运算放大器单电源供电

One of the most common applications questions on opera- tional amplifiers concerns operation from a single supply voltage. “Can the model OPAxyz be operated from a single supply?” The answer is almost always yes. Operation of op amps from single supply voltages is useful when negative supply voltages are not available. Furthermore, certain ap- plications using high voltage and high current op amps can derive important benefits from single supply operation. Consider the basic op amp connection shown in Figure la. It is powered from a dual supply (also called a balanced or split supply). Note that there is no ground connection to the op amp. In fact, it could be said that the op amp doesn’t know where ground potential is. Ground potential is some- where between the positive and negative power supply voltages, but the op amp has no electrical connection to tell it exactly where. VIN VOUT = VIN G = +1 +VS = 15V –VS = 15V VIN VOUT = VIN G = 1 +VS = 30V (a) (b) FIGURE 1. A simple unity-gain buffer connection of an op amp illustrates the similarity of split-supply op- eration (a) to single-supply operation in (b). The circuit shown is connected as a voltage follower, so the output voltage is equal to the input voltage. Of course, there are limits to the ability of the output to follow the input. As the input voltage swings positively, the output at some point near the positive power supply will be unable to follow the input. Similarly the negative output swing will be limited to somewhere close to –VS. A typical op amp might allow output to swing within 2V of the power supply, making it possible to output –13V to +13V with ±15V supplies. Figure 1b shows the same unity-gain follower operated from a single 30V power supply. The op amp still has a total of 30V across the power supply terminals, but in this case it comes from a single positive supply. Operation is otherwise unchanged. The output is capable of following the input as long as the input comes no closer than 2V from either supply terminal of the op amp. The usable range of the circuit shown would be from +2V to +28V. Any op amp would be capable of this type of single-supply operation (with somewhat different swing limits). Why then are some op amps specifically touted for single supply applications? Sometimes, the limit on output swing near ground (the “negative” power supply to the op amp) poses a significant limitation. Figure 1b shows an application where the input signal is referenced to ground. In this case, input signals of less than 2V will not be accurately handled by the op amp. A “single-supply op amp” would handle this particular application more successfully. There are, however, many ways to use a standard op amp in single-supply applications which may lead to better overall performance. The key to these applications is in understanding the limitations of op amps when handling voltages near their power supplies. There are two possible causes for the inability of a standard op amp to function near ground in Figure 1b. They are (1) limited common-mode range and (2) output voltage swing capability. These performance characteristics are easily visualized with the graphical representation shown in Figure 2. The range over which a given op amp properly functions is shown in relationship to the power supply voltage. The common- mode range, for instance, is sometimes shown plotted with respect to another parameter such as temperature. A ±15V supply is assumed in the preparation of this plot, but it is easy to imagine the negative supply as being ground. In Figure 2a, notice that the op amp has a common-mode range of –13V to +13.5V. For voltages on the input termi- nals of the op amp of more negative than –13V or more positive than +13.5V, the differential input stage ceases to properly function. Similarly, the output stages of the op amp will have limits on output swing close to the supply voltage. This will be load- dependent and perhaps temperature-dependent also. Figure 2b shows output swing ability of an op amp plotted with respect to load current. It shows an output swing capability of –13.8V to +12.8V for a l0kΩ load (approximately ±1mA) at 25°C. ® ©1986 Burr-Brown Corporation AB-067 Printed in U.S.A. March, 1986 SINGLE-SUPPLY OPERATION OF OPERATIONAL AMPLIFIERS SBOA059 One of the most common applications questions on opera- tional amplifiers concerns operation from a single supply voltage. “Can the model OPAxyz be operated from a single supply?” The answer is almost always yes. Operation of op amps from single supply voltages is useful when negative supply voltages are not available. Furthermore, certain ap- plications using high voltage and high current op amps can derive important benefits from single supply operation. Consider the basic op amp connection shown in Figure la. It is powered from a dual supply (also called a balanced or split supply). Note that there is no ground connection to the op amp. In fact, it could be said that the op amp doesn’t know where ground potential is. Ground potential is some- where between the positive and negative power supply voltages, but the op amp has no electrical connection to tell it exactly where. VIN VOUT = VIN G = +1 +VS = 15V –VS = 15V VIN VOUT = VIN G = 1 +VS = 30V (a) (b) FIGURE 1. A simple unity-gain buffer connection of an op amp illustrates the similarity of split-supply op- eration (a) to single-supply operation in (b). The circuit shown is connected as a voltage follower, so the output voltage is equal to the input voltage. Of course, there are limits to the ability of the output to follow the input. As the input voltage swings positively, the output at some point near the positive power supply will be unable to follow the input. Similarly the negative output swing will be limited to somewhere close to –VS. A typical op amp might allow output to swing within 2V of the power supply, making it possible to output –13V to +13V with ±15V supplies. Figure 1b shows the same unity-gain follower operated from a single 30V power supply. The op amp still has a total of 30V across the power supply terminals, but in this case it comes from a single positive supply. Operation is otherwise unchanged. The output is capable of following the input as long as the input comes no closer than 2V from either supply terminal of the op amp. The usable range of the circuit shown would be from +2V to +28V. Any op amp would be capable of this type of single-supply operation (with somewhat different swing limits). Why then are some op amps specifically touted for single supply applications? Sometimes, the limit on output swing near ground (the “negative” power supply to the op amp) poses a significant limitation. Figure 1b shows an application where the input signal is referenced to ground. In this case, input signals of less than 2V will not be accurately handled by the op amp. A “single-supply op amp” would handle this particular application more successfully. There are, however, many ways to use a standard op amp in single-supply applications which may lead to better overall performance. The key to these applications is in understanding the limitations of op amps when handling voltages near their power supplies. There are two possible causes for the inability of a standard op amp to function near ground in Figure 1b. They are (1) limited common-mode range and (2) output voltage swing capability. These performance characteristics are easily visualized with the graphical representation shown in Figure 2. The range over which a given op amp properly functions is shown in relationship to the power supply voltage. The common- mode range, for instance, is sometimes shown plotted with respect to another parameter such as temperature. A ±15V supply is assumed in the preparation of this plot, but it is easy to imagine the negative supply as being ground. In Figure 2a, notice that the op amp has a common-mode range of –13V to +13.5V. For voltages on the input termi- nals of the op amp of more negative than –13V or more positive than +13.5V, the differential input stage ceases to properly function. Similarly, the output stages of the op amp will have limits on output swing close to the supply voltage. This will be load- dependent and perhaps temperature-dependent also. Figure 2b shows output swing ability of an op amp plotted with respect to load current. It shows an output swing capability of –13.8V to +12.8V for a l0kΩ load (approximately ±1mA) at 25°C. ® ©1986 Burr-Brown Corporation AB-067 Printed in U.S.A. March, 1986 SINGLE-SUPPLY OPERATION OF OPERATIONAL AMPLIFIERS SBOA059 运算放大器的单电源供电 单电源电压供电是运算放大器最常见的应用问题之 一。当问及“型号为OPAxyz， 能否采用单电源供 电？”， 答案 八年级地理上册填图题岩土工程勘察试题省略号的作用及举例应急救援安全知识车间5s试题及答案 通常是肯定的。在不启用负相电源电压 时，采用单电源电压驱动运算放大器是可行的。并 且，对使用高电压及大电流运算放大器的特定应用而 言，采用单电源供电将使其切实的获益。 考虑如图 1a 所示的基本运算放大器连线图。运算放大 器采用了双电源供电（也称平衡[balanced]电源或分离 [split]电源）。注意到此处运算放大器无接地。而事实 上，可以说运算并不会确认地电位的所在。地电位介 于正相电压及负相电压之间，但运算放大器并不具有 电气接线端以确定其确切的位置。 图 1 所示电路连接为电压跟随器，因此输出电压与输 入电压相等。当然，输出跟随输入的能力是有限的。 随着输入电压正相摆幅的增大，在某些接近正相电源 的电位点上，输出将无法跟随输入。类似的，负相输 出摆幅也限制在靠近 –Vs 的某电位点上。典型的运算 放大器允许输出摆幅在电源轨的 2 V 以内，使得 ± 15V 的电源可支持 –13V 至 +13V 的输出。 图 1b 展示了同样的单位增益跟随器，采用 30 V 单电 源支持供电。运算放大器的两个电源接线端之间的总 电压仍为 30 V，但此时采用了单正相电源。从另一角 度考虑，其运行状态是不变的。只要输入介于运算放 大器电源接线端电压 2 V 以内，输入就能跟随输入。 电路可支持的输出范围从 +2V 至 +28V。 既然任意的运算放大器均能支持此类单电源供电（仅 是摆幅限制稍有不同），为何某些运算放大器特别注 明用于单电源应用呢？ 某些时候，输出摆幅在地电平（运算放大器的“负 相”电源轨）附近受到了极大的限制。如图 1b 所示， 应用的输入信号参考地电平，此时，运算放大器将无 法准确的处理小于 2 V的输入信号。而“单电源运算放 大器”则能很好的应对此类特殊的应用。尽管如此， 仍可采用许多不同的方式将 标准 excel标准偏差excel标准偏差函数exl标准差函数国标检验抽样标准表免费下载红头文件格式标准下载 的运算放大器用于单 电源应用中，并实现较好的总体性能。应对此类应用 的关键即在于对运算放大器的局限性（对其电源轨附 近的电压信号进行处理之时）的理解。 如图 1b 所示，导致标准运算放大器无法处理地电平附 近信号的原因有两个：(1)共模范围限制；(2)输出电压 摆幅能力。 此类性能特点通过图 2 的图形描述得到了很直观的展 示。电压范围与电源电压相关，指示了给定运算放大 器的正常功能。以共模范围为例，通常采用图示说明 其与诸如温度等参数的关系。图 2 假定的电源为 ± 15V，但还是很容易将负相电源轨设想为地。在图 2a 中，注意到运算放大器的共模范围为 -13 V 至 +13.5 V。对于运算放大器输入端低于 -13 V 或高于 +13.5 V 的电压信号而言，差分输入级将无法正常运作。 类似的，运算放大器的输出级也会将输出摆幅限制在 电源电压附近。此项特性取决于负载，并同时取决于 外设的温度。图 2b 展示了运算放大器的输出摆幅性能 与负载电流的关系曲线。在25°C 时，对于 l0kW 的 负载（电流约为 ±1mA）而言，输出摆幅为 –13.8V 至 +12.8V。 图 1. 简易单位增益缓冲器的运算放大器连线示意图，举例说 明了分离电源供电(a)与单电源供电(b)的相似性。 ZHCA054 ©1986 Burr-Brown Corporation Printed in U.S.A. March, 1986AB-067 2 2 FIGURE 2. The Common-mode Range of an Op Amp is Usually Dependent on Temperature. This be- havior is shown plotted in (a). Output voltage swing will be affected by output current. (b). Often the op amp load is connected to ground, so load current is always positive. Furthermore, as the output voltage approaches zero, load current approaches zero, increasing the available output swing. A split power supply voltage (normally ±15V) is assumed in preparation of these plots. 125 15 10 5 0 –5 –10 –15 15 10 5 0 –5 –10 –15 –10 –8 –6 –4 –2 0 2 4 6 8 10 Output Current (mA) O ut pu t V ol ta ge (V ) Temperature (°C) C om m on -M od e V ol ta ge (V ) –50 –25 0 25 50 75 100(a) (b) So the circuit of Figure 1b is limited to +13V output by output swing capability and –13V by negative common- mode range. A single-supply op amp is specifically designed to have a common-mode range which extends all the way to the negative supply (ground). Also, its output stage is usu- ally designed to swing close to ground. It would be convenient if all op amps were designed to have this capability, but significant compromises must be made to achieve these goals. Increased common-mode range, for instance, often comes at the sacrifice of performance charac- teristics such as offset voltage, offset drift, and noise. Gen- eral purpose applications may tolerate op amp performance with these compromises, but high accuracy or other special purpose applications may require a different approach. Fortunately, there are many ways to use high performance and special purpose op amps in single-supply applications. As demonstrated in Figure 1b, an op amp with typical common mode and output characteristics functions well on a single supply as long as the input and output voltages are constrained to the necessary limits. Circuit configurations must be used which operate within these limits. Figure 3 shows a circuit, for instance, which references the input and output to a “floating ground” created with a zener diode. The zener diode is biased with a current set by RZ. Since VIN and VOUT are both referenced to the same floating ground, the zener voltage accuracy or stability is not critical. VIN and VOUT can now be bipolar signals (with respect to floating ground). With +V = 30V and VZ = 15V, operation is similar to standard split supply operation. The load current in this circuit, however, flows to the floating ground where it will add to the zener diode current (negative load currents subtract from zener current). The zener diode must be selected to handle this additional current. If the zener current is allowed to approach zero, the floating ground voltage will fall rapidly as the zener turns off. Rl must be selected so that the zener diode current remains positive under all op amp load conditions. FIGURE 3. Bipolar Signals Can be Handled When Input and Output are Referenced to a Floating Ground. Changing load current causes a variation in zener current which must be evaluated. +VS = 12V RLOAD = RL || (RI + RF) R1 4.7kΩ RF 47kΩ RL VZ = 5.6V G = –RF/R1 = –4.7 VIN IL R1 4.7kΩ RZ VOUT + – Figure 4 shows operation in a noninverting gain configura- tion. In this circuit, the feedback components present an additional load to the op amp equal to the sum of the two resistors. This current must also be considered when plan- ning for the variation in current flowing in the zener diode. Again, the zener current should not be allowed to approach zero or exceed a safe value. Notice that in this example, a single +12V supply is shown. Often, single-supply applications use supply voltages which are considerably less than the 30V total (±15V) at which the performance of most op amps is specified. While modern op amps generally perform well at less than their character- ized voltage, this needs to be verified. Some op amps, although they are specified to operate at lower voltage, 2 FIGURE 2. The Common-mode Range of an Op Amp is Usually Dependent on Temperature. This be- havior is shown plotted in (a). Output voltage swing will be affected by output current. (b). Often the op amp load is connected to ground, so load current is always positive. Furthermore, as the output voltage approaches zero, load current approaches zero, increasing the available output swing. A split power supply voltage (normally ±15V) is assumed in preparation of these plots. 125 15 10 5 0 –5 –10 –15 15 10 5 0 –5 –10 –15 –10 –8 –6 –4 –2 0 2 4 6 8 10 Output Current (mA) O ut pu t V ol ta ge (V ) Temperature (°C) C om m on -M od e V ol ta ge (V ) –50 –25 0 25 50 75 100(a) (b) So the circuit of Figure 1b is limited to +13V output by output swing capability and –13V by negative common- mode range. A single-supply op amp is specifically designed to have a common-mode range which extends all the way to the negative supply (ground). Also, its output stage is usu- ally designed to swing close to ground. It would be convenient if all op amps were designed to have this capability, but significant compromises must be made to achieve these goals. Increased common-mode range, for instance, often comes at the sacrifice of performance charac- teristics such as offset voltage, offset drift, and noise. Gen- eral purpose applications may tolerate op amp performance with these compromises, but high accuracy or other special purpose applications may require a different approach. Fortunately, there are many ways to use high performance and special purpose op amps in single-supply applications. As demonstrated in Figure 1b, an op amp with typical common mode and output characteristics functions well on a single supply as long as the input and output voltages are constrained to the necessary limits. Circuit configurations must be used which operate within these limits. Figure 3 shows a circuit, for instance, which references the input and output to a “floating ground” created with a zener diode. The zener diode is biased with a current set by RZ. Since VIN and VOUT are both referenced to the same floating ground, the zener voltage accuracy or stability is not critical. VIN and VOUT can now be bipolar signals (with respect to floating ground). With +V = 30V and VZ = 15V, operation is similar to standard split supply operation. The load current in this circuit, however, flows to the floating ground where it will add to the zener diode current (negative load currents subtract from zener current). The zener diode must be selected to handle this additional current. If the zener current is allowed to approach zero, the floating ground voltage will fall rapidly as the zener turns off. Rl must be selected so that the zener diode current remains positive under all op amp load conditions. FIGURE 3. Bipolar Signals Can be Handled When Input and Output are Referenced to a Floating Ground. Changing load current causes a variation in zener current which must be evaluated. +VS = 12V RLOAD = RL || (RI + RF) R1 4.7kΩ RF 47kΩ RL VZ = 5.6V G = –RF/R1 = –4.7 VIN IL R1 4.7kΩ RZ VOUT + – Figure 4 shows operation in a noninverting gain configura- tion. In this circuit, the feedback components present an additional load to the op amp equal to the sum of the two resistors. This current must also be considered when plan- ning for the variation in current flowing in the zener diode. Again, the zener current should not be allowed to approach zero or exceed a safe value. Notice that in this example, a single +12V supply is shown. Often, single-supply applications use supply voltages which are considerably less than the 30V total (±15V) at which the performance of most op amps is specified. While modern op amps generally perform well at less than their character- ized voltage, this needs to be verified. Some op amps, although they are specified to operate at lower voltage, 图 2. 运算放大器的共模范围通常取决于温度，其特性如图 (a) 所示；输出电压则受输出电流的影响，如图 (b) 所 示。通常情况下，运算放大器的负载接地，其负载电流 始终为正。随着输出电压逼近零电位，负载电流也逐渐 归零，从而增大了有效输出摆幅。此处的图示假定为分 离电源电压（正常值为 ±15V）。 由上文可知，如图 1b 所示电路的上下限分别为 +13V （受限于输出摆幅性能）及 –13V（受限于负相共模范 围）。单电源运算放大器针对扩展的共模范围进行了 特别的设计，可始终包含负相电源轨（地）。同时， 其输出级也大都是针对接近地电平的摆幅而设计的。 倘若所有的运算放大器都设计为具有上述的扩展性 能，则将提供很大的便利，但须考虑的实现该目标需 要做极大的折衷。例如，增大共模范围通常需要牺牲 诸如偏置点电压、偏置点漂移及噪声等特性。一般的 应用或许能容忍此类运算放大器的性能折衷，但对于 高精度或其他特定用途的应用而言，则需要采用不同 的处理方法。 值得庆幸的是仍有多种方式可支持在单电源应用中使 用高性能及特定用途运算放大器。如图 1b 所示，运算 放大器具有典型的共模范围及输出特性，只要将输入 及输出电压约束在必要的限度以内，即可实现良好的 单电源运作。简言之，电路配置必须在适当的使用范 围内运作。 以图 3 所示的电路为例，输入及输出均以齐纳二极管 所形成的“浮动地(floating ground)”为参考。齐纳二 极管通过 Rz 偏置并设定偏置电流。由于 VIN 及 VOUT 都参考同一浮动地，齐纳电压的精度或稳定性已不是 最重要的。VIN 及 VOUT 均可采用双极型信号（以浮 动地为参考）。当 +V = 30V 且 VZ = 15V 之时，其 运行与采用标准分离电源的状况类似。在该电路中， 负载电流流入浮动地，与齐纳二极管的电流（等于齐 纳电流减去负相负载电流）相叠加。所选取的齐纳二 极管必须能应对该附加的电流。若齐纳电流允许接近 零，浮动地的电压将由于齐纳二极管的截止而快速降 低。因此，Rl 值必须小心选取以确保齐纳二极管电流 在任意运算放大器负载状况下都保持正向。 图 3. 当输入及输出均参考浮动地时，可对双极型信号进行处 理。改变负载电流将导致齐纳电流的变化，这是必须进 行估算的。 图 4 所示电路以放大器增益配置运作。在该电路中， 反馈元件施加于运算放大器的附加负载等于两电阻 (R1+Rf)之和。当设计流经齐纳二极管的电流变化量 时，此反馈电流也必须纳入考虑。此时，齐纳电流不 能为零，也不能超过安全值。 注意到该示例所采用的单电源为 +12 V，通常情况下， 单电源应用所采用的电源电压要大大低于绝大多数运 算放大器所规定的 30 V (±15V)的总体性能值。而现代 的运算放大器也通常是在小于其特征电压的范围内才 表现良好，这是需要验证的。某些运算放大器，尽管 3 3 FIGURE 5. Even Though the Impedance of the Voltage Divider is in Series with R1 to Ground, the Gain of this Noninverting Circuit is Determined Solely by R1 and R2. Since the input and output are referenced to the same floating ground, its im- pedance does not affect the voltage gain of the circuit. FIGURE 6. Many Systems Have a +5V Logic Supply or Other Appropriate Voltage Source which Can Be Used as a Floating Reference Potential for Analog Circuitry. Be sure logic noise does not enter the analog system by providing an ad- equate decoupling network or additional by- passing. +24V 0.22µF VIN G = 5.7 R1 10kΩ 1kΩ VOUT 1kΩ R2 47kΩ 10kΩ OPA27 33kΩ 0.1µF +12V G = –3.3 VI