Three Balun Designs for Push-Pull Amplifiers 1 RF Application Information Freescale Semiconductor...

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  • AN1034

    1RF Application InformationFreescale Semiconductor

    Three Balun Designs for Push-PullAmplifiersSingle RF power transistors seldom satisfy todays design

    criteria; several devices in separate packages1, or in the samepackage (balanced, push-pull or dual transistors), must becoupled to obtain the required amplifier output power. Sincehigh power transistors have very low impedance, designersare challenged to match combined devices to a load. Theyoften choose the push-pull technique because it allows theinput and output impedances of transistors to be connected inseries for RF operation.Balun-transformers provide the key to push-pull design, but

    they have not been as conspicuous in microwave circuits asat lower frequencies. Ferrite baluns2 have been applied up to30 MHz; others incorporating coaxial transmission linesoperate in the 30 to 400 MHz range3.The success of these two balun types should prompt the

    microwave designer to ask if balun-transformers can beincluded in circuits for frequencies above 400 MHz. Theoryand experimental results lead to the emphatic answer: yes!Not only will baluns function at microwave frequencies, but aspecial balun can be designed in microstrip form that avoidsthe inherent connection problems of coax.On the next six pages, you will observe the development of

    three balun-transformers culminating with the microstripversion. None of the baluns was tuned nor were the parasiticelements compensated. In this way, the deviation of theexperimental baluns from their theoretical performance couldbe evaluated more easily. The frequency limitations imposedby the parasitic elements also were observed more clearly.1. A balun transforms a balanced system that is

    symmetrical (with respect to ground) to an unbalancedsystem with one side grounded. Without balun-transformers,the minimum device impedance (real) that can be matched to

    50 ohms with acceptable bandwidth and loss is approximately0.5 ohms. The key to increasing the transistors output poweris reducing this impedance ratio. Although 3 dB hybridcombiners can double themaximum power output, they lowerthematching ratio to only 50:1. Balun transformers can reducethe original 100:1 ratio to 6.25:1 or less. The design offersother advantages: the baluns and associated matchingcircuits have greater bandwidth, lower losses, and reducedeven-harmonic levels.

    A 500-W Push-Pull Amplifier for DME Band

    180

    OUTPUTMATCHINGNETWORK

    50

    50180

    OUTPUT BALUN(BALANCE-UNBALANCE)

    TRAN-SISTORS

    INPUTMATCHINGNETWORK

    INPUT BALUN(UNBALANCE-BALANCE)

    00

    Figure 1.

    AN1034Rev. 0, 7/1993

    Freescale SemiconductorApplication Note

    Freescale Semiconductor, Inc., 1993, 2009. All rights reserved.

  • 2RF Application InformationFreescale Semiconductor

    AN1034

    2. Baluns are not free of disadvantages.Coupling a pairof push-pull amplifiers with 3-dB hybrids avoids (forfour-transistor circuits) one of these: the higher broadbandVSWRs of balun-transformers. A second disadvantage, thelack of isolation between the two transistors in each push-pullconfiguration, is outweighed by the advantages of the balundesign in reducing the critical impedance ratio.3. In this simple balun that uses a coaxial transmission

    line, the grounded outer conductor makes an unbalancedtermination, and the floating end makes a balancedtermination. Charge conservation requires that the currentson the center and the outer conductors maintain equalmagnitudes and a 180-degree phase relationship at any pointalong the line. By properly choosing the length andcharacteristic impedance, this balun can be designed tomatch devices to their loads. In the case shown, if A = 90degrees, the matching condition is:

    ZA2 = 2 x R x 50.

    4. By adding a second coaxial line, the basic balun canbe made perfectly symmetrical. In this symmetrical coaxialbalun, the bandwidth (in terms of the input VSWR) is limitedby the transformation ratio, 50/2R, and the leakages, whichare represented by lines B and C. If ZA = 50 ohms and R =25 ohms, the bandwidth is constrained only by the leakages.

    Experimental Version of a Simple BalunUsing Coaxial Lines

    50

    500

    -- 90

    QUAD--RATURECOMBINER

    0

    -- 90

    INPUT(LOW VSWR)

    OUTPUT(LOW VSWR)

    QUAD--RATURECOMBINER

    Figure 2.

    V

    (a) COAXIAL BALUN DESIGN 1

    --V

    R

    R

    (b) EQUIVALENT CIRCUIT

    R

    R

    LINE A

    LINE A

    LINE B

    50

    50

    ZP

    LINE A: ZA, ALINE B: ZB, BZP = jZB TAN B AND ZP & R

    Figure 3.

    SYMMETRICAL COAXIAL BALUN (DESIGN 1)

    R

    R

    LINE A

    LINE B

    50

    LINE A: ZA, ALINE B, LINE C: ZB = ZC; B = C

    LINE C

    Figure 4.

  • AN1034

    3RF Application InformationFreescale Semiconductor

    5. The equivalent circuit for the symmetrical balunshows the effect of the leakages (lines B and C) on itsperformance. A broadband balun can be obtained by using arelatively high characteristic impedance for these leakagelines. In theory, the construction of the baluns insures perfectbalance.

    SYMMETRICAL BALUN EQUIVALENT CIRCUIT

    RRLINE A

    50 LINE C

    LINE B

    Figure 5.

    6. The symmetric baluns input equivalent circuitfurther simplifies its configuration and allows the input VSWRto be calculated.4 In this design, line A has a characteristicimpedance of ZA = 50 ohms, a length of LA = 1799 mils, anda dielectric constant (relative) of r = 2.10. For lines B and C,Z0 = 30 ohms, L = 1799 mils, and eff = 2.23.

    SYMMETRIC BALUN INPUT EQUIVALENT CIRCUIT

    RLLINE AA

    50

    LINE BB

    LINE AA: ZAA = ZA; AA = ALINE BB: ZBB = 2ZB; BB = BRL = 2R

    Figure 6.

    7. The theoretical input VSWR has been calculated for50-ohm values of ZA and 2R, and for two other sets of valuesfor these parameters. The performance of an experimentalbalun will be compared with these theoretical results.

    THEORETICAL INPUT RESPONSE OFTHE SYMMETRIC BALUN (DESIGN 1)

    3:1

    ZA = 102R = 2ZA = 25

    2R = 12.5

    ZA = 502R = 50

    2.5:1

    2:1

    1.5:1

    1:10.7 0.8 0.9 1.0 1.1 1.2 1.3 1.4 1.5

    INPUTVSWR

    FREQUENCY -- GHz

    Figure 7.

  • 4RF Application InformationFreescale Semiconductor

    AN1034

    8. Two /16 line-section Chebyshev impedancetransformers match the experimental balun to a 50-ohm

    measurement system. The balun was tested from 0.6 to1.5 GHz.

    EXPERIMENTALCOAXIAL BALUN (DESIGN 1)

    WITH OUTPUT TRANSFORMERS

    25

    /16 LINE-SECTIONCHEBYSHEV IMPEDANCE

    TRANSFORMERMICROSTRIP

    LOAD25 LINESMICROSTRIP

    BALUNCOAXIAL/

    MICROSTRIP

    R

    R

    LINE A

    LINE B

    50

    LINE C

    LINE E LINE F

    LINE D LINE E LINE F

    25

    LINE D

    FOR LINE A: Zo = 50 L = 1799 MILS; r = 2.10LINES B AND C: Zo = 30 L = 1799 MILS; eff = 2.23LINES D AND D: Zo = 25 L = 750 MILS; eff = 2.23LINES E AND E: Zo = 61.5; L = 464 MILS; eff = 2.10LINES F AND F: Zo = 20.3; L = 441 MILS; eff = 2.31

    Figure 8.

    9. The measured phase difference and insertion lossdifference, which indicate the maximum unbalance for theDesign 1 experimental balun, are 3 degrees and 0.2 dB,respectively.

    BALUN MEASURED PERFORMANCE FOR DESIGN 1

    185

    180

    175

    0.5

    0

    0.7 0.8 0.9 1.0 1.1 1.2 1.3 1.4 1.5

    PHASEDIFFERENCE

    --0.5

    FREQUENCY -- GHz

    --DEGREES

    0.6

    INSERTIONLOSS

    DIFFERENCE--dB

    Figure 9.

    10. The maximum VSWRmeasured for the first designis 1.5:1. Note the comparison between the calculated andmeasured response. The performance shown can beconsidered valid for amplifier applications up to an octaverange.

    BALUN PERFORMANCE FOR DESIGN 1

    0.7 0.8 0.9 1.0 1.1 1.2 1.3 1.4 1.5

    INPUTVSWR

    FREQUENCY -- GHz0.6

    2:1

    1.5:1

    1:1

    CALCULATED MEASURED

    Figure 10.

  • AN1034

    5RF Application InformationFreescale Semiconductor

    11. The second balun design adds two identical coaxlines to the simple balun just described. The inputs of theidentical lines are connected in series to the output of the firstbalun. By putting their outputs in parallel, the final outputbecomes symmetrical. The output impedance is halved.

    LINE B AND LINE C: IDENTICAL

    SECOND SECTIONFIRST SECTION

    R

    RLINE A

    LINE B

    50

    LINE CLINE F

    LINE D

    LINE E

    LINE G

    LINE D AND LINE F: IDENTICAL

    LINE E AND LINE G: IDENTICAL

    COAXIAL BALUN DESIGN 2

    Figure 11.

    12. The equivalent circuit for the Design 2 balunindicates that its bandwidth, in terms of input VSWR, is limitedby the transformation ratios of the first and second sectionsand the leakages represented by lines B, C, E, and G. If thebalun is designed with ZA = 50 ohms, and ZD = ZF = 25 ohms,and if the load, 2R, is set at 2 x 6.25 ohms, all of thetransmission lines will be connected to their characteristicimpedances. In this case, the bandwidth will be limited by theleakage alone, and a broadband balun can be obtained bychoosing lines B, C, E, and G with relatively high impedanceand /4 length for the center frequency. The balun achieves atransformation from 50 ohms to twice 6.26 ohms withoutcausing a standing wave in the coaxial cables.

    DESIGN 2 BALUN EQUIVALENT CIRCUIT

    R

    LINE F

    50

    LINE ELINE C

    LINE D

    R

    LINE A

    0

    H

    180 180

    LINE GLINE B

    I

    0

    Figure 12.

    13. The performance of the Design 2 balun can becalculated using its equivalent circuit. The calculatedVSWR shows a response very close to the simple coaxialbalun (Figure 10) because the new second section has fourtimes the bandwidth of the first section. This desig