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1、大家好，欢迎来到 TI Precision Labs（德州仪器高精度实验室） 。本次视频将介绍运 算放大器的 Input & Output Limitations，即输入和输出限制。我们将会探讨运放的 Common-mode input voltage（共模输入电压） ，input and output voltage swing limitations（输入和输出电压摆幅限制） 。通过本节视频，你将学会判定电路误差是由 哪些限制引起的。 Hello, and welcome to the TI Precision Labs discussing op amp input and output
2、 limitations.In this video well discuss op amp common-mode input voltage, input and output voltage swing limitations, and show how to determine the source of circuit errors caused by these limitations. 首先，我们来看一个简单的 non-inverting buffer circuit （同相缓冲电路） ，也就是电 压跟随器。 同相输入端输入的是一个三角波信号， 幅度从-1.5V 到+1.5V。
3、正常情况下， 输出端将会得到一个一模一样的信号。但实际上由于某些原因，这个运放的输出不可 能超过 1V。这种非线性就叫做 clipping （“削波”） 。 是什么引起了这种“削波”现象呢？稍后我们会回答这个问题，现在我们先要明确一些 术语的定义。 Lets start by considering this simple non-inverting buffer circuit.An triangle-wave input signal of +/- 1.5V is applied to the non-inverting input, and one might expect the ou
4、tput to look exactly the same. For some reason, the op amp output does not increase past +1V. This type of nonlinearity is called “clipping.” What is causing this clipping behavior? Well answer this question later in the presentation, but first lets define some terms that are necessary to properly u
5、nderstand this issue. Common mode voltage（共模电压）是指放大器两个输入端的平均电压。对于运放来 说， 它的两个输入端电平几乎是一样的， 两者只相差一个很小的 offset （失调电压值） ， 所以你在每一个输入端上都能看到这个共模信号。 Common mode voltage is defined as the average voltage which is applied to the two inputs of an amplifier.In the case of an op amp, the two inputs are at the pra
6、ctically same potential, with only a small offset between them.So, effectively you can see the common mode voltage on either input. 共模输入电压范围也叫 input voltage swing（输入电压摆幅） 。它定义了放大器正常线 性工作所需的输入共模电压范围。共模输入电压范围通常是相对于正电源和负电源而 定义的。如果超过这个共模输入范围，输出信号会变成非线性。 Output voltage swing（输出电压摆幅）则是指输出信号线性工作时的输出电压范围。 输
7、出摆幅同样也是相对于供电电源而定义的。如果超过运放的输出摆幅参数指标，输 出信号将会失真或者出现非线性。 Common mode input voltage range is also known as input voltage swing.This term describes the range of input common mode voltages that can be used for normal linear operation of the amplifier.The common mode input voltage range is always defined re
8、lative to the positive supply and the negative supply.When you exceed the common mode input range, the output becomes nonlinear. Output voltage swing is the range of output voltages that allow for linear operation of output signals.Output swing is also defined relative to the power supplies.The outp
9、ut signal becomes distorted and non-linear if you exceed the op amps output swing specifications. 现在我们看一看在数据手册中共模电压和输入输出电压摆幅是如何定义的。 共模电压范围的最小值和最大值是相对于供电电源定义的。如图所示，负电源 V-是 0V，则共模输入电压的最小值为 0V 减去 0.1V， 即-0.1V。正电源 V+是 5V，则 5V 减去 3.5V，我们得到共模输入电压的最大值是 1.5V。因此，只要共模输入电压小于 -0.1V 或者大于 1.5V，输出信号都会变成非线性。 输出摆幅如表
10、中所示，同样地，它也是相对于供电电源定义的。最小输出电压是 V-+0.2V，因为这里负电源 V-是 0V，所以最小输出电压为 0.2V，而最大输出电压是 V+-0.2V，即 4.8V。任何低于 0.2V 或者高于 4.8V 的输出都会导致非线性。 Lets look at how common mode voltage and input and output voltage swing are typically defined on a data sheet. The common mode voltage range is defined here with the minimum an
11、d maximum limits given relative to the power supplies.The negative supply, V-, is zero volts in this case, so zero volts minus 0.1V gives us -0.1V for the minimum common mode limitation.The positive supply, V+, is 5V, so 5V minus 3.5V gives us 1.5V for the maximum common mode limitation.Therefore, a
12、pplying an input common mode voltage below -0.1V or above 1.5V will result in nonlinear output. The output swing is given here, and its the same type of definition which is relative to the supply voltages.The minimum output voltage is V- + 0.2V, or 0.2V in this case, and the maximum output voltage i
13、s V+ - 0.2V, or 4.8V. Driving the output below 0.2V or above 4.8V will cause the output to become nonlinear. 现在我们来探讨一下是放大器的什么因素导致了输入输出限制。 左图显示的是一个典型的 CMOS 输入级，随着共模输入信号接近正电源或者负电源， 输入晶体管会相应地 saturate（饱和）或 cutoff（截止） ，这些都是非线性工作的状态， 意味着此时放大器不能线性放大输入信号。这就是引起共模输入电压限制的原因。大 家需要记住的是，一些 CMOS 放大器的共模限制可能很接近甚至超过
14、供电电源轨。 输出级的电压摆幅限制则是由于内部晶体管的饱和和压降引起的。 CMOS 放大器的输 出电压限制比较小，因为 CMOS 晶体管的饱和电压比较低。 我们通常所说的 rail-to-rail amplifiers （“轨对轨放大器”）指的是，共模输入电压范围 可以达到供电电源的两个轨、并且输出电压接近供电电源轨的放大器。 Lets discuss what elements inside the amplifier actually cause the input and output limitations. On the left you can see a typical CMOS
15、 input stage.As the common mode input signal approaches either the positive or negative supply, the input transistors will either saturate or cutoff.Saturation and cutoff are both nonlinear modes of operation, so the amplifier cannot linearly amplify the input signal.This is what causes the common m
16、ode input voltage limitation.Please keep in mind that some CMOS amplifiers have common mode limitations which are very near or even beyond the power supply rails. The output stage voltage swing limitation caused by the saturation and diode drops on internal transistors.CMOS amplifiers tend to have better output voltage swing limitations, because CMOS transistors can have lower saturation voltages. Amplifiers which can accept common mode input voltage ranges up to
