that detect power of a signal
`output from a power amplifier, in accordance with aspects of
`the disclosure. In FIG. 6, the power amplifier 600 and the
`50 detector 602 are both located on the same chip 604. The
`power amplifier 600 can be a radio frequency RF power
`amplifier with multiple gain stages, including a final gain
`stage 606. The final gain stage 606 of the power amplifier 600
`has an input and an output and a feedback loop 608 is coupled
`55 between the input and output of the power amplifier 600. The
`detector 602 includes a detection circuit 610 that has a detec-
`tion circuit input that is electrically coupled to the final gain
`stage 606 input of the power amplifier 600 and has a detection
`circuit output (840 in the FIG. 8 example). An amplitude
`control circuit 612 and a phase control circuit 614 that are
`electrically coupled between the final gain stage 606 output
`and a second detection circuit 610 input. The amplitude con(cid:173)
`trol circuit 612 and phase control circuit 614 are electrically
`coupled in series and their order can be reversed in other
`examples. The amplitude control circuit 612 and the phase
`control circuit 614 produce a signal that is received by the
`second detection circuit 610 input so that the detection circuit
`
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`US 8,947,164 B2
`
`5
`
`5
`610 produces an output signal that is proportional to the
`forward power output of the power amplifier 600.
`The detection circuit 610 of the power amplifier 600 shown
`in FIG. 6 can be independent of the power amplifier 600
`design parameters, which produces VSWR insensitive for-
`ward power detection in the power amplifier 600 output sig(cid:173)
`nal. The signal detected by the detection circuit 610 input on
`the input of the final gain stage 606 of the power amplifier 600
`and the output signal that has been corrected for both ampli(cid:173)
`tude and phase signal mismatch by the amplitude control 10
`circuit 612 and the phase control circuit 614 are summed in
`the detection circuit 610 independently of the performance
`characteristics inherent in the power amplifier 600. The
`detection circuit 610 operates separately from the power
`amplifier 600 in this example.
`FIG. 7 shows another embodiment of a power amplifier
`700 with a power detector 702 that detects power of a signal
`output from the power amplifier 700, in accordance with
`aspects of the disclosure. Similar to FIG. 6, the power ampli(cid:173)
`fier 700 and the detector 702 are both located on the same chip 20
`704. In this example, a final gain stage 706 of the power
`amplifier 700 has an input and an output and a feedback loop
`708 coupled between the input and output. The detector 702
`includes a detection circuit 710 that has a detection circuit
`input that is electrically coupled to a first phase control circuit 25
`712 and a first amplitude control circuit 714. The input to the
`first phase control circuit 712 and the first amplitude control
`circuit 714 is electrically coupled to the input of the final gain
`stage 706 of the power amplifier 700.
`The detection circuit 710 also includes a detection circuit 30
`output that is electrically coupled to a second phase control
`circuit 716 and a second amplitude control circuit 718. The
`output of the second phase control circuit 716 and the second
`amplitude control circuit 718 is electrically coupled to the
`output of the final gain stage 706 of the power amplifier 700.
`The order of both the first and second phase control 712, 716
`and amplitude control circuits 714, 718 can be reversed. In a
`similar manner described above with reference to FIG. 6, the
`signal produced by the first phase and amplitude control
`circuits 712, 714 and the signal produced by the second phase
`and amplitude control circuits 716, 718 are summed in the
`detection circuit 710 to produce a summed RF signal. The
`summed RF signal has a power proportional to a forward
`power output of the power amplifier 700.
`The phase control circuit 712 may be omitted while having
`the amplitude control circuit 714 remain electrically coupled
`to the final gain stage 706 input. The final gain stage of the
`power amplifier has an input and an output and a feedback
`loop coupled between the input and output, as described
`above. The detector includes a detection circuit that has a
`detection circuit input that is electrically coupled to a first
`amplitude control circuit. The input to the first amplitude
`control circuit is electrically coupled to the input of the final
`gain stage of the power amplifier. This detection circuit also
`includes a detection circuit output that is electrically coupled
`to a phase control circuit and a second amplitude control
`circuit that are electrically coupled together in series. The
`output of the phase control circuit and the second amplitude
`control circuit is electrically coupled to the output of the final
`gain stage of the power amplifier. The order of the phase and 60
`second amplitude control circuits can be reversed. The detec(cid:173)
`tion circuit is electrically coupled between a first amplitude
`control circuit and a combined second amplitude control cir(cid:173)
`cuit and a phase control circuit in this example. The gain stage
`output or the signal output by the power amplifier exhibits a 65
`VSWR that is greater than zero in any of the example power
`amplifiers and detectors described above.
`
`6
`Referring now to FIG. 8, an example summing detection
`circuit is disclosed that illustrates the detector shown in FIG.
`7. The summing detection circuit includes a first phase and
`amplitude control circuit 804, a second phase and amplitude
`control circuit 806, a summing node 808, and a detection
`circuit 810. The input to the first phase and amplitude control
`circuit 804 is electrically coupled to the input 812 of the final
`gain stage of the power amplifier. The output 814 of the
`second phase and amplitude control circuit is electrically
`coupled to the output of the final gain stage of the power
`amplifier. The first phase and amplitude control circuit 804
`includes a capacitor 816, a variable or selectable blocking
`capacitor 818, a transistor 820, and a current source 822. The
`second phase and amplitude control circuit 806 includes two
`15 capacitors 824, 826, a variable or selectable blocking capaci(cid:173)
`tor 828, and a programmable resistor 830. The output of the
`first phase and amplitude control circuit 804 is electrically
`coupled or summed together with the input to the second
`phase and amplitude control circuit 806 at the summing node
`808 of the detection circuit 810. FIG. 8 shows an example of
`a summing node 808, although summing the output of the first
`phase and amplitude control circuit 804 and the second phase
`and amplitude control circuit 806 can be accomplished in
`var10us manners.
`The detection circuit 702 shown in FIG. 8 includes a rec(cid:173)
`tifier 834, such as a diode or any other circuit element that can
`transform an RF signal into a DC voltage, a current source
`836, and a capacitor 838. The output 840 of the detection
`circuit 702 is proportional to the RF signal output of the
`power amplifier. The detection circuit creates a direct current
`(DC) voltage that is proportional to the amplitude of the RF
`signal at the summing node 808. The DC voltage produced by
`the detection circuit 702 is also proportional to the voltage
`associated with the power of the signal output from the power
`35 amplifier. Because the signal is now a low frequency signal,
`i.e., a DC signal, it can be accurately communicated to other
`elements of the RF transmission system.
`FIG. 9 shows an example of the second phase and ampli(cid:173)
`tude control circuit 806 shown in FIG. 8. The blocking
`40 capacitor 828 includes a series of three capacitors 932, 934,
`936 and respective switches that provide different capacitor
`values depending on which switches 938, 940, 942 are open
`and closed. The three capacitors 932, 934, 936 are electrically
`coupled together in parallel. The programmable resistor 832
`45 includes a series of three resistors 944, 946, 948 and respec(cid:173)
`tive switches 950, 952, 954, which can be gate-controlled
`FETs in some examples. When all of the resistor switches
`950, 952, 954 are open, the total resistance equals the sum of
`the values of all three resistors 944,946,948. When all of the
`50 resistor switches 950, 952, 954 are closed, the total resistance
`is the line resistance and the three resistors 944, 946, 948 add
`no resistance to the circuit. The resistance across the pro(cid:173)
`grammable resistor can be varied by opening and closing one
`or more of the switches 950,952,954, as desired. Because of
`55 the programmable nature of the phase and amplitude control
`circuit 806 shown in FIG. 9, the detector can be programmed
`in response to variations in power of an output signal of the
`power amplifier. For example, any of the phase and amplitude
`control circuits discussed above in reference to FIGS. 6-8 can
`have programmable components that can be programmed in
`response to variations in power of the output signal of the
`power amplifier.
`FIG. 10 shows graphical representations of the perfor(cid:173)
`mance improvement of forward power detection using the
`disclosed power amplifiers with power detectors. For each
`graph, the forward power is sampled along the X-axis and a
`constant voltage is plotted along the Y-axis. Each line repre-
`
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`US 8,947,164 B2
`
`7
`sents a different load condition with a 3:1 VSWR (the mag(cid:173)
`nitude of the mismatch) with a variable phase (60 degree
`steps). The first graph 1000 shows the final detector response
`to forward power and represents the amplitude of the RF
`signal input to the final gain stage of the power amplifier. The 5
`second graph 1002 represents the amplitude of the RF output
`voltage signal. The output voltage signal has much higher
`amplitude and greater variation for the same forward power
`than the amplitude and variation of the RF signal input to the
`final gain stage of the power amplifier.
`The first graph 1000 shows a constant RF amplitude of 125
`m V has nearly 1.9 dB of forward power variation, which
`represents forward power detection of a power amplifier
`without the disclosed detectors at 1004 and with the disclosed
`detectors at 1006. A detector solution that relies solely on
`detecting RF amplitude at the input of the final gain stage of
`the power amplifier, such as the detector shown in FIG. 5,
`cannot improve on this variation. The variation of the final
`detector output voltage, V DET[op is only 1.4 dB for the same
`RF amplitude when the disclosed detector is used with the
`power amplifier. Likewise, the second graph shows a constant
`RF output amplitude having an 8 dB forward pow