运算放大器：输入和输出限制资料下载.pdf

1、我们将会探讨运放的Common-mode input voltage（共模输入电压），input and output voltage swinglimitations（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 outputlimitations.In this video well discuss op amp common-mode input voltage,inputand output voltage sw

2、ing limitations,and show how to determine the source ofcircuit 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。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

3、一些术语的定义。Lets start by considering this simple non-inverting buffer circuit.An triangle-waveinput signal of+/-1.5V is applied to the non-inverting input,and one might expectthe output to look exactly the same.For some reason,the op amp output does notincrease past+1V.This type of nonlinearity is call

4、ed“clipping.”What is causing this clipping behavior?Well answer this question later in thepresentation,but first lets define some terms that are necessary to properlyunderstand 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。C

5、ommon mode voltage is defined as the average voltage which is applied to the twoinputs of an amplifier.In the case of an op amp,the two inputs are at the practicallysame potential,with only a small offset between them.So,effectively you can seethe common mode voltage on either input.共模输入电压范围也叫 input

6、 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.Thisterm describes the range of i

7、nput common mode voltages that can be used fornormal linear operation of the amplifier.The common mode input voltage range isalways defined relative to the positive supply and the negative supply.When youexceed the common mode input range,the output becomes nonlinear.Output voltage swing is the rang

8、e of output voltages that allow for linear operation ofoutput signals.Output swing is also defined relative to the power supplies.Theoutput signal becomes distorted and non-linear if you exceed the op amps outputswing 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

9、 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

10、 output voltage swing aretypically defined on a data sheet.The common mode voltage range is defined here with the minimum and maximumlimits given relative to the power supplies.The negative supply,V-,is zero volts inthis case,so zero volts minus 0.1V gives us-0.1V for the minimum common modelimitati

11、on.The positive supply,V+,is 5V,so 5V minus 3.5V gives us 1.5V for themaximum common mode limitation.Therefore,applying an input common modevoltage 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 relativeto the supp

12、ly voltages.The minimum output voltage is V-+0.2V,or 0.2V in thiscase,and the maximum output voltage is V+-0.2V,or 4.8V.Driving the outputbelow 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 cut

13、off（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 act

14、ually cause the input and outputlimitations.On the left you can see a typical CMOS input stage.As the common mode inputsignal approaches either the positive or negative supply,the input transistors willeither saturate or cutoff.Saturation and cutoff are both nonlinear modes ofoperation,so the amplif

15、ier cannot linearly amplify the input signal.This is whatcauses the common mode input voltage limitation.Please keep in mind that someCMOS amplifiers have common mode limitations which are very near or evenbeyond the power supply rails.The output stage voltage swing limitation caused by the saturati

16、on and diode dropson internal transistors.CMOS amplifiers tend to have better output voltage swinglimitations,because CMOS transistors can have lower saturation voltages.Amplifiers which can accept common mode input voltage ranges up to the powersupply rails,and can swing the output voltage near the supply rails,are referred toas rail-to-rail amplifiers.下面给出两个电路的例子，两者有不同的共模输入考虑。左图显示的是 inverting configuration（反