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