【正文】 它們是金屬外殼，帶有延長的底部平面，底部平面上還有兩個安裝孔。集電極引腳 到基集引腳的間距也許比發(fā)射極到基集引腳的間距要大 。 PNP 型晶體管的符號在發(fā)射極上有一個指示電流方向的箭頭，總是指向基極。 本電路的突出優(yōu)點是相對小的基極電流能控制和激發(fā)出一個比它大得多的集電極電流 (或更恰當?shù)卣f，一個小的輸入功率能夠產(chǎn)生一個比它大得多的輸出功率 )。 在生產(chǎn)晶體管的過程中，通過控制不同層的摻雜度，經(jīng)過負載電阻流過第二個電路電流的導電能力非常顯著。要使工作電路運行，晶體管需與兩個外部電壓或極性連接。第二個定律來自于運算放大器的內(nèi)部電路結(jié)構(gòu)，此結(jié)構(gòu)使得基本上沒有電流流入任何一個輸入端。那么對負輸入端利用基爾霍夫定律可得 I1 = I2， 利用等式 (12A2) ，并設(shè) I1 =I2 =I， U0 = (R1 +R2) I (12A3) 根據(jù)電流參考方向和接零管腳電位為零伏特的事實，利用歐姆定律，可得負極輸入電壓 U： (U0)/ R1=I 因此 U=IR1 ，并由式 (12A3)可得： U =[R1/(R1+R2)] U0 因為現(xiàn)在已有了 U+ 和 U的表達式，所以式 (12A1)可用于計算輸出電壓， U0 = A（ U+U） =A[USR1U0/(R1+R2)] 綜合上述等式，可得： U0 =[1+AR1/(R1+R2)]= AUS (12A4) 最后可得： AU = U0/US= A(R1+R2)/( R1+R2+AR1) (12A5a) 這是電路的增益系數(shù)。也就是說，等式 (12A1)中的數(shù) A約為 100,000 或更多 (例如 ，五個晶體管放大器串聯(lián)，每一個的增益為 10，那么將會得到此數(shù)值的 A)。在實際電路中使用運算放大器時，后者是必要的，但在本文中討論理想的運算放大器的應用時則不必考慮后者。在這里我們將詳細研究一個例子，然后給出兩個運算放大器定律并說明在許多實用電路中怎樣使用這兩個定律來進行分析。 reading 1,2,3 from top to bottom with the flat side to the right looking at the base. With TO92 subtype a (TO92a): 1=emitter 2=collector 3=base With TO92 subtype b (TO92b): 1=emitter 2=base 3=collector To plicate things further, some transistors may have only two emerging leads (the third being connected to the case internally)。 for positive, with a PNP transistor). In the case of an NPN transistor, exactly the same working principles apply but the polarities of both supplies are reversed (Fig. 12B4). That is to say, the emitter is always made negative relative to base and collector (39。P39。UNIT 2 A: The Operational Amplifier One problem with electronic devices corresponding to the generalized amplifiers is that the gains, Au or A~, depend upon internal properties of the twoport system (p, fl, R~, Ro, etc.)?~ This makes design difficult since these parameters usually vary from device to device, as well as with temperature. The operational amplifier, or OpAmp, is designed to minimize this dependence and to maximize the ease of design. An OpAmp is an integrated circuit that has many ponent part such as resistors and transistors built into the device. At this point we will make no attempt to describe these inner workings. A totally general analysis of the OpAmp is beyond the scope of some texts. We will instead study one example in detail, then present the two OpAmp laws and show how they can be used for analysis in many practical circuit applications. These two principles allow one to design many circuits without a detailed understanding of the device physics. Hence, OpAmps are quite useful for researchers in a variety of technical fields who need to build simple amplifiers but do not want to design at the transistor level. In the texts of electrical circuits and electronics they will also show how to build simple filter circuits using OpAmps. The transistor amplifiers, which are the building blocks from which OpAmp integrated circuits are constructed, will be discussed. The symbol used for an ideal OpAmp is shown in Fig. 12A1. Only three connections are shown: the positive and negative inputs, and the output. Not shown are other connections necessary to run the OpAmp such as its attachments to power supplies and to ground potential. The latter connections are necessary to use the OpAmp in a practical circuit but are not necessary when considering the ideal 0pAmp applications we study in this chapter. The voltages at the two inputs and the output will be represented by the symbols U+, U, a