晶体管特性文献翻译

1、黄河科技学院毕业设计（文献翻译） 第 18 页 晶体管特性第一章中已经指出，晶体管能够放大电流。因此，晶体管在电子线路中应用很广，例如音频放大器，助听器，扩音机放大器，无线电接收机和电视接收机，测量仪表和工业控制。另外，晶体管也可以用作“电子开关”，它可使电流通路或者呈现高电阻或者呈现低电阻。这就是晶体管有可能在计算机电路和控制系统中获得广泛应用。对每一项应用都必须进行细致的电路设计，在能够系统地进行设计工作之前，应对晶体管这一电路元件的特性有个详尽的了解，知道什么是最佳工作电压和电流，对信号的阻抗有多大，晶体管的放大倍数有多大，什么是晶体管输出端内阻抗等等。这些特性资料可从各类晶体管有关数据

2、中获得，这些数据由制造厂商提供，印成“数据表”发行，使晶体管使用者能根据此进行初步设计，而不必自行量测。首要的问题是取得晶体管电压电流关系曲线。通常要提供出两组曲线；发射结正向电压电流特性曲线，通常称为发射极特性曲线或输入特性曲线；以及集电结反向电压电流曲线，通常称为集电极特性曲线或输出特性曲线。1 基特性曲线首先研究共基极电路，图1所示为发射极基极正向特性曲线，它描述了发射极电流如何随发射极基极电压从零正向增高而增大。图上所示的特性曲线是小型锗管的典型曲线。由图可看出，最初电流随电压增高而增加，但增加的幅度很小。在这期间，外加电压逐渐克服pn结的势垒。势垒一旦被中和，电流就迅速增加。图1 发

3、射极基极正向特性曲线现在来研究发射极电流变化时集电极电路出现的情况。首先，发射极电流为零时集电极基极特性如图2中IE=0的曲线所示。它和前面图所示的pn结反向特性曲线相似。图中的小股电流称为集电结的漏电流。现在使发射结电流增加到1毫安（图1中的A点），并使之维持不变。我们看到，几乎全部发射极电流都传送到集电极，通过集电极的电流量与集电极电压的高低无关。这样，集电极电压电流特性曲线就成了图2中IE=2,3,4和5毫安时的集电极曲线B,C,D和E。集电极特性曲线几乎处于水平位置这一性质突出了高输出电阻这一特性，因而集电极电压发生大幅度变化时，电流变化很小。 图2 集电极基极特性由图中可以看出，甚至

4、在集电极电压为零时仍然存在集电极电流。这是因为基极电流在通过基区电阻时在集电极基极回路中产生一小电势差，从而在集电极两端形成很小的反向偏压。要使集电极电流减为零，就需要外加一很小的正向集电极电压，如图2所示。图2中集电极特性曲线画在第三象限，以使人们注意到集电极反向偏置。现在一般把它画在第一象限，如图3所示，这在某种程度上是由于热离子管的输出特性曲线本身就是这样处理的。图3 集电极特性曲线 2 共基极放大器的相位关系基极发射极回路是正向偏置。以pnp晶体管为例，它的发射极与基极相比为正。信号正向半周与Vee串联连接时，发射极比以前更正，使发射极基极电流增大。在晶体管中，发射极基极电流的增大使集

5、电极电流相应增加。由于RL中的电流方向朝上，此电阻器上端与它的下端相比较就比以前更正了。因此，正向半周输入信号引出正向半周输出信号。这就是说共基极晶体管放大器没有倒相问题。图4 共基极放大器的倒位关系在许多类型的多级放大器、振荡器和电视用视频放大器中，相位关系是考虑的重要问题。对今后的应用来说，重要的是要记住我们如何判断倒相与否的方法。 3 单电源共基极电路 图5 单电池共基极电路共基极电路的设计通常避免使用发射极电池Vee。要做到这一点，只需加上一基极电阻（Rb），并使此电阻的低端成为输入和输出回路两者的公共端。此电路除了省去发射极电池外，还使一个输入端和一个输出端处于低电位。这样，输入和输

6、出两个回路现在都有一个公共的参考电位（低电位）。流过Rb的小股电流Ico和地连接，它相对于基极是负的（这是npn晶体管）。因而不用电池就获得了较小的正向偏压。偏置电阻Rb可以旁路，这样在可能出现交流信号时Rb的电压降仍保持不变。4 共发射极特性曲线共发射极连接时可得出与共基极连接时相类似的特性曲线。首先是输入特性曲线，它表明基极电流随发射极基极结两端电压正向升高而变化的情况，如图6所示；其次是输出特性曲线，它表明集电极电流随不同基极电流下的集电极电压变化已定的情况下，基极电流变化比发射极电流变化小。图7的输出特性曲线和共基极特性曲线很相似，只是电流曲线有明显的坡度，即电流随电压而增大。这说明它

7、的输出电阻比基极电路低，但它仍然是相当高的。此外，集电极电压为零时集电极电流也为零，这是因为基极电流所形成的电势并没有出现在集电极反射极回路。图6 共发射极特性曲线图7 2N78输出特性曲线5 集电极曲线的应用当你观察集电极曲线时，首先映入眼中使你感兴趣的是电流并不随集电极电压的变化而急剧增大。它是一组十分近似于水平的直线，特别在基极电流很低的区域；即使在基极电流较高的区域，坡度也很平缓的。我们可以说：在制造厂商所推荐的区域范围内，晶体管的集电极电流相对独立于集电极电压。图8 IbIc曲线与之相反，在集电极电势恒定的条件下，基极电流稍有变化就可使用集电极电流发生较大的变化。基极电流作等值增长时

8、，集电极电流是否相应地等值上升呢？为了弄清这一点，我们选定一集电极电势，譬如说5伏，然后沿这条5伏线上升。注意观察基极电流每增加25毫安时集电极电流的变化情况。为帮助你估算集电极电流的变化量，我们绘制了集电极电势为5伏的IbIc曲线。虽然这条曲线并不完全是一条直线，但它确实很近似于直线，如果晶体管用于例如音频放大电路，我们就可以说：如果Ib的变化局限在相当小的范围内，集电极电流就随Ib作线性变化。这一提法可进一步解释如下：如馈入基极的输入电流是弱音频电流，则集电极回路的电流变化比输入电流的变化大，但其波形保持不变。这样，我们得到一种度量我们对这类晶体管所能要求的保真度的方法，或者反过来说，度量

9、晶体管固有畸变的方法。图9 晶体管畸变曲线只要IbIc曲线是条直线，其固有畸变就为零。曲线的曲率越大，晶体管本身所造成的畸变越严重。前面所有讨论的前提是，电路元件选择恰当，才能使所用的晶体管产生正常的偏压。不然，就可能出现与晶体管固有特性无关的畸变。图10 Ic的改变影响Ib的改变我们可利用集电极特性曲线或者IbIc曲线在几秒钟内即可估算出晶体管的值。晶体管集电极特性一般可在制造厂商的参数表中取得。让我们利用它来校核晶体管2N78的ß值，即常说的所谓的基极电流增益。晶体管2N78通常在5伏集电极电位下工作，我们先找出5伏线，然后沿此线选出基极电流变化的一般区域范围内，例如从100微安

10、到125微安。利用我们已经知道的等式=IC/Ib，取Ib从100微安到125微安的变化为Ib，它等于25微安。然后，我们可注意到在Ib的变换范围内Ic从2.9毫安变为3.5毫安。将这些值代入等式就得到：=246 单电池共发射极电路关于这个电路，让我们先回顾第三节双电池共基极放大器改成单电池电路的部分。你可能会发现，为了提供所需要的偏置电位，需要在基极回路上外加一电阻和电容。电阻Rb使基极相对于发射极（正向偏置）和集电极（反向偏置）都具有正确的极性。如果我们进一步分析双电池共基极电路，显然可知，需要外加偏压元件的原因在于发发射极和集电极电流的流动方向。我们用pnp晶体管为例，可知Ie和IC在接向

11、基极的公共引线中流动方向相反。由于用一个电池，不论放在电路的哪个部分，要在一个元件中产生两股方向相反的电流是决不可能的，所以为了建立正常的偏置电位，借助于人为的辅助元件即偏置电阻是完全必要的。图11 双电池共发射极电路现在我们来分析双电池共发射极电路。Vee使电子流向通过Ri(或通过信号源本身，假设信号源是连续直流的)和基极到发射极，再向下经过公共导线回到电池的正端，如深色箭头所示。Vee使电子流向通过RL和集电极到发射极，再向下经过公共连接线回到电池的正端，如浅色箭头所示。由此可知，从发射极到两组电池正极结点的公共导线中电流的流向相同。这就很容易用一组电池来完成原来两组电池的工作。不管我们把

12、电池接到那里，一定要记住，pnp晶体管的基极相对于发射极应是负的，但负的程度不如集电极。 图12 发射极和集电极电流的流动同向图13 单电池基极发射极放大电路实际上去掉第二组电池很容易，这不能不使我们感到奇怪，为什么在共发射极电路中曾经使用过两组电池。我们只注意到Vee和Vcc相对于发射极都是负的。全面检查线路的连接，我们很快就可看出，集电极电流的通路与过去完全相同；从电池的负端通过集电极，离开发射极，又回到正端。与此同时，同一电池按深色箭头指示的方向把电流（虽然可能很小）送入基极回路。这股电流的流动和采用两组电池时的情况完全相同。剩下要作的只是选择适当的电阻值，使基极电压大小合适。由于Ri阻

13、值大小在很大程度上取决于晶体管类型、Vcc的电势和环境温度，所以我们无法提供某一定值。晶体管2N78在标准中频电路室温下的Ri典型阻值为10,000欧姆左右。 来自晶体管原理附：英文原文Transistor CharacteristicsIt has been shown in the previous chapter that the transistor is cable of amplifying electric currents. As a result, it can be used for many applications in electronic circuits, suc

14、h as audio amplifiers, hearing aids, pubic address amplifiers, radio and television receivers, instrumentation and industrial control. Also, the transistor can be used as an "electronic switch ", that is it can present either a high or a low resistance to the passage of current. This opens

15、 up the possibility of wide use in computer circuits and control systems.For each application careful circuit design work must be carried out. Before this can be done systematically, it is necessary to have detailed knowledge of the characteristics of the characteristics of the transistor as a circu

16、it element, that is, to know what is the best operating voltage and current, what impedance is presented to the signal, what amplification the transistor will give, what is the internal impedance of the transistor at the output, and so on. Data from which information of this nature can be obtained i

17、s prepared by the manufacturer on each type of transistor and published as “data sheets” so that the user can carry out his preliminary design without having to make measurements himself. First, it is important to derive groups of voltage-current relationships for the transistor. Two sets of curves

18、are normally presented, the forward voltage-current characteristics of the emitter junction, referred to as the emitter characteristics curves of the collector junction, called the collector characteristics or the output characteristics.1 Common Base CharacteristicsConsidering first the common base

19、arrangement, Fig. 1 shows the emitter to base forward characteristic, that is, how the emitter current increaser as the emitter to base voltage is increased positively from zero. The characteristic shown is typical of a small germanium transistor. It will be seen that at first the current increases

20、only very slightly as the voltage is increased. During this region the applied voltage is overcoming the potential barrier of the junction. Once the barrier has been neutralized, the current increases rapidly.Now consider what happens in the collector circuit when the emitter current is varied. At f

21、irst, with zero emitter current, the collector to base characteristic is shown as the cure for in Fig. 2. This is similar to the reverse characteristic of a pn junction shown previously in Fig. The small current is known as the leakage current of the collector junction. Now let the emitter current b

22、e increased to 1 mApoint A in Fig. 1 and held constant at that value. We have seen that nearly all of the emitter current passes to the collector, the amount of current crossing the collector junction not being dependent on the collector voltage. Thus the collector voltage current characteristic wil

23、l now be curve A for IE=1 in Fig. 2.Similarly, as the emitter current is increased in further steps collector curves B,C,D and E are obtained for emitter current IE=2,3,4 and 5 mA. The almost horizontal nature of the collector characteristics emphasizes the high output resistance, a large change of

24、collector voltage producing only a very small change of current.It will be seen that the collector current is maintained even at zero collector voltage. This is because the base current, in flowing out through the resistance of the base region, sets up a small potential which appears in the collecto

25、r-base circuit, and constitutes a small reverse bias across the collector junction. To reduce the collector current to zero it is necessary to apply a small forward collector voltage as shown in Fig. 2.In Fig. 2 the collector characteristics have been shown in the third quadrant as a reminder that t

26、he collector junction is biased in the reverse direction. It is now customary to present them in the first quadrant as shown in Fig. 3, to some extent because the output characteristics of thermionic valves were always drawn in this way.2 Hase Relations in a Common-Base AmplifierThe base-emitter cir

27、cuit is forward-biased. In the case of the pnp transistor used as an example, this means that the emitter is more positive than the base. When a positive-going half-cycle is now inserted in series with Vee ,the emitter becomes more positive than before, increasing the emitter-base current. In a tran

28、sistor, an increase of emitter-base current produces a corresponding increase of collector current. Since the direction of current flow is upward in RL, the top terminal of this resistor must become more positive than it was before with respect to the bottom terminal. Hence a positive-going input ha

29、lf-cycle gives rise to a positive-going output half-cycle. This means that there is no phase inversion in the common base transistor amplifier.Phase relations are important considerations in many types of multistage amplifiers. In oscillators, and in video amplifiers for television. It is important

30、to remember how we determine whether phase inversion does or does not occur for future use.3 Single-battery Common-base Circuit A common-base circuit is normally designed to do away with the need for an emitter battery Vee. To do this we need merely add a base resistor(Rb) and make the lower termina

31、l of this resistor common to both input and output circuits. In addition to doing away with the emitter battery, this circuit makes it possible to maintain one input and one output terminal at ground potential . There is now a common reference potential(ground) for both input and output. The small l

32、eakage current ICO flowing through Rb places the base at a slightly higher positive potential than ground. Since the emitter is connected to ground through Re, this element must be negative with respect to the base(this is an npn transistor). Thus, the small amount of for-ward-bias is obtained witho

33、ut the need for a battery. The bias resistor Rb may be bypassed to maintain the voltage drop across it constant, despite the possible presence of alternating signal currents.4 Common Emitter CharacteristicsWith the common emitter connection, similar characteristics can be prepared. First, the input

34、characteristic, which shows how the base current varies as the voltage across the emitter-base junction is increased in the forward direction, as shown in Fig. 7. It will be seen from Fig. 6 that the input resistance is higher than for the common base arrangement, the change of base current being sm

35、aller than the change of emitter current for a given change of emitter to base voltage .The output characteristics in Fig. 7 are similar to the common base characteristics except that there is now a noticeable slope on the current lines, the current increasing with voltage. This indicates that the o

36、utput resistance is lower than in the common base arrangement; but nevertheless it is still high. Also, the collector current is now zero for zero collector voltage since the potential produced by the base current does not appear in the collector to emitter circuit.5 Using the Collector CurvesOne of

37、 the first interesting things that strikes you as you look at the collector curves is that the current does not rise very rapidly with changes of collector voltage. Notice how horizontal the graph lines are, especially when the base current is low. Even for higher base currents, the slopes are very

38、shallow. We can say it this way: over the range recommended by the manufacturer, the collector current of a transistor is relatively independent of the collector voltage.On the other hand, a small change of base current always produces a relatively large change in collector current, if the collector

39、 potential is maintained constant. Does the collector current rise in equal steps for equal increments of base current? To see this, choose a certain collector potential, say 5 volts, then follow the 5-volt line upward and note how the current changes, we have drawn IbIc curve for a collector potent

40、ial of 5 volts.Although this curve is not a perfect straight line, it certainly does approach it closely. Of the transistor is used in an audio amplifier circuit, for example, we might then say: If the range of variation of Ib is held within reasonably small limits, the collector current varies line

41、arly with it.This may be interpreted as follows: if the input current to the base is a weak audio current, the current variations in the collector circuit will be large but will have the same waveform. Thus, we arrive at a measure of the fidelity or, conversely, the inherent distortion that we may e

42、xpect from a transistor of this type. As long as the IbIc curve is a straight line, the inherent distortion will be zero. The greater the curvature of the graph, the greater will be the distortion that can be attributed to the transistor itself.All of the foregoing presupposes that the circuit compo

43、nents have been selected to produce the proper bias for the specific transistor used. If this is not the case, distortion may occur that has no connection with the inherent characteristics of the transistor.Either the collector characteristic curves or the IbIc curve may be used to estimate the beta

44、 of a transistor in just a few seconds. Since the collector characteristics are those generally found in manufacturers' rating sheets, suppose we use these to check the beta, or base current gain as it is often called, of the 2N78.Since this transistor normally operates at a collector potential

45、of 5 volts, we first locate the 5-volt line. Then we select an average region of base current change along this line, say from 100 microamperes to 125 microamperes. Using our knowledge that beta =Ic/Ib ,we can take the change of Ib from 100A to 125A for a total of Ib=25A . We then note that Ic goes

46、from 2.9 ma to 3.5 ma over this range of Ib .Substituting these values in the equation we have:=246 The Single-Battery Common-Emitter CircuitAt this point, let us refer back to where we converted the two-battery common-base amplifier into a single-battery circuit. You will find that it was necessary

47、 to add a special resistance and capacitor in the base circuit to provide the required biasing potentials. That is, Rb permits the base to have the correct polarity with respect to the emitter(forward bias) and with respect to the collector (reverse bias).If we analyze the two-battery common-base ci

48、rcuit further, it is apparent that the reason for the need of an extra biasing component lies in the directions of flow of the emitter and collector currents. Using a pnp transistor as an example (the npn is analyzed the same way except that all current directions are reversed), we see that Ie and I

49、c flow in opposite directions in the common lead going to the base. Since one battery, placed anywhere at all in the circuit, could never produce two oppositely-directed currents in a common element, it is necessary to create the correct bias potentials by means of an artificial aid -the bias resistor.Now let us analyze the two-battery common-emitter circuit. Vee forces current up through Ri (or through the signal source itself if the source has d-c continuity), through the base to the emitter, then downward through the common lead back to the positive terminal o