Biasing Decoupling Amps Single Supply Applications Charles Kitchi

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Biasing Decoupling Amps Single Supply Applications Charles Kitchi

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AN-581 APPLICATION NOTE
Biasing Decoupling Amps Single Supply Applications
Charles Kitchin
SINGLE DUAL SUPPLY? Battery-powered applications such those found automotive marine equipment have only single available power source. Other applications, such computers, operate from power lines still have only single polarity power source, such Therefore, often practical necessity power circuits from single polarity supply. single supply operation does have drawbacks: requires additional passive components each stage and, properly executed, lead serious instability problems. COMMON PROBLEMS WITH RESISTOR BIASING Single supply applications have inherent problems that usually found dual supply circuits. fundamental problem that dual supply device some type biasing, using external components, must used center amp's output voltage midsupply. This allows maximum input output voltage swing given supply voltage. some gain applications, where input signals very small, amp's output lifted above ground only most cases, clipping needs avoided output needs centered around midsupply. circuit Figure shows simple single supply biasing method. This noninverting, ac-coupled, amplifier circuit uses resistor divider with biasing resistors, voltage noninverting equal VS/2. shown, input signal, VIN, capacitively coupled noninverting input terminal. This simple circuit some serious limitations. that amp's power supply rejection almost entirely gone, change supply voltage will directly change VS/2 biasing voltage resistor divider. Power Supply Rejection (PSR) very important (and frequently overlooked) characteristic.
Since volt change supply line causes one-half volt change output divider, circuit's only normally high power supply rejection provided modern amp, which greatly reduces signals (and power supply hum) from feeding into supply line, gone.
100k
COUT VOUT
100k
RLOAD
*STAR GROUND (1/2RA) RLOAD
BW1=
SIGNALS, VOUT (R2/R1)) WHERE
Figure Potentially Unstable Single Supply Circuit
REV.
Analog Devices, Inc., 2002
AN-581
Even worse, instability often occurs circuits where must supply large output currents into load. Unless power supply well regulated (and well bypassed), large signal voltages will appear supply line. With amp's noninverting input referenced directly supply line, these signals will directly back into often initiating "motor boating" other forms instability. While extremely careful layout, multicapacitor power supply bypassing, star grounds, printed circuit board "power plane," provide circuit stability, easier reintroduce some reasonable amount power supply rejection into design. DECOUPLING BIASING NETWORK FROM SUPPLY solution modify circuit, shown Figure point voltage divider bypassed signals capacitor restoring some PSR. Resistor provides return path VS/2 reference voltage also sets circuit's (ac) input impedance.
Many published applications circuits show k/100 voltage divider with similar capacitance value However, bandwidth this network parallel combination Capacitor equal
(50, 000)(0.1 Farads
Motor boating other forms instability still occur, circuit essentially power supply rejection frequencies below signals below that present supply line, very easily find their back input amp. practical solution this problem increase value capacitor needs large enough effectively bypass voltage divider frequencies within circuit's passband. good rule thumb this pole one-tenth input bandwidth, RIN/CIN R1/C1. Note that circuit gain unity. Even amp's input bias currents need considered. RA/RB voltage divider adds considerable resistance series with amp's positive input terminal, equal parallel combination resistors. Maintaining amp's output close midsupply requires "balancing" this resistance increasing resistance minus input terminal equal amount. Current feedback amps often have unequal input bias currents, which further complicates design. Therefore, designing single supply circuit design that considers input bias current errors well power supply rejection, gain, input output circuit bandwidth, etc., become quite involved. However, design greatly simplified using "cookbook" approach. common voltage feedback operating from single supply, resistor divider using resistors reasonable compromise between supply current consumption input bias current errors. supply, resistors reduced lower value such Finally, some applications need operate from standard. applications, essential that "rail-to-rail" device biased very close midsupply; biasing resistors further reduced value around
100k 100k VS/2
COUT
100k
VOUT VS/2
RLOAD (1/2RA) RLOAD *STAR GROUND 150k
1/10TH BW2, BW3, SIGNALS, VOUT (R2/R1)) WHERE <<R1 MINIMIZE INPUT BIAS CURRENT ERRORS, SHOULD EQUAL (1/2
Figure Decoupled Single Supply Biasing Circuit
REV.
AN-581
Note that current feedback amps typically designed high frequency low-pass filter formed stray circuit capacitance, which severely reduce circuit's bandwidth. Therefore, current feedback amps normally need fairly resistance value such AD811, which designed video speed applications, typically will have optimum performance using resistor Therefore, these types applications need much smaller resistor values RA/RB voltage divider minimize input bias current errors. Instead bipolar device, modern input will greatly reduce input bias current errors unless circuit required operate over very wide temperature range. that case, balancing resistance amp's input terminals still wise precaution. Table provides typical component values circuit Figure several different gains bandwidths. Table Typical Component Values Circuit Figure Where Input Output CIN* Gain (Hz) (Hz)
100k
VS/2
COUT VOUT
100k
VS/2
RLOAD
*STAR GROUND
(1/2 RLOAD
<<XC1
16.5 7.87 16.5
COUT 0.05
RLOAD
SIGNALS, VOUT (R2/R1) WHERE <<R1 MINIMIZE INPUT BIAS CURRENT ERRORS, SHOULD EQUAL
Figure Decoupled Single Supply Inverting Amplifier Circuit
Figure shows circuit similar Figure inverting amplifier.
*Capacitance values rounded next highest common value. Since CIN/RIN pole C1/R1 poles same frequency, both affect input each capacitor larger than would otherwise single pole RC-coupled input. selected provide corner frequency 1/10th that input
Table provides typical component values several different gains bandwidths. Table Typical Component Values Circuit Figure Where Input (Hz) Output (Hz) COUT 0.05 RLOAD
Gain
*Capacitance values rounded next highest common value. Since C1/R1 pole C2/R A/RB poles same frequency, both affect input each capacitor larger than would otherwise single pole RC-coupled input.
REV.
AN-581
ZENER DIODE BIASING Although resistor divider biasing technique cost, always keeps amp's output voltage VS/2, amp's common-mode rejection entirely dependent upon time constant formed RA/RB capacitor Using value that provides least times time constant input coupling network (R1/C1 RIN/CIN) will help ensure reasonable common-mode rejection ratio. With resistors practical values kept fairly small long circuit bandwidth low. However, another provide necessary VS/2 biasing single supply operation Zener diode regulator. Just such scheme shown Figure Here, current flows through resistor Zener. Capacitor helps prevent Zener-generated noise from feeding into amp. noise circuits need larger value than specified.
Zener should chosen that operating voltage close VS/2. Resistor needs selected provide high enough Zener current operate Zener stable rated voltage keep Zener output noise low. also important minimize power consumption (and heating) prolong life Zener. amp's input current essentially zero, it's good idea choose power Zener. device best more common types also acceptable. ideal Zener current varies with each manufacturer practical levels between (250 Zener) (500 Zener) usually good compromise this application. Within operating limits Zener, circuit Figure basically restores amp's power supply rejection. this does come without price: amp's output Zener voltage rather than VS/2. power supply voltage drops, nonsymmetrical clipping occur large signals. Furthermore, circuit consumes more power. Finally, input bias currents still need considered. Resistors should close same value prevent input bias currents from creating large offset voltage error. Figure inverting amplifier circuit using same Zener biasing method.
COUT VOUT
100k
RLOAD
100k
*STAR GROUND
SELECT PROVIDE DESIRED ZENER OPERATING CURRENT, TEXT. ZENER RLOAD
SIGNALS, VOUT (R2/R1)) WHERE <<R1 MINIMIZE INPUT BIAS CURRENT ERRORS, SHOULD EQUAL RIN.
Figure Noninverting Single Supply Amplifier Using Zener Diode Biasing
REV.
AN-581
100k
COUT VOUT
Table used with circuits provide practical resistor values with some common Zener diodes. Note that lowest possible circuit noise, optimum Zener current should selected referring Zener product data sheet. Table III. Recommended Values Motorola Zener Diode Part Numbers with Figures Supply Voltage Zener Voltage Zener Type 1N4100 1N4693 1N4627 1N4691 1N4623 1N4687 1N4617 1N4682 Zener Current (IZ) Value 11.5 1.15 9.31 5.23
ZENER RLOAD
100k
*STAR GROUND
SELECT PROVIDE DESIRED ZENER OPERATING CURRENT, TEXT. VZENER RLOAD
Tables provide practical component values Figures several different circuit gains bandwidths. Table Typical Component Values Circuit Figure Where Select from Table Input Gain (Hz) Output CIN* (Hz) 11.0 5.23 11.0 0.47 COUT 0.05 RLOAD
SIGNALS, VOUT (R2/R1) WHERE <<R1 MINIMIZE INPUT BIAS CURRENT ERRORS, SHOULD EQUAL RIN.
Figure Inverting Single Supply Amplifier Using Zener Diode Biasing
*Capacitance values rounded next highest common value. Since CIN/RIN pole C1/R1 poles same frequency, both affect input each capacitor larger than would otherwise single pole C-coupled input.
Table Typical Component Values Circuit Figure Where Select from Table Input Gain (Hz) Output (Hz) 0.05 COUT 0.05 RLOAD
*Capacitance values rounded next highest common value. Since C1/R1 pole C2/R poles same frequency, both affect input each capacitor larger than would otherwise single pole C-coupled input.
REV.
AN-581
BIASING USING LINEAR VOLTAGE REGULATOR circuits operating from standard, 1.65 biasing voltage needed. Zener diodes commonly available only down easiest provide this biasing voltage linear voltage regulator, such ADM663A ADM666A devices. This shown Figure
SENSE VOUT
VS/2 110k *STAR GROUND
220k
ADM663A ADM666A
VSET 1.3V ADJUSTABLE OUTPUT
VS/2 VS/2
COUT VOUT RLOAD
220k
*STAR GROUND
Figure Single Supply Biasing Circuit Using Linear Voltage Regulator
Although Zener diode usually cheapest voltage regulator available, linear voltage regulator lower drift over temperature than Zener less noise. Resistors selected provide desired VS/2 voltage reference; consult AD663A datasheet. DC-COUPLED BATTERY-POWERED CIRCUITS far, only ac-coupled circuits have been discussed. Although with suitably large input output coupling capacitors, ac-coupled circuit operate frequencies well below some applications require true response. Battery-powered applications permit "phantom ground" circuit shown Figure This provides dual supply voltages, both positive negative with respect ground, from single battery. used buffer output VS/2 voltage divider. voltage battery such used, should "rail-to-rail" device able operate effectively from this supply voltage. also needs able supply output current large enough power load circuit. Capacitor bypasses voltage divider output enough prevent resistor noise from feeding into amp. This capacitor does need provide power supply rejection because load current flows directly ground signal currents flow equally from both sides battery. Resistors selected provide desired VS/2 voltage reference; consult AD663A datasheet.
Figure Using Provide "Phantom Ground" Battery-Powered DC-Coupled Applications
NOISE ISSUES Some applications need noise amplifier noise amplifier circuits require resistance values signal path. Johnson (resistor) noise equals times square root resistance value While Johnson noise resistor only nV/Hz, this increases nV/Hz resistor nV/Hz resistor. Even though RA/RB resistor divider bypassed ground with capacitor (C2), these resistors limit minimum value that used amp's feedback resistor and, larger this greater Johnson noise. noise applications need much smaller biasing resistor values than specified here. However, lower value resistors divider mean higher power supply current reduced battery life. Fortunately, Zener diode biasing method supplies VS/2 without need large resistors. long Zener bypassed keep noise circuit, both noise supply current kept low. linear voltage regulator even better, noise output impedance both very low.
REV.
AN-581
CIRCUIT TURN-ON TIME ISSUES final issue that needs considered circuit turn-on time. approximate turn-on time will equal time constant lowest filter being used. circuits shown here call RA/RB, voltage divider network have times longer time constant than that input output circuit. This simplify circuit design (since three different poles input BW). This long time constant also helps keep biasing network from "turning before amp's input output networks and, therefore, amp's output gradually climbs from zero volts VS/2 without "railing" positive supply line. value supplied this table corner frequency that 1/10th that R1/C1 RLOAD/COUT. example: Figure circuit gain Table recommends value which provides bandwidth Fifty thousand ohms (the parallel combination times microFarads equals time constant 0.15 seconds. amp's output will take 0.15 seconds (approximately) settle VS/2. input output networks will charge times faster. some applications, where circuit's frequency bandwidth very low, circuit turn-on time become excessively long. that case, Zener biasing method better choice. INPUT "HEADROOM" CONSIDERATIONS Some specialty amps designed voltage operation. When these operated from voltage, single supply, such input headroom limitations introduced. This happen amplifier's input stage does limit symmetrically. example: AD8061 designed have input common-mode voltage range that extends down "ground" negative supply line). However, inputs only swing within positive supply voltage without introducing errors limiting device bandwidth. this amplifier operated from single supply amplifier's positive input biased VS/2 (2.5 input voltage swing negative direction full (down zero volts). But, positive direction, only swing before clipping. Note that this problem amplifier being operated gain higher, maximum output swing will reached before input stage limits. However, amplifier being operated lower gain, positive input needs biased below VS/2, allow symmetrical input stage limiting. case AD8061, biasing positive input will allow input swing without clipping. Refer individual product data sheet determine optimum single supply biasing voltage.
Table Rail-to-Rail Amps Recommended Designs Type High Speed Single AD8031 AD8061 AD8051 AD8063 AD8591 AD8531 AD820 AD8551 AD8601 OP184 OP162 AD8605 AD8628 OP196 AD8541 OP777 Dual AD8032 AD8062 AD8052 AD823 AD8592 AD8532 AD822 AD8552 AD8602 OP284 OP262 AD8606 OP296 AD8542 OP727 Quad
AD8054 AD8594 AD8534 AD824 AD8554 AD8604 OP484 OP462 AD8608 OP496 AD8544 OP747
High Output JFET Input Auto Zero Digital Trim Noise
Power Precision
REV.
E02493-0-10/02(0)
PRINTED U.S.A.

Biasing Decoupling Amps Single Supply Applications Charles Kitchi

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