SECTION 1 SINGLE-SUPPLY AMPLIFIERS Rail-to-Rail Input stages Rail-to-Rail Output Stages Single-Supply Instrumentation Amplifiers
1 SECTION 1 SINGLE-SUPPLY AMPLIFIERS Rail-to-Rail Input Stages Rail-to-Rail Output Stages Single-Supply Instrumentation Amplifiers
SECTION 1 SINGLE-SUPPLY AMPLIFIERS Adolfo garcia Over the last several years, single-supply operation has become an increasingly important requirement as systems get smaller, cheaper, and more portable nd total circuit consumption is an important and often dominant design issue, and in some instances, more important than cost. This makes low-voltage/low supply current operation critical; at the same time, however, accuracy and precision requirements have forced IC manufacturers to meet the challenge of" doing more with less"in their amplifier design SINGLE-SUPPLY AMPLIFIERS ■ Single Supply Offers Lower Power Battery Operated Portable Equipment Simplifies Power Supply Requirements ■ But Watch out for: Signal-swings limited, therefore more sensitive to errors caused by offset voltage, bias current, finite open-loop gain, noise, etc. o More likely to have noisy power supply because of sharing with digital circuits DC coupled, multi-stage single-supply circuits can get very tricky! Rail-to-rail op amps needed to maximize signal swIngs In a single-supply application, the most immediate effect on the performance of an amplifier is the reduced input and output signal range. As a result of these lower input and output signal excursions, amplifier circuits become more sensitive to internal and external error sources. Precision amplifier offset voltages on the order of 0. 1mv are less than a 0.04 LSB error source in a 12-bit, 10v full-scale system. In a single-supply system, however, a"rail-to-rail "precision amplifier with an offset voltage of Imv represents a 0. 8LSB error in a 5V FS system, and 1.6LSB error in a 2.5V FS system Furthermore, amplifier bias currents, now flowing in larger source resistances to keep current drain from the battery low, can generate offset errors equal to or greater than the amplifiers own offset voltage Gain accuracy in some low voltage single-supply devices is also reduced, so device millions typically operate on dual supplies: for example, the OP07 family typee the selection needs careful consideration. Many amplifiers having open- loop gain However, many single-supply/rail-to-rail amplifiers for precision application
2 SECTION 1 SINGLE-SUPPLY AMPLIFIERS Adolfo Garcia Over the last several years, single-supply operation has become an increasingly important requirement as systems get smaller, cheaper, and more portable. Portable systems rely on batteries, and total circuit power consumption is an important and often dominant design issue, and in some instances, more important than cost. This makes low-voltage/low supply current operation critical; at the same time, however, accuracy and precision requirements have forced IC manufacturers to meet the challenge of “doing more with less” in their amplifier designs. SINGLE-SUPPLY AMPLIFIERS Single Supply Offers: Lower Power Battery Operated Portable Equipment Simplifies Power Supply Requirements But Watch Out for: Signal-swings limited, therefore more sensitive to errors caused by offset voltage, bias current, finite open-loop gain, noise, etc. More likely to have noisy power supply because of sharing with digital circuits DC coupled, multi-stage single-supply circuits can get very tricky! Rail-to-rail op amps needed to maximize signal swings In a single-supply application, the most immediate effect on the performance of an amplifier is the reduced input and output signal range. As a result of these lower input and output signal excursions, amplifier circuits become more sensitive to internal and external error sources. Precision amplifier offset voltages on the order of 0.1mV are less than a 0.04 LSB error source in a 12-bit, 10V full-scale system. In a single-supply system, however, a "rail-to-rail" precision amplifier with an offset voltage of 1mV represents a 0.8LSB error in a 5V FS system, and 1.6LSB error in a 2.5V FS system. Furthermore, amplifier bias currents, now flowing in larger source resistances to keep current drain from the battery low, can generate offset errors equal to or greater than the amplifier’s own offset voltage. Gain accuracy in some low voltage single-supply devices is also reduced, so device selection needs careful consideration. Many amplifiers having open-loop gains in the millions typically operate on dual supplies: for example, the OP07 family types. However, many single-supply/rail-to-rail amplifiers for precision applications
typically have open- loop gains between 25,000 and 30, 000 under light loading (10kohm) Selected devices, like the OPX13 family, do have high open-loop gains (i.e, >1V/u V) Many trade-offs are possible in the design of a single-supply amplifier: speed versus power, noise versus power, precision versus speed and power, etc. Even if the noise floor remains constant (highly unlikely), the signal-to-noise ratio will drop as the signal amplitude decreases Besides these limitations, many other design considerations that are otherwise minor issues in dual-supply amplifiers become important. For example, signal-to- noise(SNR) performance degrades as a result of reduced signal swing. "Ground devices, but not others. System noise increases as operating supply current dropo e reference!"is no longer a simple choice, as one reference voltage may work for so and bandwidth decreases. Achieving adequate bandwidth and required precision with a somewhat limited selection of amplifiers presents significant system design challenges in single-supply, low-power applications Most circuit designers take"ground"reference for granted. Many analog circuits scale their input and output ranges about a ground reference. In dual-supply applications, a reference that splits the supplies(0v) is very convenient, as there is equal supply headroom in each direction, and ov is generally the voltage on the low impedance ground plane In single-supply/rail-to-rail circuits, however, the ground reference can be chosen anywhere within the supply range of the circuit, since there is no standard to follow. The choice of ground reference depends on the type of signals processed and the amplifier characteristics. For example, choosing the negative rail as the ground reference may optimize the dynamic range of an op amp whose output is designed to swing to oV. On the other hand, the signal may require level shifting in order to be compatible with the input of other devices(such as ADCs) that are not designed to operate at oV input RAIL-TO-RAIL AMPLIFIERS ■ What exactly is“rail-to-rai| Does the input common range(for guaranteed CMMR)include: oV, +Vs, b or neither Output Voltage Swing(how close to the rails can you get under load?) ■ Where is“ ground"? Complementary bipolar processes make rail-to-rail inputs and outputs feasible(within some fundamental physical mutations) a Implications for precision single-supply instrumentation
3 typically have open-loop gains between 25,000 and 30,000 under light loading (>10kohm). Selected devices, like the OPX13 family, do have high open-loop gains (i.e., >1V/µV). Many trade-offs are possible in the design of a single-supply amplifier: speed versus power, noise versus power, precision versus speed and power, etc. Even if the noise floor remains constant (highly unlikely), the signal-to-noise ratio will drop as the signal amplitude decreases. Besides these limitations, many other design considerations that are otherwise minor issues in dual-supply amplifiers become important. For example, signal-tonoise (SNR) performance degrades as a result of reduced signal swing. "Ground reference" is no longer a simple choice, as one reference voltage may work for some devices, but not others. System noise increases as operating supply current drops, and bandwidth decreases. Achieving adequate bandwidth and required precision with a somewhat limited selection of amplifiers presents significant system design challenges in single-supply, low-power applications. Most circuit designers take "ground" reference for granted. Many analog circuits scale their input and output ranges about a ground reference. In dual-supply applications, a reference that splits the supplies (0V) is very convenient, as there is equal supply headroom in each direction, and 0V is generally the voltage on the low impedance ground plane. In single-supply/rail-to-rail circuits, however, the ground reference can be chosen anywhere within the supply range of the circuit, since there is no standard to follow. The choice of ground reference depends on the type of signals processed and the amplifier characteristics. For example, choosing the negative rail as the ground reference may optimize the dynamic range of an op amp whose output is designed to swing to 0V. On the other hand, the signal may require level shifting in order to be compatible with the input of other devices (such as ADCs) that are not designed to operate at 0V input. "RAIL-TO-RAIL" AMPLIFIERS What exactly is “rail-to-rail” Does the input common mode range (for guaranteed CMMR) include: 0V, +Vs, both, or neither? Output Voltage Swing (how close to the rails can you get under load?) Where is “ground”? Complementary bipolar processes make rail-to-rail inputs and outputs feasible (within some fundamental physical limitations) Implications for precision single-supply instrumentation amps
Early single-supply "zero-in, zero-out"amplifiers were designed on bipolar processes which optimized the performance of the nPn transistors. The PNP transistors were either lateral or substrate PNPs with much poorer performance than the NPNs Fully complementary processes are now required for the new-breed of single supply/rail-to-rail operational amplifiers. These new amplifier designs do not use lateral or substrate PNP transistors within the signal path, but incorporate parallel NPN and PNP input stages to accommodate input signal swings from ground to the positive supply rail. Furthermore, rail-to-rail output stages are designed with bipolar NPN and PNP common-emitter, or N-channel/P-channel common-source amplifiers whose collecto tter saturation voltage or drain-source channel on-resistance determine output signal swing with the load current. The characteristics of a single-supply amplifier input stage(common-mode rejection input offset voltage and its temperature coefficient, and noise)are critical in precision, low-voltage applications. Rail-to-rail input operational amplifiers must resolve small signals, whether their inputs are at ground, or at the amplifiers positive supply Amplifiers having a minimum of 60dB common-mode rejection over the entire input common-mode voltage range from oV to the positive supply (VpoS) are good candidates. It is not necessary that amplifiers maintain common-mode rejection for signals beyond the supply voltages: what is required is that they do not self-destruct for momentary overvoltage conditions. Furthermore, amplifiers that have offset voltages less than lmv and offset voltage drifts less than 2u v/C are also very good candidates for precision applications. Since input signal dynamic range and SNR are equally if not more important than output dynamic range and SNR, precision single-supply/rail-to-rail operational amplifiers should have noise levels referred-to-input (RTi) less than 5u Vp-p in the 0. 1Hz to 10Hz band Since the need for rail-to-rail amplifier output stages is driven by the need to maintain wide dynamic range in low-supply voltage applications, a single-supply/rail- to-rail amplifier should have output voltage swings which are within at least 100mv of either supply rail (under a nominal load). The output voltage swing is very dependent on output stage topology and load current, but the voltage swing of a good output stage should maintain its rated swing for loads down to 10kohm The “zero- scale”or“ full-scale” output voltage, should be determined by an amplifier; smaller the VoL and the larger the VOH, the better System parameters, such VOL (for zero-scale) and VOH (for full-scale) Since the majority of single-supply data acquisition systems require at least 12-to 14-bit performance, amplifiers which exhibit an open-loop gain greater than 30, 000 for all loading conditions are good choices in precision applications SINGLE- SUPPLY/RAIL-TO-RAIL OP AMP INPUT STAGES With the increasing emphasis on low-voltage, low-power, and single-supply operation, there is some demand for op amps whose input common- mode range includes both supply rails. Such a feature is undoubtedly useful in some applications, but engineers should recognize that there are relatively few applications where it is absolutely essential. These should be carefully distinguished
4 Early single-supply “zero-in, zero-out” amplifiers were designed on bipolar processes which optimized the performance of the NPN transistors. The PNP transistors were either lateral or substrate PNPs with much poorer performance than the NPNs. Fully complementary processes are now required for the new-breed of singlesupply/rail-to-rail operational amplifiers. These new amplifier designs do not use lateral or substrate PNP transistors within the signal path, but incorporate parallel NPN and PNP input stages to accommodate input signal swings from ground to the positive supply rail. Furthermore, rail-to-rail output stages are designed with bipolar NPN and PNP common-emitter, or N-channel/P-channel common-source amplifiers whose collector-emitter saturation voltage or drain-source channel on-resistance determine output signal swing with the load current. The characteristics of a single-supply amplifier input stage (common-mode rejection, input offset voltage and its temperature coefficient, and noise) are critical in precision, low-voltage applications. Rail-to-rail input operational amplifiers must resolve small signals, whether their inputs are at ground, or at the amplifier’s positive supply. Amplifiers having a minimum of 60dB common-mode rejection over the entire input common-mode voltage range from 0V to the positive supply (VPOS) are good candidates. It is not necessary that amplifiers maintain common-mode rejection for signals beyond the supply voltages: what is required is that they do not self-destruct for momentary overvoltage conditions. Furthermore, amplifiers that have offset voltages less than 1mV and offset voltage drifts less than 2µV/°C are also very good candidates for precision applications. Since input signal dynamic range and SNR are equally if not more important than output dynamic range and SNR, precision single-supply/rail-to-rail operational amplifiers should have noise levels referred-to-input (RTI) less than 5µVp-p in the 0.1Hz to 10Hz band. Since the need for rail-to-rail amplifier output stages is driven by the need to maintain wide dynamic range in low-supply voltage applications, a single-supply/railto-rail amplifier should have output voltage swings which are within at least 100mV of either supply rail (under a nominal load). The output voltage swing is very dependent on output stage topology and load current, but the voltage swing of a good output stage should maintain its rated swing for loads down to 10kohm. The smaller the VOL and the larger the VOH, the better. System parameters, such as “zero-scale” or “full-scale” output voltage, should be determined by an amplifier’s VOL (for zero-scale) and VOH (for full-scale). Since the majority of single-supply data acquisition systems require at least 12- to 14-bit performance, amplifiers which exhibit an open-loop gain greater than 30,000 for all loading conditions are good choices in precision applications. SINGLE-SUPPLY/RAIL-TO-RAIL OP AMP INPUT STAGES With the increasing emphasis on low-voltage, low-power, and single-supply operation, there is some demand for op amps whose input common-mode range includes both supply rails. Such a feature is undoubtedly useful in some applications, but engineers should recognize that there are relatively few applications where it is absolutely essential. These should be carefully distinguished
from the many applications where common- mode range close to the supplies or one that includes one of the supplies is necessary, but input rail-rail operation is not supply rails(usually ground). Amplifiers which will handle zero-volt inputs are e In many single-supply applications, it is required that the input go to only one of relatively easily designed using either PNP transistors(see OP90 and the oPX93 in Figure 1.3 or N-channel JFETs(see AD820 family in Figure 1.4). P-channel JFETs can be used where inputs must include the positive supply rail (but not the negative rail)as shown in Figure 1.4 for the OP282/0P482 OP90 AND OPX93 INPUT STAGE ALLOWS INPUT TO GO TO THE NEGATIVE RAIL 2k·4k Figure 1.3 input signals approach and exceed the amplifiers linear input common-mode s In the FET-input stages of Figure 1.4, the possibility exists for phase reversal voltage ranges. As described in Section 7, internal amplifier stages saturate, forcing subsequent stages into cutoff. Depending on the structure of the input stage, phase reversal forces the output voltage to one of the supply rails. For n-channel JFET input stages, the output voltage goes to the negative output rail during phase reversal For p-channel JFET-input stages, the output is forced to the positive output rail. New FET-input amplifiers, like the AD820 family of amplifiers, incorporate design improvements that prevent output voltage phase reversal for signals within the rated supply voltage range. Their input stage and second gain stage even offer protection against output voltage phase reversal for input signals 200mv more positive than the positive supply voltage
5 from the many applications where common-mode range close to the supplies or one that includes one of the supplies is necessary, but input rail-rail operation is not. In many single-supply applications, it is required that the input go to only one of the supply rails (usually ground). Amplifiers which will handle zero-volt inputs are relatively easily designed using either PNP transistors (see OP90 and the OPX93 in Figure 1.3) or N-channel JFETs (see AD820 family in Figure 1.4). P-channel JFETs can be used where inputs must include the positive supply rail (but not the negative rail) as shown in Figure 1.4 for the OP282/OP482. OP90 AND OPX93 INPUT STAGE ALLOWS INPUT TO GO TO THE NEGATIVE RAIL Figure 1.3 In the FET-input stages of Figure 1.4, the possibility exists for phase reversal as input signals approach and exceed the amplifier’s linear input common-mode voltage ranges. As described in Section 7, internal amplifier stages saturate, forcing subsequent stages into cutoff. Depending on the structure of the input stage, phase reversal forces the output voltage to one of the supply rails. For n-channel JFETinput stages, the output voltage goes to the negative output rail during phase reversal. For p-channel JFET-input stages, the output is forced to the positive output rail. New FET-input amplifiers, like the AD820 family of amplifiers, incorporate design improvements that prevent output voltage phase reversal for signals within the rated supply voltage range. Their input stage and second gain stage even offer protection against output voltage phase reversal for input signals 200mV more positive than the positive supply voltage
AD820/AD822/AD824 INPUT INCLUDES NEGATIVE RAIL OP-282/OP-482 INCLUDES POSITIVE RAIL AD820AD822/AD824 oP282oP482 INPUTS MAY P-CHANNEL N-CHANNEL JFETS IFETS Y GO TO-V Figure 1.4 As shown in Figure 1.5, true rail-to-rail input stages require two long-tailed pairs, one of nPn bipolar transistors(or N-channel FETs), the other of PNP transistors (or p-channel FETs). These two pairs exhibit different offsets and bias currents, so when the applied input common-mode voltage changes, the amplifier input offset voltage and input bias current does also. In fact, when both current sources(ll and 12) remain active throughout the entire input common-mode range, amplifier input ffset voltage is the average offset voltage of the npn pair and the pnp pair. In those designs where the current sources are alternatively switched off at some point along the input common- mode voltage amplifier input offset voltage is dominated by the pnp pair offset voltage for signals near the negative supply, and by the npn pair offset voltage for signals near the positive supply Amplifier input bias current, a function of transistor current gain, is also a function of the applied input common-mode voltage. The result is relatively poor common mode rejection(CMR), and a changing common- mode input impedance over the common-mode input voltage range, compared to familiar dual supply precision devices like the OPO7 or OP97. These specifications should be considered carefully when choosing a rail- rail input op amp, especially for a non- inverting configuration Input offset voltage, input bias current, and even CMr may be quite good over part of the common-mode range, but much worse in the region where operation shifts between the npn and pnp devices
6 AD820/AD822/AD824 INPUT INCLUDES NEGATIVE RAIL, OP-282/OP-482 INCLUDES POSITIVE RAIL Figure 1.4 As shown in Figure 1.5, true rail-to-rail input stages require two long-tailed pairs, one of NPN bipolar transistors (or N-channel FETs), the other of PNP transistors (or p-channel FETs). These two pairs exhibit different offsets and bias currents, so when the applied input common-mode voltage changes, the amplifier input offset voltage and input bias current does also. In fact, when both current sources (I1 and I2) remain active throughout the entire input common-mode range, amplifier input offset voltage is the average offset voltage of the NPN pair and the PNP pair. In those designs where the current sources are alternatively switched off at some point along the input common-mode voltage, amplifier input offset voltage is dominated by the PNP pair offset voltage for signals near the negative supply, and by the NPN pair offset voltage for signals near the positive supply. Amplifier input bias current, a function of transistor current gain, is also a function of the applied input common-mode voltage. The result is relatively poor commonmode rejection (CMR), and a changing common-mode input impedance over the common-mode input voltage range, compared to familiar dual supply precision devices like the OP07 or OP97. These specifications should be considered carefully when choosing a rail-rail input op amp, especially for a non-inverting configuration. Input offset voltage, input bias current, and even CMR may be quite good over part of the common-mode range, but much worse in the region where operation shifts between the NPN and PNP devices
RAIL-TO-RAIL INPUT STAGE TOPOLOGY OS 12 Figure 1.5 Many rail-to-rail amplifier input stage designs switch operation from one differential pair to the other differential pair somew here along the input common- mode voltage range. Devices like the oPX91 family and the oP279 have a common-mode crossover threshold at approximately 1 v below the positive supply. In these devices the PNP differential input stage remains active; as a result, amplifier input offset g voltage, input bias current, CMR, input noise voltage/current are all determined by the characteristics of the PNp differential pair At the crossover threshold, however amplifier input offset voltage becomes the average offset voltage of the NPN/PNP pairs and can change rapidly. Also, amplifier bias currents, dominated by the PNP differential pair over most of the input common-mode range, change polarity and magnitude at the crossover threshold when the npn differential pair becomes ctive. As a result, source impedance levels should be balanced when using such devices, as mentioned before, to minimize input bias current offsets and distortion. An advantage to this type of rail-to-rail input stage design is that input stage transconductance can be made constant throughout the entire input common-mode voltage range, and the amplifier slews symmetrically for all applied signals Operational amplifiers, like the OP284/0P484, utilize a rail-to-rail input stage design where both PNP and nPn transistor pairs are active throughout the entire input common-mode voltage range, and there is no common-mode crossover threshold Amplifier input offset voltage is the average offset voltage of the npn and the PNp stages. Amplifier input offset voltage exhibits a smooth transition throughout the entire input common-mode voltage range because of careful laser-trimming of resistors in the input stage. In the same manner, through careful input stage current balancing and input transistor design, amplifier input bias currents also exhibit a smooth transition throughout the entire common-mode input voltage range. The exception occurs at the extremes of the input common-mode range
7 RAIL-TO-RAIL INPUT STAGE TOPOLOGY Figure 1.5 Many rail-to-rail amplifier input stage designs switch operation from one differential pair to the other differential pair somewhere along the input common-mode voltage range. Devices like the OPX91 family and the OP279 have a common-mode crossover threshold at approximately 1V below the positive supply. In these devices, the PNP differential input stage remains active; as a result, amplifier input offset voltage, input bias current, CMR, input noise voltage/current are all determined by the characteristics of the PNP differential pair. At the crossover threshold, however, amplifier input offset voltage becomes the average offset voltage of the NPN/PNP pairs and can change rapidly. Also, amplifier bias currents, dominated by the PNP differential pair over most of the input common-mode range, change polarity and magnitude at the crossover threshold when the NPN differential pair becomes active. As a result, source impedance levels should be balanced when using such devices, as mentioned before, to minimize input bias current offsets and distortion. An advantage to this type of rail-to-rail input stage design is that input stage transconductance can be made constant throughout the entire input common-mode voltage range, and the amplifier slews symmetrically for all applied signals. Operational amplifiers, like the OP284/OP484, utilize a rail-to-rail input stage design where both PNP and NPN transistor pairs are active throughout the entire input common-mode voltage range, and there is no common-mode crossover threshold. Amplifier input offset voltage is the average offset voltage of the NPN and the PNP stages. Amplifier input offset voltage exhibits a smooth transition throughout the entire input common-mode voltage range because of careful laser-trimming of resistors in the input stage. In the same manner, through careful input stage current balancing and input transistor design, amplifier input bias currents also exhibit a smooth transition throughout the entire common-mode input voltage range. The exception occurs at the extremes of the input common-mode range
forward-biasing of parasitic p-n junctions This occurs for input voltages within Sht where amplifier offset voltages and bias currents increase sharply due to the sligh approximately 1v of either supply rail When both differential pairs are active throughout the entire input common-mode range, amplifier transient response is faster through the middle of the common- mode range by as much as a factor of 2 for bipolar input stages and by a factor of the square root of 2 for FEt input stages. Input stage transconductance determines the slew rate and the unity-gain crossover frequency of the amplifier, hence response time degrades slightly at the extremes of the input common-mode range when either the PNP stage(signals approaching VpoS) or the NPN stage(signals approaching GND) are forced into cutoff. The thresholds at which the transconductance changes occur approximately within 1v of either supply rail, and the behavior is similar to that of the input bias current Applications which initially appear to require true rail-rail inputs should be carefully evaluated, and the amplifier chosen to ensure that its input offset voltage, input bias current, common-mode rejection, and noise(voltage and current)are suitable. a true rail-to-rail input amplifier should not generally be used if an input range which includes only one rail is satisfactory. SINGLE-SUPPLY/RAIL-TO-RAIL OP AMP OUTPUT STAGES The earliest IC op amp output stages were NPN emitter followers with NPn current sources or resistive pull-downs, as shown in Figure 1.6. Naturally, the slew rates were greater for positive-going than for negative- going signals. while all modern op amps have push-pull output stages of some sort, many are still asymmetrical, and have a greater slew rate in one direction than the other. This asymmetry, which generally results from the use of IC processes with better npn than PNP transistors, may also result in the ability of the output to approach one supply more closely than the other. In many applications, the output is required to swing only to one rail, usually the negative rail (i.e, ground in single-supply systems). A pulldown resistor to the negative rail will allow the output to approach that rail (provided the load impedance is high enough, or is also grounded to that rail), but only slowly. Using an FEt current source instead of a resistor can speed things up, but this adds complexity
8 where amplifier offset voltages and bias currents increase sharply due to the slight forward-biasing of parasitic p-n junctions. This occurs for input voltages within approximately 1V of either supply rail. When both differential pairs are active throughout the entire input common-mode range, amplifier transient response is faster through the middle of the commonmode range by as much as a factor of 2 for bipolar input stages and by a factor of the square root of 2 for FET input stages. Input stage transconductance determines the slew rate and the unity-gain crossover frequency of the amplifier, hence response time degrades slightly at the extremes of the input common-mode range when either the PNP stage (signals approaching VPOS) or the NPN stage (signals approaching GND) are forced into cutoff. The thresholds at which the transconductance changes occur approximately within 1V of either supply rail, and the behavior is similar to that of the input bias currents. Applications which initially appear to require true rail-rail inputs should be carefully evaluated, and the amplifier chosen to ensure that its input offset voltage, input bias current, common-mode rejection, and noise (voltage and current) are suitable. A true rail-to-rail input amplifier should not generally be used if an input range which includes only one rail is satisfactory. SINGLE-SUPPLY/RAIL-TO-RAIL OP AMP OUTPUT STAGES The earliest IC op amp output stages were NPN emitter followers with NPN current sources or resistive pull-downs, as shown in Figure 1.6. Naturally, the slew rates were greater for positive-going than for negative-going signals. While all modern op amps have push-pull output stages of some sort, many are still asymmetrical, and have a greater slew rate in one direction than the other. This asymmetry, which generally results from the use of IC processes with better NPN than PNP transistors, may also result in the ability of the output to approach one supply more closely than the other. In many applications, the output is required to swing only to one rail, usually the negative rail (i.e., ground in single-supply systems). A pulldown resistor to the negative rail will allow the output to approach that rail (provided the load impedance is high enough, or is also grounded to that rail), but only slowly. Using an FET current source instead of a resistor can speed things up, but this adds complexity
OP AMP OUTPUT STAGES USING COMPLEMENTARY DEVICES ALLOW PUSH-PULL DRIVE NPN NPN NPN ONLY CLASS A NPN/PNP CLASS B Figure 1.6 An IC process with relatively well-matched (AC and dc)PNP and NPn transistors lows both the output voltage swing and slew rate to be reasonably well matched. However, an output stage using B Ts cannot swing completely to the rails, but only to within the transistor saturation voltage(V cesaR of the rails(see Figure 1.7) For small amounts of load current (less than 100u A), the saturation voltage may be as low as 5 to 10mv, but for higher load currents, the saturation voltage can increase to several hundred mv(for example, 500mv at 50mA) On the other hand, an output stage constructed of CMOS FETs can provide true rail-to-rail performance, but only under no-load conditions. If the output must source or sink current, the output swing is reduced by the voltage dropped across the FEts internal"on"resistance(typically, 100ohms)
9 OP AMP OUTPUT STAGES USING COMPLEMENTARY DEVICES ALLOW PUSH-PULL DRIVE Figure 1.6 An IC process with relatively well-matched (AC and DC) PNP and NPN transistors allows both the output voltage swing and slew rate to be reasonably well matched. However, an output stage using BJTs cannot swing completely to the rails, but only to within the transistor saturation voltage (VCESAT) of the rails (see Figure 1.7). For small amounts of load current (less than 100µA), the saturation voltage may be as low as 5 to 10mV, but for higher load currents, the saturation voltage can increase to several hundred mV (for example, 500mV at 50mA). On the other hand, an output stage constructed of CMOS FETs can provide true rail-to-rail performance, but only under no-load conditions. If the output must source or sink current, the output swing is reduced by the voltage dropped across the FETs internal "on" resistance (typically, 100ohms)
RAIL-TO-RAIL OUTPUT STAGE SWING IS LIMITED BY Vcesat, Ron, AND LOAD CURRENT NMOS SWINGS TO RAILS LIMITED SWINGS T。 RAILS LIMITED BY SATURATION VOLTAGE BY FET" ON"RESISTANCE (-1002 Figure 1.7 In summary, the following points should be considered when selecting amplifiers for single-supply/rail-to-rail applications First, input offset voltage and input bias currents can be a function of the applied input common- mode voltage (for true rail-to-rail input op amps). Circuits using this class of amplifiers should be designed to minimize resulting errors. An inverting amplifier configuration with a false ground reference at the non- inverting input prevents these errors by holding the input common-mode voltage constant. If the inverting amplifier configuration cannot be used, then amplifiers like the OP284/0P484 which do not exhibit any common-mode crossover thresholds should use econd, since input bias currents are not always small and can exhibit different polarities, source impedance levels should be carefully matched to minimize additional input bias current-induced offset voltages and increased distortion. Again, consider using amplifiers that exhibit a smooth input bias current transition throughout the applied input common-mode voltage Third, rail-to-rail amplifier output stages exhibit load-dependent gain which affects amplifier open-loop gain, and hence closed-loop gain accuracy. Amplifiers with open-loop gains greater than 30,000 for resistive loads less than 10kohm are good choices in precision applications. For applications not requiring full rail-rail swings, device families like the OPX13 and OPX93 offer DC gains of 0. 2V/u V or more Lastly, no matter what claims are made, rail-to-rail output voltage swings are functions of the amplifier's output stage devices and load current. The saturation voltage(VCESAT), saturation resistance(RsaT), and load current all affect the amplifier output voltage swing
10 RAIL-TO-RAIL OUTPUT STAGE SWING IS LIMITED BY Vcesat, Ron, AND LOAD CURRENT Figure 1.7 In summary, the following points should be considered when selecting amplifiers for single-supply/rail-to-rail applications: First, input offset voltage and input bias currents can be a function of the applied input common-mode voltage (for true rail-to-rail input op amps). Circuits using this class of amplifiers should be designed to minimize resulting errors. An inverting amplifier configuration with a false ground reference at the non-inverting input prevents these errors by holding the input common-mode voltage constant. If the inverting amplifier configuration cannot be used, then amplifiers like the OP284/OP484 which do not exhibit any common-mode crossover thresholds should be used. Second, since input bias currents are not always small and can exhibit different polarities, source impedance levels should be carefully matched to minimize additional input bias current-induced offset voltages and increased distortion. Again, consider using amplifiers that exhibit a smooth input bias current transition throughout the applied input common-mode voltage. Third, rail-to-rail amplifier output stages exhibit load-dependent gain which affects amplifier open-loop gain, and hence closed-loop gain accuracy. Amplifiers with open-loop gains greater than 30,000 for resistive loads less than 10kohm are good choices in precision applications. For applications not requiring full rail-rail swings, device families like the OPX13 and OPX93 offer DC gains of 0.2V/µV or more. Lastly, no matter what claims are made, rail-to-rail output voltage swings are functions of the amplifier’s output stage devices and load current. The saturation voltage (VCESAT), saturation resistance (RSAT), and load current all affect the amplifier output voltage swing
