自动化专业外文翻译运算放大器.docx

1、自动化专业外文翻译运算放大器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 two-port system (p, fl, R, Ro, etc.)? This makes design difficult since these parameters usually vary f

2、rom device to device, as well as with temperature. The operational amplifier, or Op-Amp, is designed to minimize this dependence and to maximize the ease of design. An Op-Amp is an integrated circuit that has many component part such as resistors and transistors built into the device. At this point

3、we will make no attempt to describe these inner workings. A totally general analysis of the Op-Amp is beyond the scope of some texts. We will instead study one example in detail, then present the two Op-Amp laws and show how they can be used for analysis in many practical circuit applications. These

4、 two principles allow one to design many circuits without a detailed understanding of the device physics. Hence, Op-Amps 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

5、 circuits and electronics they will also show how to build simple filter circuits using Op-Amps. The transistor amplifiers, which are the building blocks from which Op-Amp integrated circuits are constructed, will be discussed. The symbol used for an ideal Op-Amp is shown in Fig. 1-2A-1. Only three

6、connections are shown: the positive and negative inputs, and the output. Not shown are other connections necessary to run the Op-Amp such as its attachments to power supplies and to ground potential. The latter connections are necessary to use the Op-Amp in a practical circuit but are not necessary

7、when considering the ideal 0p-Amp applications we study in this chapter. The voltages at the two inputs and the output will be represented by the symbols U+, U-, and Uo. Each is measured with respect t ground potential. Operational amplifiers are differential devices. By this we mean that the output

8、 voltage with respect to ground is given by the expression Uo =A(U+ -U-) (1-2A-l) where A is the gain of the Op-Amp and U+ and U - the voltages at inputs. In other words, the output voltage is A times the difference in potential between the two inputs. Integrated circuit technology allows constructi

9、on of many amplifier circuits on a single composite chip of semiconductor material. One key to the success of an operational amplifier is the cascading of a number of transistor amplifiers to create a very large total gain. That is, the number A in Eq. (1-2A-1) can be on the order of 100,000 or more

10、. (For example, cascading of five transistor amplifiers, each with a gain of 10, would yield this value for A.) A second important factor is that these circuits can be built in such a way that the current flow into each of the inputs is very small. A third important design feature is that the output

11、 resistance of the operational amplifier (Ro) is very small. This in turn means that the output of the device acts like an ideal voltage source. We now can analyze the particular amplifier circuit given in Fig. 1-2A-2 using these characteristics. First, we note that the voltage at the positive input

12、, U +, is equal to the source voltage, U + = Us. Various currents are defined in part b of the figure. Applying KVL around the outer loop in Fig. 1-2A-2b and remembering that the output voltage, Uo, is measured with respect to ground, we have -I1R1-I2R2+U0=0 (1-2A-2) Since the Op-Amp is constructed

13、in such a way that no current flows into either the positive or negative input, I- =0. KCL at the negative input terminal then yields I1 = I2 Using Eq. (1-2A-2) and setting I1 =I2 =I, U0=(R1+R2)I (1-2A-3) We may use Ohms law to find the voltage at the negative input, U-, noting the assumed current d

14、irection and the fact that ground potential is zero volts: (U-0)/ R1=ISo, U-=IR1and from Eq. (1-2A-3), U- =R1/(R1+R2) U0Since we now have expressions for U+ and U-, Eq. (1-2A-l) may be used to calculate the output voltage,U0 = A（U+-U-）=AUS-R1U0/(R1+R2)Gathering terms, U0 =1+AR1/(R1+R2)= AUS (1-2A-4)

15、and finally, AU = U0/US= A(R1+R2)/( R1+R2+AR1) (1-2A-5a)This is the gain factor for the circuit. If A is a very large number, large enough that AR (R1+R2),the denominator of this fraction is dominated by the AR term. The factor A, which is in both the numerator and denominator, then cancels out and

16、the gain is given by the expression AU =(R1+R2)/ R1 (1-2A-5b)This shows that if A is very large, then the gain of the circuit is independent of the exact value of A and can be controlled by the choice of R1and R2. This is one of the key features of Op-Amp design the action of the circuit on signals

17、depends only upon the external elements which can beeasily varied by the designer and which do not depend upon the detailed character of the Op-Amp itself. Note that if A=100 000 and (R1 +R2)/R1=10, the price we have paid for this advantage is that we have used a device with a voltage gain of 100 00

18、0 to produce an amplifier with a gain of 10. In some sense, by using an Op-Amp we trade off power for control. A similar mathematical analysis can be made on any Op-Amp circuit, but this is cumbersome and there are some very useful shortcuts that involve application of the two laws of Op-Amps which

19、we now present. 1) The first law states that in normal Op-Amp circuits we may assume that the voltage difference between the input terminals is zero, that is, U+ =U- 2) The second law states that in normal Op-Amp circuits both of the input currents may be assumed to be zero: I+ =I- =0 The first law

20、is due to the large value of the intrinsic gain A. For example, if the output of an Op- Amp is IV and A= 100 000, then ( U+ - U- )= 10-SV. This is such a small number that it can often be ignored, and we set U+ = U-. The second law comes from the construction of the circuitry inside the Op-Amp which

21、 is such that almost no current flows into either of the two inputs. B: Transistors Put very simply a semiconductor material is one which can be doped to produce a predominance of electrons or mobile negative charges (N-type); or holes or positive charges (P- type). A single crystal of germanium or

22、silicon treated with both N-type dope and P-type dope forms a semiconductor diode, with the working characteristics described. Transistors are formed in a similar way but like two diodes back-to-back with a common middle layer doped in the opposite way to the two end layers, thus the middle layer is

23、 much thinner than the two end layers or zones. Two configurations are obviously possible, PNP or NPN (Fig. 1-2B-l). These descriptions are used to describe the two basic types of transistors. Because a transistor contains elements with two different polarities (i.e., P and N zones), it is referred

24、to as a bipolar device, or bipolar transistor. A transistor thus has three elements with three leads connecting to these elements. To operate in a working circuit it is connected with two external voltage or polarities. One external voltage is working effectively as a diode. A transistor will, in fa

25、ct, work as a diode by using just this connection and forgetting about the top half. An example is the substitution of a transistor for a diode as the detector in a simple radio. It will work just as well as a diode as it is working as a diode in this case. The diode circuit can be given forward or

26、reverse bias. Connected with forward bias, as in Fig.l-2B-2, drawn for a PNP transistor, current will flow from P to the bottom N. If a second voltage is applied to the top and bottom sections of the transistor, with the same polarity applied to the bottom, the electrons already flowing through the

27、bottom N section will promote a flow of current through the transistor bottom-to-top. By controlling the degree of doping in the different layers of the transistor during manufacture, this ability to conduct current through the second circuit through a resistor can be very marked. Effectively, when

28、the bottom half is forward biased, the bottom section acts as a generous source of free electrons (and because it emits electrons it is called the emitter). These are collected readily by the top half, which is consequently called the collector, but the actual amount of current which flows through t

29、his particular circuit is controlled by the bias applied at the center layer, which is called the base. Effectively, therefore, there are two separate working circuits when a transistor is working with correctly connected polarities (Fig. 1-2B-3). One is the loop formed by the bias voltage supply en

30、compassing the emitter and base. This is called the base circuit or input circuit. The second is the circuit formed by the collector voltage supply and all three elements of the transistor. This is called the collector circuit or output circuit. (Note: this description applies only when the emitter

31、connection is common to both circuits known as common emitter configuration.) This is the most widely used way of connecting transistors, but there are, of course, two other alternative configurations - common base and common emitter. But, the same principles apply in the working of the transistor i

32、n each case. The particular advantage offered by this circuit is that a relatively small base current can control and instigate a very much larger collector current (or, more correctly, a small input power is capable of producing a much larger output power). In other words, the transistor works as an amplifier. With this mode of working the base-emitter circuit is the input side; and the emitter through base to collector circuit the output side. Although these have a common path through base and emitter, the two circuits are effectively separa