An electric circuit (or network) is an interconnection of physical electrical device. The purpose of electric circuits is to distribute and convert energy into some other forms. Accordingly, the basic circuit components are an energy source (or sources), an energy converter (or converters) and conductors connecting them.
电路（或者网络）是物理电气设备的一种互相连接。电路的目的是为了将能量分配和转换到另外一种形式中。因此，基本的电路元件包括电源、电能转换器以及连接它们的导体。
An energy source (a primary or secondary cell, a generator and the like) converts chemical, mechanical, thermal or some other forms of energy into electric energy. An energy converter, also called load (such as a lamp, heating appliance or electric motor), converts electric energy into light, heat, mechanical work and so on.
电源（原生电池或者再生电池、发电机等类似装备）将化学能量、机械能量，热能或者其他形式的能量转换成电能。电能转换器（也称为负载，如灯泡、电热器或者电动机）将电能转换成光、热、机械运动等等。 Events in a circuit can be defined in terms of e.m.f. (or voltage) and current. When electric energy is generated, transmitted and converted under conditions such that the currents and voltages involved remain constant with time, one usually speaks of direct-current (D.C.) circuits.
电路属性可以根据电动势和电流来定义。当电能在产生、传输和变换时，若电路中相关的电流和电压不随时间而变化，我们便称其为直流电路
With time-invariant currents and voltages, the magnetic and electric fields of the associated electric plant are also time-invariant. This is the reason why no e.m.f.s of self-(or mutual-)induction appear in D.C. circuits, nor are there any displacement currents in the dielectric surrounding the conductors.
在电流和电压时不变的情况下，相关电气设备的电磁场也是时不变的。这就是为什么直流电路中没有自（互）感电动势、以及围绕在导体附近的电介质没有位移电流。
Fig.1.1 shows in simplified form a hypothetical circuit with a storage battery as the source and a lamp as the load. The terminals of the source and load are interconnected by conductors (generally but not always wires).
图1.1给出了一个假设电路的简单形式：一个蓄电池作为电源、以及一个灯泡作为负载。电源和负载的终端用导体互相连接，通常这种导体是导线，但少数情况下也有例外。
As is seen, the source, load and conductors form a closed conducting path. The e.m.f. of the source causes a continuous and unidirectional current to circulate round this closed path.
如图所示，电源、负载和导体构成了一个闭合导电回路。电源的电动势产生一个连续的单方向电流在闭合回路中流通。
The simple circuit made up of a source, a load and two wires is seldom, if ever, met with in practice. Practical circuits may contain a large number of sources and loads interconnected in a variety of ways.
这种由一个电源、一个负载和两根导线组成的简单电路在实践中即使有时能遇到，也是很少见的。实际的电路包括很多用不同方法连接起来的电源和负载。
To simplify analysis of actual circuits, it is usual to show them symbolically in a diagram called a circuit diagram, which is in fact a fictitious or, rather, idealized model of an actual circuit of network. Such a diagram consists of interconnected symbols called circuit elements or circuit parameters.
为了简化分析，通常用电路图来象征性地表示实际的电路。实际上，电路图是实际电路的一个假设模型，或相当于一个理想模型。这样的电路图包括电路元件或者电路参数等互联符号。
Two elements are necessary to represent processes in a D.C. circuit. These are a source of e.m.f. E and of internal (or ―source‖) resistance RS, and the load resistance (which includes the resistance of the conductors) R (Fig.1.2). 在直流电路中，有两个元件是有必要描绘出来处理的。这就是电源的电动势E、内阻RS，以及负载电阻R（其中包含了导体电阻）。
Whatever its origin (thermal, contact, etc.), the source e.m.f. E(Fig.1.2(a)) is numerically equal to the potential difference between terminals 1 and 2 with the external circuit open, that is, when there is no current flowing through the source.
无论图1.2(a)中的电动势E的原动力是什么（即不论是热的、机械的还是其它什么形式），其大小就等于1、2两端之间的开路电压，也就是电源没有电流通过的情况。
The source e.m.f. is directed from the terminal at a lower potential to that at a higher one. On diagram, this is shown by arrows.
电动势从较低电压端指向较高电压端，这在图表中用箭头表示。
When a load is connected to the source terminals (the circuit is then said to be loaded) and the circuit is closed, a current begins to flow round it. Now the voltage between source terminals 1 and 2 (called the terminal voltage) is not equal to its e.m.f. because of the voltage drop VS inside the source, that across the source resistance RS .
当一个负载加在电源两端（也就是说电路被加上负载），同时电路闭合，就有电流开始流通。这时在电源1、2两端之间的电压（称为端电压）不等同于电源的电动势，因为这时电源内阻RS两端将产生电压降VS。 Fig.1.3 shows a typical so-called external characteristic of a loaded source (hence another name is the load characteristic of a source). As is seen, increase of current from zero to I~I1 causes the terminal voltage of the source to decrease linearly
图1.3表示带负载电源的一个典型的外特性（由于电源带负载，所以也称为电源的负载特性）。如图所示，当电流从0增加到I1时，引起电源端电压的线性减小。
In other words, the voltage drop VS across the source resistance rises in proportion to the current. This goes on until a certain limit is reached. Then as the current keeps rising, the proportionality between its value and the voltage drop across the source is upset, and the external characteristic ceases to be linear.
换句话说，横跨电源内阻的电压降与电流成比例的增长，这种增长趋势一直持续到一个特定的限值。然后当电流继续增加时，电流值与横过电源的电压降之间的比例关系被扰乱，使得外特性终止线性关系。
This decrease in voltage may be caused by a reduction in the source voltage, by an increase in the internal resistance, or both. The power delivered by a source is given by the equality PS=EI, where PS is the power of the source.
电源1、2两端电压的减小可能是由于电源电压的减小或电源内阻的增加引起的，也可能是两种情况共同引起的。电源提供的功率由等式PS=EI表示，其中PS是电源的功率。
It seems relevant at this point to dispel a common misconception about power. Thus one may hear that power is generated, delivered, consumed, transmitted, lost, etc. In point of fact, however, it is energy that can be generated, delivered, consumed, transmitted or lost.
这个公式恰当地消除了对功率的一个普遍的误解。例如有人可能听过功率被产生、释放、消耗、传输、损耗等等。然而，实际上是能量才能够被产生、释放、消耗、传输或损耗。
Power is just the rate of energy input or conversion, that is, the quantity of energy generated, delivered, transmitted etc per unit time. So, it would be more correct to use the term energy instead of power in the above context. Yet, we would rather fall in with the tradition.
功率只是能量输入或者转换的比率，也就是说，单位时间内产生能量、释放能量、传输能量的数量。因此，在上文中用能量这个术语代替功率会更准确些。但是，我们一般采用传统的说法。
The load resistance R(Fig.1.2(b)), as a generalized circuit element, gives an idea about the consumption of energy, that is, the conversion of electric energy into heat, and is defined as P=I2R.In the general case, the load resistance depends solely on the current through the load, which in fact is symbolized by the function R(I).
负载电阻R作为一个普遍的电路元件，给出了能量消耗的概念，也就是电能转化成热能，定义为P=I2R。一般情况下，负载电阻只由流过负载的电流决定，用方程R(I)表示。
By Ohm‘s law, the voltage across a resistance is V=RI.In circuit analysis, use is often made of the reciprocal of the resistance, termed the conductance, which is defined as g=1/R.
从欧姆定律可知，电阻两端的电压表示为V=RI。在电路分析中，经常使用电阻的倒数，称为电导，定义为g=1/R.
In practical problems, one often specifies the voltage across a resistance as a function of current V(I),or the inverse relation I(V) have come to be known as volt-ampere characteristics. 在实际问题中，通常规定横跨电阻的电压为关于电流的方程V(I),或者是反比关系I(V)，这就是众所周知的伏安特性。
Fig.1.4 shows volt-ampere curves for a metal-filament lamp V1(I), and for a carbon-filament lamp V2(I). As is seen, the relation between the voltage and the current in each lamp is other than linear （nonlinear）. The resistance of the metal-filament lamp increases (with increase of current), and that of the carbon-filament lamp decreases with increase of current.
图1.4给出了金属丝灯泡和碳丝灯泡的伏安曲线。如图所示，每个灯泡的电压和电流关系并不是线性的，当电流升高时，金属丝灯泡的电阻增大，碳丝灯泡的内阻减小。
Electric circuits containing components with non-linear characteristic are called non-linear.
If the e.m.f and internal resistances of sources and associated load resistances are assumed to be independent of the current and voltage, respectively, the external characteristic V(I) of the sources and the volt-ampere characteristic V1(I) of the loads will be linear(Fig.1.5).
包含非线性特性元件的电路称为非线性电路。如果假定电源的电动势和内阻以及相连的负载电阻分别与电流和电压无关，那么电源的外特性V(I)和负载的伏安特性V1(I)将是线性的（如图1.5所示）。
Electric circuits containing only elements with linear characteristic are called linear.Most practical circuits may be classed as linear. Therefore, a study into the properties and analysis of linear circuits is of both theoretical and applied interest.
只包含具有线性特性的元件的电路称为线性电路。很多应用电路都归类为线性电路，因此，对线性电路的特性和分析的学习具有理论和实际应用的双重意义。
An operational amplifier may be treated as a single electronic component with input and output terminals, rather as transistor is. The amplifier itself consists of a number of transistor stages such as those described in other lesson, fabricated and interconnected on a single substrate, and the user has access to a limited number of terminal points. Thus, of most interest from the applications point of view are the terminal characteristics.
运算放大器可以看做一个具有输入端口和输出端口的单个电子元件，就像晶体管那样。放大其本身由多级晶体管组成，这样的多极晶体管在前述课程中已经介绍，他们制作和连接在一块基板上，有几个引出端可供客户使用。从使用的角度看，人们感兴趣的是端口特性。
The name operational amplifier came about from the use to which early versions of the amplifiers were put, which was to provide electronic analogs of mathematical operations such as addition, Subtraction, Multiplication, Integration etc. Present-day usage is very much wider scope but the popular name ―op-amp‖ persists. 运算放大器这个名字起源于早期放大器的用途，他被用来进行一些电子模拟数学运算，例如，加减乘积分等，如今它的使用范围已大大扩展，但运算放大器这个通用名字却流传下来。
The operational voltage amplifier is represented schematically by the triangular symbol. A0 is the voltage gain from differential input to single-ended output and is always a positive number. Phase reversals are taken into account at the input terminals, which is the reason why these are labeled + and -. The voltage at each terminal, including the output, may be referred to common reference, usually ground, and unless otherwise stated, this common reference will be assumed. Thus, letting V(+) represent the voltage of the positive input terminal, we may define the differential input voltage as Vid=V(+)-V(-), and the output voltage is V0=A0Vid. If, however, the differential input voltage is defined as Vid=V(-)-V(+), the output voltage is V0=-A0Vid.
电压运算放大器可用三角形符号来表示。A0表示从差动输入端到单一输出端的电压增益，并恒为正值，考虑到在输入端可能会有反向输入的情况，所以要标上+号与-号。每个端口包括输出端口的电压，都可以选一个共同的参考点，通常选大地。除非另有说明，否则所假定的参考点就是地。这样用V+代表正相输入端对参考点的电压，而V—则代表负相输入点对参考点的电压，我们可以将差动电压定义为Vid=V(+)+V(-),输出电压V0=A0Vid。然而，如果差动输入电压定为Vid=V(-)-V(+);则输出电压V0=-A0Vid。
Because no phase reversal takes place in the circuit, the positive input terminal is termed the noninverting terminal, and because a phase reversal dose take place in the circuit, the negative input terminal is termed the inverting terminal. These terminals will always be specified in the manufacturer‘s data sheet for an amplifier. One or other of the input terminals may be connected to the common line, and the phase relationships still hold as shown. Note that the inverting terminal convention does not mean that the positive input must be connected to the noninverting and the negative input to the inverting; the amplifier may be used either way up, so to speak.
因为当信号从正输入端输入时电路不发生反相，所以称之为同相端，而从负输入端输入时，电路发生反相，因此称之为反相端。这些端口在放大器的铭牌上已注明。输入端口的一端或其他的端都可连到参考点，可以看出其他相位关系仍保持不变。注意，称反相端和同相端的惯例，并不意味着正信号输入一定要与同相端相连，而负信号输入一定要与反相端相连，可以说放大器可以采取其中任何一种使用方式。
The basic equivalent circuit for the operational voltage amplifier is shown in fig. Ri is the differential input resistance, and will always be high, usually in the megohm range. High input resistance may be achieved through the use of Darlington connected pairs in the input stage , or by using an FET differential input pair, as described in other lesson. R0 is the output resistance, and always be low, usually less then 100, for the operational voltage amplifier. A0 is the voltage gain as defined in the previous section, and this will always be large, of the order of 10^6:1. For many applications, the amplifier may be represented by an voltage amplifier, for which ??
电压放大器的基本等值电路如图所示。Ri为差动输入电阻，切总是很大，经常在兆欧级范围内。高输入电阻可以通过在输入端使用达林顿连接对来实现，或如前所述，通过使用一个FET差动输入对来实现。R0为输出电阻，并且总是很小。对电压运算放大器来说，通常不到100欧。A0为前几节所定义的电压放大倍数，他总是很大。大约属于10^6:1的数量级。在许多实际应用中，放大器可用一理想放大器来代表，这时： The concept of infinite voltage gain, or , for that matter, a very high but finite voltage gain, requires some explanation,. A voltage gain of 10^6:1 does not mean that a one-volt input signal would be amplified up to one million volts at the out put! In fact, the maximum out put voltage is limited by the bias supply voltage, typically ±15V. However, an input signal of one microvolt will be amplified up to one volt at the output, and, because the amplifier is always used in circuits in which a large fraction of the output voltage is fed back to the input, the differential input voltage may be assumed negligible in comparison with the feedback component, or in other words, Vid is assumed zero. This gives rise to the odd situation in which a zero input voltage, multiplied by an infinite voltage gain, results in a finite and very ordinary level output. The technique for analyzing this situation is illustrated in the next section.
对无穷大电压增益（此处即指非常大却有限的电压增益的概念）在此需要一些解释。10^6:1的电压增益并不代表将1V的电压输入信号放大成10^6V的输出信号，实际上最大输出信号电压由偏置电源电压所限制，典型值为正负15V。然而，一个微伏级输入信号在输出端将放大到1V，同时因为放大器一般用于输出电压有一大部分反馈到输入端的电路中，所以差动输入电压与反馈部分比可忽略不计，或者换句话说，Vid假定为0。这样就产生一个奇特的现象，输入电压为0，乘以一个无穷大的电压增益，得到一个有限的常规输出电压，这种情况的分析技巧将在下一节中阐述。
The basic inverting amplifier circuit is shown in fig, in which ideal operational voltage amplifier is assumed. The input voltage Vi is fed to the inverting terminal through a resistor R1, and the noninverting terminal is connected to ground (or the common reference line). Feedback from output to input occurs through resistor R2. Now, because Vid=0 for the ideal amplifier, the inverting terminal is at the same potential as the noninverting terminal, and is termed a virtual ground. Because of the virtual ground, the input loop voltage equation may be written as Vid=i1R1. For the same reason, the output voltage loop equation may be written as V0=i2R2. Because Ri is assumed infinite, the input current ii is zero, and, therefore i1=-i2. Collecting and rearranging gives for the terminal voltage gain:?? 基本反向放大器的电路如图所示，其中假设了一个理想的电压运算放大器，输入电压Vi通过一电阻R1输到反相端，并且同相端直接接地（或者接到公共参考线上）。输出端到输入端的反馈电压出现在电阻R2上。因为对理想放大器来说，Vid=0，因此其反相端与同相端电势相等，并称为虚地。因为是虚地，输入回路电压方程便可写成??同样输出电压方程可写成。。。因为已经令r1无限大输入电流为0，所以整理后得端口电压增益
The significance of this result is that the terminal voltage gain, which is the usable voltage gain, is independent of the parameters of the amplifier, and depends only on the external components R1 and R2. Had A0 been assumed large but finite, the terminal gain would be reduced approximately by a factor(1-1/A0), and it can be seen that with A0 of the order of 10^6:1 this factor is very close to unity. It is also assumed in the ideal amplifier that the output resistance is zero so that zero voltage drop results, internal to the amplifier. A0 is known as the open loop gain because it is the voltage gain which would result with the feedback loop open; the results of feedback with amplifiers of finite A0 are described in other lesson.
这个结论的意义在于端电压增益（这是很有用的电压增益）与放大器的参数无关，而只取决于外部元件R1和R2.若A0真的假定为很大但有限，端口电压增益就会大致下降一个系数（1-1/A0）。可以看出，由于A0属于10^6:1数量级，因此这个系数非常接近于1.在理想放大器中，假定输出电阻为0，这样在放大器内部电压将为0.因为A0是在反馈回路开路时得出的电压增益，所以他定义为开环增益。对于A0为有限的放大器反馈结果将在其他课中讲述。
Practical comstraints limit the ration of R2/R1 to about 10^6:1 maximum. Offset problems, discussed in section 2.15, place an upper limit on R2 and R1 must be large enough compared with the signal source resistance for the latter to be ignored. In practical circuits, R1 usually ranges between 1.0k and 10.0k.
In carrying out the signal analysis it is not necessary to show the DC bias supply, but of course this must be provided.
实际条件限制了R2/R1的值最大只能约为1000:1. 2.15节中讨论的偏置问题限制了R2的上限，而R1与信号源的电阻相比要足够大，以便后者可以忽略不计。在实际电路中，R1通常在1.0-10.0k范围内。在进行信号分析时，没有必要画出直流偏置电压，但这个电压当然是必须的。
The integrator circuit produces an output voltage which is proportional to the integral of the input voltage. This is the inverse mathematical operation to that of differentiation.
The lower limit on the integration is taken as zero time, which of course is arbitrary, and the initial voltage on the capacitor, Vc(0), takes into account all the charge accumulated prior to the chosen time origin. Since the capacitor may be allowed to charge for any arbitrary time t, the upper limit on the integral is the time variable t so that Vc itself is a function of time. This is often shown by use of a ―dummy variable‖ for the time in the integrand. Denoting the dummy variable by the symbol t‘, we find that Eq.(2.5) may be written as??
积分器电路产生一个与输入电压的积分成正比例的输出电压，他是微分运算的逆运算。式中积分下限为0，当然这不是强求的。电容的初始电压计及了起始时间之前的所有充电结果。由于电容允许充电到任意时刻t，因此积分上限为时间变量t，这样Vc便为时间t的函数。这一点可以从积分时间中常用哑变量表示看出来。若用符号t‘表示哑变量，可以写成??
However, where there is no risk of confusion, the simpler notation of Eq.(2.5) will be used.
Figure (omitted) shows how the operational voltage amplifier may be arranged as an integrator circuit. Application of the virtual ground concept gives…and….
然而在不发生混淆的情况下，可以使用比较简单的式子。图显示了电压运算放大器怎样被组成积分电路的，由虚地概念可得出。以及与方程联立可得
Thus, the output voltage is proportional to the integral of the input voltage, the constant of proportionality being1/(-RC). This is also the gain of the integrator, and as with the differentiator, it keeps the equation dimensionally correct.
因此，输出电压与输入电压的积分成正比，比例系数为。这也就是积分器增益，对于微分器其量纲也是相同的。
A microcomputer interface converts information between two forms .Outside the microcomputer the information handled by an electronic system exists as a physical signals, but within the program , it is represented numerically . The function of any interface can be broken down into a number of operations which modify the data in some way ,so than the process of conversion between the external and internal forms is carried out in a number or steps. 微机接口实现两种信息形式的交换。在计算机之外，由电子系统所处理的信息以一种物理信号形式存在，但实际程序中 ，它是用数字表示的。任一接口的功能都可分为以某种形式进行数据变换的一些操作，所以外部和内部形式的转换是由许多步骤完成的。
This can be illustrated by means of an example such as than or Fig 1,which shows an interface between a microcomputer and a transducer producing a continuously variable analog signal. transducers often produce very small out requiring amply frication, or they may generate signals .in a form that needs to be converted again before being handled by the rest of the system .For example ,many transducers these variable resistance which must be converted to a voltage by a special circuit. This process of converting the transducer output into a voltage4 signal which can be connected to the rest of the system is called signal conditioning .In the example of Figure 18.1, the sigma conditioning section translates the range lf voltage or current signals from the transducer to one which can be converted to digital forum by an analog-to-digital converter.
用图一所示的情况为例加以说明，图中展示了微型计算机和产生连续变化信号的传感器之间的接口。但传感器产生的信号非常小，需要放大，或者产生的信号形式被系统的其他部分处理之前需要再次转换。举例来说，许多传感器具有电阻变化，这必须由一专门电路转化为电压。这种将传感器输出转化成电压信号，并与系统的其他部分相连接的过程，称为信号调理。如图所示例子中，信号调理部分将源自传感器的电压或电流信号范围转换成可以用模拟数字转换器变成数字形式的信号范围。
Analog-to-digital –digital converter (ADC) is used to convert a continuously variable signal to a corresponding digital forum which can take any one of a fixed number of possible binary values .If the output lf the transducer does not vary continuously ,no ADC is necessary. In this case the signal conditioning section must convert the incoming signal to a form which can be connected directly to the next part of the interface, the input/output section lf the microcomputer itself.
模拟数字转换器（ADC）用来将连续变化的信号变成相应的数字量，这数字量可是可能的二进制数值中的一固定值。如果传感器输出不是连续变化的，就不需要模拟数字转换。这种情况下，信号调理单元必须将输入信号变换成为另一信号。也可直接与接口的下一部分，即微型计算机本身的输入输出单元相连接。
The I/O section converts digital ―on/off‖ voltage signals to a form which can be presented to the processor via the via the system buses .Here the state of each input line whether it is ―on‖ or ―off‖, is indicated by a corresponding ―1‖ or ―0‖.In the line inputs which have been converted to digital form, the patterns of ones and zeros in the internal representation will form binary numbers corresponding to the quantity being converted. 输入输出单员将数字开，关电压信号转换成能通过系统总线传送到计算机的信号形式。这里每一根线的状态，无论是开或关，用相应的1或0 表示。对于已经转换成数字形式的模拟输入量，内部表示中用1和0组成的排列形式形成与被转换量相对应的二进制数。
The ―raw‖ numbers from the interface are limited by the design of the interface circuitry and they often require linearization and scaling to produce values suitable for use in the main program. For example ,the interface night be rise to convert temperatures in the range –20 to – +50 dress, buy the numbers produced by an 8-bit converter will lie in the range 0 to 255.Obviously it is easier , the programmer?s point of view to deal directly with temperature rather than to work out the equivalent of any given temperature in terms of the numbers produced by the ADC .Every time the interface is used to read a transducer ,the same operations must be carried out to convert the input number into a more convenient form .Addtionarly ,the operation of some interfaces requires control signals to be passed between the microcomputer and components of the interface ,For these reasons it is normal to use a subroutine to loot after the detailed operation of the interface and carry out any scaling and /or linearization which might be needed.
从接口得到的原数值会受到接口电路设计的限制而且经常需要线性化和量程调整才能形成适合于在主程序中使用的数值。举例来说，接口可用于转换范围在-20度到50度的温度，而八位转换器所产生的数值则在0-255之间。显然，从程序员的观点，对温度进行直接的处理要比使用由ADC产生的与一给定温度相一致的值要容易。接口总是用于读取传感器的值，同时还要将输入数值转换成更便易的形式。而且，接口操作需要将控制信号在微机和接口元件之间进行传送。根据这些理由，通常使用子程序来监督接口的具体操作，并完成任何所需的量程调整和/或线性变化。
Output interfaces take a similar form (Fig.18.2), the biopic difference being that here the flow of information is in the opposite direction; it is passed from the program to the outside world. In this case the program may call an output subroutine which supervises the operation of the interface and performs the scaling numbers which may be needed for a digital-to-analog converter (DAC) .This subroutine passes information in term to an out analog form using a DAC .Finally the signal is conditioned (usually amplified ) to a form suitable for operating an actuator.
输出接口采用相似的形式，明显的差别在于信息流的方向相反；是从程序到外部世界。这种情况下，程序可称为输出程序，它监督接口的操作并完成数字模拟转换器所需数字的标定。该子程序依次送出信息给输出器件，产生相应的电信号，由DAC转换成模拟形式。最后，信号经调理（通常是放大）以形成适应于执行器操作的形式。
The signals used within microcomputer circuits are almost always too small to be connected directly to the ―outside world ‖and some king of interface must be used to translate them to a more appropriate form .The design of section of interface circuits is one of the most important tasks facing the engineer wishing to apply microcomputers. We have seen that in microcomputers information is represented as discrete patterns of bits ;this digital form is most useful when the microcomputer is to be connected to equipment which can only be switched on or off, where each bit might represent the state of a switch or actuator. 在微机电路中使用的信号几乎总是太小而不能直接连接到外部世界，因而必须用某种形式将其转换成更适宜的形式。接口电路部分的设计是使用微机的工程师所面临最重要的任务之一。我们已经了解到微机中，信息以离散的位形式表示。当微机要与只有打开或关闭操作的设备相连接时，这种数字形式是最有用的，这里每一位都可以表示一开关或一执行器的状态。
Care must be taken when connecting logic circuits to ensure that their logic levels and current ratings are compatible .The output voltages produced by a logic circuit are normally specified in terms of worst case values when sourcing or sinking the maximum rated currents .Thus VOH is the guaranteed minimum ―high ‖ voltage when sourcing the maximum rate ―high‖ output current IOH ,while VOL is the guaranteed ―low‖ output voltage when sinking the maximum rated ―low ‖output current IOL .There are corresponding specifications for logic inputs which specify the minimum input voltage which will be recognized as a logic ―high‖ state VIH ,and the maximum input voltage which will be regarded as a logic ―low‖ state VIL.
连接逻辑电路时，必须小心翼翼，以保证他们的逻辑电平和电流额定值是兼容的。有逻辑电路产生的输出电压通以拉出或灌入最大额定电流时，按最弱情况下数值所定义。这样VOH是当拉出最大额定高输出电流IOH时所允许的最小高电压。而VOH则是当灌入最大额定地输出电流IOL时所允许的最低电压。对逻辑输入也有相应的参数，规定最小输入电压为逻辑高状态VIH,以及最大输出电压为逻辑低状态VIL。