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Analog Applications Journal Analog and MixedSignal Products November Analysis of fully differential amplifiers Introduction The August issue of Analog Applications Journal introduced the fully diff

This article explores the topic more deeply by analyzing gain and noise The fully differential amplifier has multiple feedback paths and circuit analysis requires close attention to detail Care must be taken to include the OCM pin for a complete ana

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Analog Applications Journal Analog and MixedSignal Products November Analysis of fully differential amplifiers Introduction The August issue of Analog Applications Journal introduced the fully diff

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Presentation on theme: "Analog Applications Journal Analog and MixedSignal Products November Analysis of fully differential amplifiers Introduction The August issue of Analog Applications Journal introduced the fully diff"— Presentation transcript:

48Analog Applications Journal introduced the fully differential amplifiers from Texas OCMOCVV= )VV(A)V()V(NPFOUTOUTŠ=ŠŠ+ 2)V()V(VOUTOUTOCŠ++= )V()V(VOUTOUTODŠŠ+= 2)V()V(VININICŠ++= )V()V(VININIDŠŠ+= Texas Instruments Incorporated VNVPR4R3V+OUTV Ð OUTVOCMR2R1VÐINV+IN ++ÐÐAF Figure 1. Analysis circuit ++ÐÐÐ1Ð\f21Ð\f121 \rVNVPV + OUTV Ð OUTVÐINV+INAF\f\f Figure 2. Block diagram Texas Instruments Incorporated 49Analog Applications Journal By settingandVWith Equations 7 and 8, a block diagram of the main differential amplifier can be constructed, like that shown in With the ÒidealÓ assumption ATo calculate V AA21)(V2)]1)(V()1)(V[(2)(1V2F1F21OCM2IN1IN21OD\b\t\n\f+\f+\fŠ\f+\fŠŠŠ\fŠ+\f+\f= .)()(V2)1)(V()1)(V()V(212OCM2IN1INOUT\f+\f\f++\fŠŠ+\fŠ+Š=Š .AA21A1V2)1)(V()1)(V()(1)V(2F1F2FOCM2IN1IN21OUT\b\t\n\f+\f+\b\t\n\f++\fŠŠ+\fŠ+Š\f+\f=Š .)(V2)1)(V()1)(V()V(211OCM2IN1INOUT\f+\f\f+\fŠŠŠ\fŠ+=+ .AA21A1V2)1)(V()1)(V()(1)V(2F1F1FOCM2IN1IN21OUT\b\t\n\f+\f+\b\t\n\f++\fŠŠŠ\fŠ+\f+\f=+ ,)]1)(V()1)(V[(A)A1(V2)AA2)(V(2IN1INF1FOCM2F1FOUT\fŠŠŠ\fŠ+=\f+Š\f+\f++ .)]1)(V()1)(V[(A)A1)(V()A1)((V2IN1INF1FOUT2FOUT\fŠŠŠ\fŠ+=\f+ŠŠ\f++ .))(V()1)(V(V1OUT1INP\fŠ+\fŠ+= ,))(V()1)(V(V2OUT2INN\f++\fŠŠ= ,R2R1R12\b\t\n+=\f \b\t\n+=\fR4R3R31 .R4R3R3)V(R4R3R4)V(VOUTINP\b\t\n+Š+\b\t\n++= \b\t\n+++\b\t\n+Š=R2R1R1)V(R2R1R2)V(VOUTINN (10)(12)(14)Continued on next page Texas Instruments Incorporated 50Analog Applications Journal Analog and Mixed-Signal ProductsNovember 2000Again, assuming AF\f1 1 and AF\f2 1, this reduces to(15) .)()(V2)]1)(V()1)(V[(2V2121OCM2IN1INOD\f+\f\fŠ\f+\fŠŠŠ\fŠ+= Continued from previous page R4R3++ÐÐR2R1V+OUTVÐOUTVOCMV+INAF Figure 3. Single-ended to differential amplifier ++ÐÐR2R1 V+OUTVÐOUTVOCMV+INAF Figure 4. \f1= 0 R4R3++ÐÐ V+OUTVÐOUTVOCMV+INAF With a slight variation of Figure 1 as shown in Figure 3,reducing the dynamic range of the amplifier. To stripdifferently, making V .)()(V2)1)(V(2V2121OCM1INOD\f+\f\fŠ\f+\fŠ+= ,)()1)(V(V2)V(211IN2OCMOUT\f+\f\fŠ+Š\f=Š ,)(V2)1)(V()V(211OCM1INOUT\f+\f\f+\fŠ+=+ .R1R21=\f\fŠ .A1111)A1(A)V()V()V()V(FFFININOUTOUT\b\t\n\f+×\f\fŠ=\f+=ŠŠ+ŠŠ+ Texas Instruments Incorporated 51Analog Applications Journal differential amplifier, and matching of the open-loop gainsWith = 0, this circuit is similar to a noninverting amplifier. ,V2)1)(V()V(OCM11INOUT+\f\fŠ+=+ .V2)V(2VOCM2INODŠ\f+= ,)V(V2)V(2INOCMOUT\f+Š=Š ,)V()V(2INOUT\f+=+ With = 0.333; or, with This circuit realizes a gain of 2 with no resistor. .]V)V(2[VOCMINODŠ+= ,)V(V2)V(INOCMOUT+Š=Š ),V()V(INOUT+=+ .)1()1(V2)1)(V(2V11OCM1INOD+\fŠ\f+\fŠ+= ,1)1)(V(V2)V(11INOCMOUT+\f\fŠ+Š=Š ,1V2)1)(V()V(11OCM1INOUT+\f\f+\fŠ+=+ .V2)1)(V(2VOCM11INOD+\f\fŠ+= ,)1)(V()V(11INOUT\f\fŠ+Š=Š R4R3++ÐÐV+OUTVÐOUTVOCMV+INAF Figure 6. \f2= 1 ++ÐÐV+OUTVÐOUTVOCMV+INAF Figure 7. \f1= 0, and \f2= 1Continued on next page in a similar manner, as shown below.the resistor number, k is BoltzmannÕs constant (1.38 x .R4R3R3R4REQ2+= R2R1R1R2REQ1+= Texas Instruments Incorporated 52Analog Applications Journal /2), and half is/2). Therefore,/2) and (ÐE/2) are correlated to one another andTo combine noncorrelated noise voltages, a sum-of-squaresthe square of the individual RMS voltages added together.the amplifierÕs noise sources. .)(E2E21INOD\f+\f= .)(A211E2E21F21INOD\b\b\t\n\f+\f+\b\t\n\f+\f= .2)E(2)E(EAE2OD1ODINFOD\f+Š\fŠ+= ).EA(E2E2EIDODODOD==\b\t\nŠŠ\b\t\n+ Continued from previous page VÐVIx RINEQ1Ix RINEQ2 +++ÐÐ\r Figure 9. Block diagram of the amplifierÕs R4R3R2R1EINECMER1ER3ER4ER2++ÐÐIÐINI+IN V+OUTVÐOUTVOCMAF Figure 8. Noise analysis circuit Texas Instruments Incorporated 53Analog Applications Journal Similarly, the noise contributions from Iandrespectively.differential noise voltageequal to)to a maximum absolute value of 2 (given = 0,or earlier, the differential output noise contribution isfor each resistor respectively. .EE)()]1)(E(2[)]1)(E(2[)](E2[)RI2()RI2()E2(E2R42R222121R322R1221CM2EQ2IN2EQ1IN2INOD++\f+\f\fŠ+\fŠ+\fŠ\f+×+×+= )()1)(E(2211R3\f+\f\fŠ )()1)(E(2212R1\f+\f\fŠ .)()(E22121CM\f+\f\fŠ\f ,)(R2I21EQ2IN\f+\f× )(RI221EQ1IN\f+\f× ,EEINOUT\f= resistor, the same as a standard single-ended voltage feedback amplifier. ,RR11GGFn+=\f= .2G21n\f+\f= loadan Acrobat Reader file at www-s.ti.com/sc/techlit/the materials listed below.Document TitleTI Lit. #1.Jim Karki, ÒFully differential amplifiers,Ó 2000), pp. 38-41 . . . . . . . . . . . . . . . . . . . . . . .slytRelated Web siteamplifier.ti.com IMPORTANT NOTICE Texas Instruments Incorporated and its subsidiaries (TI) reservestandard warranty. 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