www.ednmag.comMarch 21, 2002|edn83IT MAY SEEM LIKE A SIMPLE TASK TO MODIFY YOUROP-AMP DESIGN TO WORK FROM A SINGLE VOLTAGEPOWER SUPPLY, BUT THE CHANGE IN PERFORMANCEWILL SURPRISE YOU.Battery-powered op-amp applications, such asthose found in automotive and marine equip-ment, have only a single available power source.Other applications, such as computers, may oper-ate from the ac power lines but still have only a sin-gle polarity power source, such as 5 or 12V dc.Therefore, it is often a practical necessity to powerop-amp circuits from a single polarity supply. Butsingle-supply operation has its drawbacks: It re-quires additional passive components in each stageand, improper execution of the design can lead to se-rious instability problems.Single-supply applications have inherent prob-lems that dual-supply op-amp circuits often over-come. The fundamental problem is that an op ampis a dual-supply device, so you must employ biasingusing external components to center the op amp’soutput voltage at midsupply. This approach allowsthe maximum input and output voltage swing for agiven supply voltage. In some low-gain applicationshaving low input signals, the op amp’s output can beonly 2 or 3V above ground. But in most cases, youmust avoid clipping, and thus you must center theoutput around midsupply.The circuit of Figure 1 shows a simple sin-gle-supply biasing method. This noninvert-ing, ac-coupled, amplifier circuit uses a resistor di-vider with two biasing resistors, RAand RB, to set thevoltage on the noninverting input equal to VS/2. Theinput signal, VIN, is capacitively coupled to the non-inverting input terminal.This simple circuit has some serious limitations.The first limitation is that the op amp’s power-sup-ply rejection is almost entirely missing, because anychange in supply voltage directly changes the VS/2biasing voltage, which the resistor divider sets. Pow-er-supply rejection is an important and frequentlyoverlooked op-amp characteristic. Normally highpower-supply rejection that any modern op ampprovides greatly reduces the problem of ac signalsand power-supply hum feeding into the op amp viaits supply line. Because a 1V change on the supplyline causes a 0.5V change at the output of the divider,the circuit’s power-supply rejection is only 6 dB.Even worse, instability often occurs in circuits inwhich the op amp must supply large output currentsinto a load. Unless the power supply is well-regulat-ed and well-bypassed, large signal voltages appear onthe supply line. With the op amp’s noninverting in-put referenced directly off the supply line, these sig-nals will feed directly back into the op amp, often ini-tiating instability. The use of careful layout,multicapacitor-power-supply bypassing, stargrounds, and a pc-board power plane may providecircuit stability. However, it is easier to reintroducesome reasonable amount of power-supply rejectioninto the design.RESISTOR-DIVIDER BIASINGOne way to increase power-supply rejection is tomodify the circuit (Figure 2). Capacitor C2 now by-passes the tap point on the voltage divider, restor-Demystifying single-supply op-amp design+VS/2VSRA100kRB100kVSVS/2CINR1C1R2COUTRLOADVOUTVIN0.1 F1 F****BW1=2 ( RA)CIN1BW3=2 RROADCOUTFOR RA=RB,FOR AC SIGNALS, VOUT=VIN(1+(R2/R1)),WHERE XC1<<R1.1BW2=2 R1C11...*STAR GROUNDS.12/A first-cut single-supply op-amp design yields a potentiallyunstable circuit.Figure 1designfeatureBy Charles Kitchin, Analog Devices IncdesignfeatureOp-amp design84 edn|March 21, 2002www.ednmag.coming some ac power-supplyrejection. Resistor RINprovides a dc returnpath for the VS/2 referencevoltage and sets the circuit’sac input impedance. Manypublished applications cir-cuits show a 100/100-kvoltage divider for RAand RBwith a 0.1-F or similar ca-pacitance value for C2.How-ever, the parallel combina-tion of RAand RBand C2 setthe 3-dB bandwidth ofthis network, which is equalto:3dB BW1/2 (50,000)(0.1106F)30 Hz.Instability can still occur,because the circuit has es-sentially no power-supplyrejection for low frequen-cies. So, any signals lowerthan 30 Hz on the supplyline can easily find their wayback to the positive input ofthe op amp. A practi-cal solution to thisproblem is to increase thevalue of capacitor C2.Itneeds to be large enough toeffectively bypass the voltagedivider at all frequencieswithin the circuit’s pass-band. A good rule of thumbis to set this pole at one-tenth the 3-dB inputbandwidth, set by RIN/CINand R1/C1. Even though thedc circuit gain is unity, youneed to consider the opamp’s input-bias currents.The RA/RBvoltage divideradds considerable resistancein series with the op amp’spositive input terminal,equal to the parallel combination of thetwo resistors. Maintaining the op amp’soutput close to midsupply requires bal-ancing this resistance by increasing theresistance in the negative input terminalby an equal amount. Current-feedbackop amps often have unequal input-biascurrents, which further complicates thedesign.A single-supply op-amp circuit designthat considers input-bias current errorsas well as power-supply rejection, gain,input- and output-circuit bandwidth,and other factors can becomecomplex. However, you cangreatly simplify the design byusing a “cookbook” ap-proach. For a common volt-age-feedback op amp operat-ing from a single 15 or 12Vsupply, a resistor divider us-ing two 100-k resistors is areasonable compromise be-tween supply-current con-sumption and input-biascurrent errors. You can re-duce the resistors for a 5Vsupply to a lower value, suchas 42 k. In addition, someapplications need to operatefrom the new 3.3V standard.For 3.3V applications, the opamp must be a rail-to-rail de-vice, and you must bias itclose to midsupply. You canfurther reduce the biasing re-sistors to approximately 27k.Current-feedback op ampstypically target high-frequen-cy use; the lowpass filter thatR2and stray circuit capaci-tance form can severely re-duce the circuit’s 3-dB band-width. Therefore, current-feedback op amps normallyneed to use a low resistancevalue for R2. An op amp suchas the AD811, which targetsuse in video applications,typically will have optimumperformance using a 1-kresistor for R2. Therefore,these high-speed applicationsneed to use smaller resistorvalues in the RA/RBvoltagedivider to minimize input-bias-current errors. Unlessthe circuit must operate over+VS/2VSRA100kRIN100kRB100kVSVS/2R1C1C2CINR2150kCOUTRLOADVOUTVIN0.1 F1