1、晶體管特性第一章中已經指出，晶體管能夠放大電流。因此，晶體管在電子線路中應用很廣，例如音頻放大器，助聽器，擴音機放大器，無線電接收機和電視接收機，測量儀表和工業控制。另外，晶體管也可以用作“電子開關，它可使電流通路或者呈現高電阻或者 呈現低電阻。這就是晶體管有可 能在計算機電路和控制系統中獲得廣泛應用。對每一項應用都必須進行細致的電路設計，在能夠系統地進行設計工作之前，應對晶體管這一電路元件的特性有個詳盡的了解，知道什么是最正確工作電壓和電流，對信號的阻抗有多大，晶體管的放大倍數有多大，什么是晶體管輸出端內阻抗等等。這些特性資料可從各類晶體管有關數據中獲得，這些數據由制造廠商提供，印成“數據表

2、發行,使晶體管使用者能根據此進行初步設計，而不必自行量測。首要的問題是取得晶體管電壓電流關系曲線。通常要提供出兩組曲線；發射結正向電壓電流特性曲線，通常稱為發射極特性曲線或輸入特性曲線；以及集電結反向電壓電流曲線，通常稱為集電極特性曲線或輸出特性曲線。1基特性曲線首先研究共基極電路，圖1所示為發射極基極正向特性曲線，它描述了發射極電流如何隨發射極基極電壓從零正向增高而增大。圖上所示的特性曲線是小型鍺管的典型曲線。由圖可看出，最初電流隨電壓增高而增加，但增加的幅度很小。在這期間，外加電壓逐漸克服pn結的勢壘。勢壘一旦被中和，電流就迅速增加。發射極電流皿現在來研究發射極電流變化時集電極電路出現的情

3、況。首先，發射極電流為零時集電極基極特性如圖2中Ie=O的曲線所示射結電流增加到1毫安圖1中的A點，并使之維持不變。我們看到，幾乎全部發射極電流都傳送到集電極，通過集電極的電流量與集電極電壓的上下無關。這樣,集電極電壓電流特性曲線就成了圖2中Ie=2, 3,4和5毫安時的集電極曲線 B,C, D和E。集電極壓發生大幅度變化時，電流變化很小。特性曲線幾乎處于水平位置這一性質突出了高輸出電阻這一特性，因而集電極電A* 1ET嚴匚E=3D)DE=4E-V-4-3'2-1圖2集電極基壓特性由圖中可以看出，甚至在集電極電壓為零時仍然存在集電極電流。這是因為基極電流在通過基區電阻時在集電極基極回路

4、中產生一小電勢差，從而在集電極兩端形成很小的反向偏壓。要使集電極電流減為零，就需要外加一很小的正向集電極電壓J2如圖2所示。就是這樣處理的。圖2中集電極特性曲線畫在第三象限，以使人們注意到集電極反向偏置。現在一般把它畫在第一象限，如圖3所示，這在某種程度上是由于熱離子管的輸出特性曲線本身&>&(?護|E=2IE J7Ie=0200 10集電極電壓 V圖3集電極特性曲線2共基極放大器的相位關系基極發射極回路是正向偏置。以 pnp晶體管為例，它的發射極與基極相比為正。信號正向半周與Vee串聯連接時，發射極比以前更正，使發射極基極電流增大。在晶體管中，發射極基極電流的增大使集電

5、極電流相應增加。由于Rl中的電流方向朝上，此電阻器上端與它的下端相比擬就比以前更正了。因此，正向半周輸入信號引出正向半周輸出信號。這就是說共基極晶體管放大器沒有倒相問題。護.-滬P輸廠亠宀 輸出I>xM “A'農emm -Vcc圖4共基極放大器的倒位關系在許多類型的多級放大器、振蕩器和電視用視頻放大器中，相位關系是考慮的重要問題。對今后的應用來說，重要的是要記住我們如何判斷倒相與否的方法。3單電源共基極電路N-P-NRL共基極電路的設計通常防止使用發射極電池Vee。要做到這一點，只需加上一基極電阻Rb,并使此電阻的低端成為輸入和輸出回路兩者的公共端。 此電路除了省去發 射極電池外

6、， 還使一個輸入端和一個輸出端處于低電位。這樣，輸入和輸出兩個回路現在都有一個公共的參考電位低電位。流過 Rb的小股電流Ico和地連接，它相對于基 極是負的這是npn晶體管。因 而不用電池就獲得了較小的正向偏壓。偏置電阻Rb可以旁路，這樣在可能出現交流信號時 Rb的電壓降仍保持不變。4共發射極特性曲線共發射極連接時可得出與共基極連接時相類似的特性曲線。首先是輸入特性曲線，它說明基極電流隨發射極基極結兩端電壓正向升高而變化的情況，如圖6所示；其次是輸出特性曲線，它說明集電極電流隨不同基極電流下的集電極電壓變化已定 的情況下， 基極電流變化比發射極電流變化小。圖 7的輸出特性曲線和共基極特性曲線

7、很相似，只是電流曲 線有明顯的坡度，即電流隨電壓而增大。這說明它的輸出電阻比基 極電路低，但它仍然是相當高的。/L7 此外，集電極電壓為零時集電極電流也為零，這是因為基極電流所形成的電勢并沒有出現在集電極反射極回路。5集電極電壓V152010圖72N78輸出特性曲線5集電極曲線的應用當你觀察集電極曲線時，首先映入眼中使你感興趣的是電流并不隨集電極電壓的變化而急劇增大。它是一組十分近似于水平的直線，特別在基極電流很低的區域；即使在 基極電流較 高的區域，坡度也很平緩的。我們可以說：在制造廠商所推薦的區域范圍內 ,晶體管的集電極電流 相對獨立于集電極電壓。圖8 lb Ic曲線與之相反，在集電極電勢

8、恒定的條件下，基極電流稍有變化就可使用集電極電流發生較大的變化。基極電流作等值增長時，集電極電流是否相應地等值上升呢？為了弄清這一點，我們選定一集電極電勢，譬如說5伏，然后沿這條5伏線上升。注意觀察基極電流每增加25毫安時集電極電流的變化情況。為幫助你估算集電極電流的變化量，我們繪制了集電極電勢為5伏的lb Ic曲線。雖然這條曲線并不完全是一條直線，但它確實很近似于直線，如果晶體管用于例如音頻放大電路，我們就可以說：如果lb的變化局限在相當小的范圍內，集電極電流就隨lb作線性變化。這一提法可進一步解釋如下：如饋入基極的輸入電流是弱音頻電流，那么集電極回路的電流變化比輸入電流的變化大，但其波形保

9、持不變。這樣，我們得到一種度量我們對這類晶體管所能要求 的保真度的方法，或者反過來說，度量晶體管固有畸變的方法。輸出無畸變-* 傳輸輸入 畸變傳輸、 JF| |輸入圖9晶體管畸變曲線只要lb Ic曲線是條直線，其固有畸變就為零。曲線的曲率越大，晶體管本身所造成的畸變越嚴重 可能出現與晶體管固有特性無關的畸變。4nA=0.125mA一一一3mA2 9_ ,_0.1mA5V集電極電壓圖10 Ic的改變影響lb的改變我們可利用集電極特性曲線或者 lb Ic曲線在幾秒鐘內即可估算出晶體管的 B值。晶體管集 電極特性一般可在制造廠商的參數表中取得。讓我們利用它來校核晶體管2N78的？值，即常說的所謂的基

10、極電流增益。晶體管2N78通常在5伏集電極電位下工作，我們先找出5伏線，然后沿此線選出 基極電流變化的一般區域范圍內，例如從 100微安到125微安。利用我們已經知道的等 式B = Ic/A lb,取lb從100微安到125微安的變化為 lb,它等于25微安。然后，我 們可注意到在lb的變換范_35-290.6卩=0.025 = 0.025 =24圍內lc從2.9毫安變為3.5毫安。將這些值代入等式就得到：6單電池共發射極電路關于這個電路，讓我們先回憶第三節雙電池共基極放大器改成單電池電路的局部。你可能會發現，為了提供所需要的偏置電位，需要在基極回路上外加一電阻和電容。電阻R.使基極相對于發射

11、極正向偏置和集電極反向偏置都具有正確的極性。如果我們進一步分析雙電池共基極電路，顯然可知，需要外加偏壓元件的原因在于發發射極和集電極電流的流動方向。我們用 pnp晶體管為例，可知le和lc在接向基極的 公共引線中流動方 向相反。由于用一個電池，不管放在電路的哪個局部，要在一個元件中產生兩股方向相反的電流是決不可能的，所以為了建立正常的偏置電位，借助于人為的輔助元件即偏置電阻是完全必要的。1 H11+ +1111Vrp Vcc圖11雙電池共發射極電路現在我們來分析雙電池共發射極電路。Vee使電子流向通過 Ri 或通過信號源本身，假設信號源是連續直流的和基極到發射極，再向下經過公共導線回到電池的正

12、端，女口深色箭頭所示。Vee使電子流向通過Fl和集電極到發射極，再向下經過公共連接線回到電池的正端，如淺色箭頭所示。由此可知，從發射極到兩組電池正極結點的公共導線中電流的流向相同。這就很容易用一組電池來完成原來兩組電池的工作。不管我們把電池接到那里，一定要記住，pnp晶體管的基極相對于發射極應是負的，但負的程度不如集電極。信輸Vpr Vcc圖12發射極和集電極電流的流動同向圖13單電池基極發射極放大電路實際上去掉第二組電池很容易，這不能不使我們感到奇怪，為什么在共發射極電路中曾經使用過兩組電池。我們只注意到 Vee和Vcc相對于發射極都是負的。全面檢查線 路的連接，我們很 快就可看出，集電極電

13、流的通路與過去完全相同；從電池的負端通過集電極，離開發射極，又回到正端。與此同時，同一電池按深色箭頭指示的方向把電流雖然可能很小送入基極回路。這股電流的流動和采用兩組電池時的情況完全相同。剩下要作的只是選擇適當的電阻值，使基極電壓大小適宜。由于Ri阻值大小在很大程度上取決于晶體管類型、Vcc的電勢和環境溫度，所以我們無法提供某一定值。晶體管2N78在標準中頻電路室溫下的 Ri典型阻值為 10,000 歐姆左右。來自?晶體管原理?附：英文原文Transistor CharacteristicsIt has been shown in the previous chapter that the t

14、ransistor is cable of amplifying electric currents. As a result, it can be used for many applications in electronic circuits, such as audio amplifiers, hearing aids, pubic address amplifiers, radio and television receivers, in strume ntati on and in dustrial con trol. Also, the tran sistor can be us

15、ed as an "electro nic switch ", that is it can prese nt either a high or a low resista nee to the passage of curre nt. This ope ns 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 b

16、e done systematically, it is necessary to have detailed knowledge of the characteristics of the characteristics of the transistor as a circuit 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 g

17、ive, what is the internal impedance of the transistor at the output, and so on. Data from which in formatio n of this n ature can be obta ined is prepared by the manu facturer on each type of tran sistor and published as prelim inarydatadesig n without hav ing to make measureme nts himself.shtb etss

18、e caHoFirst, it is importa nt to derive groups of voltage-curre nt relati on ships for the tran sistor. Two sets of curves are no rmally prese nted, the forward voltage-curre nt characteristics of the emitter junction, referred to as the emitter characteristics curves of the collector junction, call

19、ed the collector characteristics or the output characteristics.1 Common Base CharacteristicsCon sideri ng first the com mon base arran geme nt, Fig. 1 shows the emitter to base forward characteristic, that is, how the emitter curre nt in creaser as the emitter to base voltage is in creased positivel

20、y from zero. The characteristic show n is typical of a small germa nium tran sistor. It will be see n that at first the curre nt in creases 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

21、bee n n eutralized, the curre nt in creases rapidly.EMITTER-BASE VOLTAGE, VOLTS Fig 1 In put charactcrijric of com mon ba?c circuit.Now consider what happens in the collector circuit when the emitter current is varied. At first, with zero emitter curre nt, the collector to base characteristic is sho

22、w n 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 curre nt be in creased to 1 mA point A in Fig. 1 and held con sta nt at that value. W

23、e have see n that n early all of the emitter current passes to the collector, the amount of current crossing the collector junction not being depe ndent on the collector voltage. Thus theAJr e= iE0Ci嚴DLE)iE=scollector voltage current characteristic will now be curve A fore=1 in Fig. 2.Similarly, as

24、the emitter curre nLs in creased in further steps collector curves B,C,D and E LECTOare obtained for emitter current I e=2,3,4 and 5 mA. The almost horizontal nature of the collector characteristics emphasizes the high output resista nee, a large cha nge of collector voltage produc ing only a very s

25、mall cha nge of curre nt.It will be seen that the collector current is maintained even at zero collector voltage. Thisis because the base curre n t, i n flow ing out through the resista nee of the base regi on, sets up a small pote n tial which appears in the collector-base circuit, and con stitutes

26、 a small reverse bias across the collector junction. To reduce the collector curre nt to zero it is n ecessary to apply arIe=jfrfsmall forward collector voltage as shown in Fig. 2.In Fig. 2 the collector chafactBHst杞s°have bee n show甲 ilh the third quadra nt as a remin dera coitit no ti-base ti

27、amsistor ain plifieithat the collector ju n ctio n is biased in the reverse direct ion. It is now customary to prese nt them in the first quadra nt as show n in Fig. 3, to some exte nt because the output characteristics ofHMHaME Ho-L3mthermio n ic valves were alw/ays draw n in this way.OUTPUT2 Hase

28、Relati ons in a Common-Base AmplifierF-ST-FR|dXCOLLEcTQRFig? Collctor<VOLATAGE, VOLTStic, coi mnon basJiccui+ThTil卜INPUT._.The base-emitter circuit is forward-biased. In the case of the pnp tran sistor used as an example, this means that the emitter is more positive tha n the base. Whe n a positi

29、ve-go ing half-cycle is now inserted in series with V ee ,the emitter becomes more positive than before, increas ing the emitter-base curre nt. In a tran sistor, an in crease of emitter-base curre nt produces a corresp onding in crease of collector curre nt. Since the direct ion of curre nt flow is

30、upward in Rl,the top terminal of this resistor must become more positive than it was before with respect to the bottom termi nal. Hence a positive-go ing in put half-cycle gives rise to a positive-go ing output half-cycle. This means that there is no phase inv ersi on in the com mon base tran sistor

31、 amplifier.Phase relati ons are importa nt con siderati ons in many types of multistage amplifiers .In oscillators, and in video amplifiers for television. It is important to remember how we determ ine whether phase inv ersi on does or does not occur for future use.3 Sin gle-battery Common-base Circ

32、uitFigj The cottirti on-base circuitVrrA com mon-base circuit is no rmally desig ned to do away with the n eed for an emitter battery V ee To do this we n eed merely add a base resistor(R and make the lower term inal of this resistor com mon to both in put and output circuits. In additi on to doing

33、away with the emitter battery, this circuit makes it possible to main ta in one in put and one output term inal at ground pote n tial . There is now a com mon refere nee pote n tial(gro und) for both in put and output. The small leakage current I co flowing through R b places the base at a slightly

34、higher positive pote ntial tha n ground. Since the emitter is conn ected to ground through R e, this eleme nt must be n egative with respect to the base(this is an npn transistor). Thus, the small amount of for-ward-bias is obta ined without the n eed for a battery. The bias resistor R b may be bypa

35、ssed to maintain the voltage drop across it constant, despite the possible presence of alter nat ing sig nal curre nts.4 Com mon Emitter CharacteristicsWith the com mon emitter conn ecti on, similar characteristics can be prepared. First, the input characteristic, which shows how the base curre nt v

36、aries as the voltage across the emitter-base jun cti on is in creased in the forward direct ion, as show n in Fig. 7. It will be see n152tDD5D- D5 H 115 乩 2 Q. 25BASE VOLATAGE, VOLTS?oaFigg Iput characteristic of com mon emitter ciicuitfrom Fig. 6 that the in put resista nee is higher tha n for the

37、com mon base arran geme nt, the cha nge of base curre nt being smaller tha n the cha nge of emitter curre nt for a give n cha nge of emitter to base voltage .The output characteristics in Fig. 7 are similar to the com mon base characteristics except that there is now a noticeable slope on the curren

38、t lines, the current in creas ing with voltage. This in dicates that the output resista nce is lower tha n in the com mon base arrangement; but nevertheless it is still high. Also, the collector current is now zero for zero collector voltage since the pote n tial produced by the base curre nt does n

39、ot appear in the collector to emitter circuit.COLLECTOR VOLATAGE, Y OLTSF1E7 Collctar charat eristic, conunon 亡 rtiittM circuit5 Usi ng the Collector CurvesOne of the first in terest ing things that strikes you as you look at the collector curves is that the current does not rise very rapidly with c

40、hangesof collector voltage. Notice how horiz on tai the graph lines are, especially whe n the base curre nt is low. Eve n for higher base curre n ts, the slopes are very shallow. We can say it this way: over the range recomme n ded by the manufacturer, the collector current of a transistor is relati

41、vely independent of the collectorvoltage.On the other hand, a small cha nge of base curre nt always produces a relatively large cha nge in collector curre n t, if the collector pote n tial is maintained con sta nt. Does the collector curre nt rise in equal steps for equal in creme nts of base curre

42、nt? To see this, choose a certa inVc 二5V/collector potential, say 5 volts, then follow the 5-volt line upward and note how the current changes, we have draw n b Ic curve for a collector pote ntial of 5 volts.E54Although this curve is not a perfect straight line, it certainly does approach it closely

43、. Of the transistor is used in an audio amplifier circuit, for example, we might then say: If the range of variati on of I b is held within reas on ably small limits, the collector curre nt varies lin early with it.a SD IDO 15D 也口 25n 3DQBASE CUTTENT8 Ib-Ic curve for 2N7S tran sistor with Vc at 5 vo

44、lts.This may be in terpreted as follows: if the in put curre nt 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, conv ersely, the in here nt distortio n that we may exp

45、ect from a transistor of this type. As long as the b Ic curve is a straight line, the in here nt distortio n will be zero. The greater the curvature of the graph, the greater will be the distortion that can be attributed to the transistor itself.kamsistorOUTPUTINPUTtramsistorFig 9 氏 curved iiamsisto

46、r charate nsticmay in troduce distrotio nAll of the forego ing presupposesthat the circuit comp onents have bee n selected to produce the proper bias for the specific tran sistor used. If this is not the case, distorti on may occur that has no conn ecti on with the in here nt characteristics of the

47、tran sistor.COLLECTOR VOLATAGE, VOLTS3mAD. 125mA 今巴&b二0 25mA 6 1mAFig 10 A small part of the Collctor charate nsticEither the collector characteristic curves or the lbIc curve may be used to estimate thebeta of a tran sistor in just a few sec on ds. Since the collector characteristics are those

48、gen erallyfound in manu facturers' rati ng sheets, suppose we use these to check the beta, or base curre ntgain as it is ofte n called, of the 2N78.Since this transistor normally operates at a collector potential of 5 volts, we first locate the5-volt line. Then we select an average regi on of ba

49、se curre nt cha nge along this line, say from100 microamperes to 125 microamperes. Using our kno wledge that beta = Ic/ lb ,we can takethe change of I b from 100 卩 A to 125 卩 A for a total of Ib=25 卩 A . We then note that I c goes from 2.9 ma to 3.5 ma over this range of I b .Substitut ing these val

50、ues in the equati on we have: 3.5-2.90.6P 0.0250.025=246 The Sin gle-Battery Com mon-Emitter CircuitINPUTDISTORTIONAt this poin t, let us refer back to where we con verted the two-battery com mon-base amplifier into a single-battery circuit. You will find that it was necessaryto add a special resist

51、ance and capacitor in the base circuit to provide the required biasing potentials. That is, R b 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 an alyze the two-battery com mon-base circuit further, it is

52、appare nt that the reas on 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 curre nt direct ions are reversed), we see tha e and Ic flow in opposite

53、direct ions in the com mon lead going to the base. Since one battery, placed any where at all in the circuit, could n ever produce two oppositely-directed curre nts in a com mon eleme nt, it is n ecessary to create the correct bias potentials by means of an artificial aid the bias resistor.Now let u

54、s an alyze the two-battery com mon-emitter circuit. V ee forces curre nt up through R i (or through the signal source itself if the source has d-c continuity), through the base to the emitter, the n dow nward through the com mon lead back to the positive termi nal of the battery as shown by the dark arrows. V drives