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1、1需要购买毕业论文/毕业设计，请登陆 360 毕业 设计网- 或加 QQ：604664738Analog and Digital Ideal Operational Amplifiers and Practical Limitations In order to discuss the ideal parameters of operational amplifiers, we must first define the terms, and then go on to describe what we regard as the ideal values for those terms. A
2、t first sight, the specification sheet for an operational amplifier seems to list a large number of values, some in strange units, some interrelated, and often confusing to those unfamiliar with the subject. The approach to such a situation is to be methodical, and take the necessary time to read an
3、d understand each definition in the order that it is listed. Without a real appreciation of what each means, the designer is doomed to failure. The objective is to be able to design a circuit from the basis of the published data, and know that it will function as predicted when the prototype is cons
4、tructed.1 It is all too easy with linear circuits, which appear relatively simple when compared with todays complex logic arrangements, to ignore detailed performance parameters which can drastically reduce the expected performance. Let us take a very simple but striking example. Consider a requirem
5、ent for an amplifier having a voltage gain of 10 at 50 KHz driving into a 10 k load. A common low-cost, internally frequency-compensated op amp is chosen; it has the required bandwidth at a closed- loop gain of 10, and it would seem to meet the bill. The device is connected, and it is found to have
6、the correct gain. But it will only produce a few volts output swing when the data clearly shows that the output should be capable of driving to within two or three volts of the supply rails. The designer has forgotten that the maximum output voltage swing is severely limited by frequency, and that t
7、he maximum low-frequency output swing becomes limited at about 10 KHz. Of course, the information is in fact on the data sheet, but its relevance has not been appreciated. This sort of problem occurs regularly for the inexperienced designer. So the moral is clear: always take the necessary time to w
8、rite down the full operating requirements before attempting a design. Attention to the detail of the performance specification will always be beneficial. It is suggested the following list of performance details be considered: 1. Closed loop gain accuracy, stability with temperature, time and supply
9、 voltage. 2. Power supply requirements, source and load impedances, power dissipation. 3. Input error voltages and bias currents. Input and output resistance, drift with time and temperature. 4. Frequency response, phase shift, output swing, transient response, slew rate, frequency stability, capaci
10、tive load driving, overload recovery. 5. Linearity, distortion and noise. 6. Input, output or supply protection required. Input voltage range, common-mode rejection. 7. External offset trimming requirement. Not all of these terms will be relevant, but it is useful to remember that it is better to co
11、nsider them initially rather than to be forced into retrospective modifications. Never forget this fact. How many times has a circuit been designed using typical values, only to find that the circuit does not work because the device used is not typical? The above statement thus poses a tricky questi
12、on: when should typical values and when should worst-case values be used in the design? This is where the judgment of the experienced designer must be brought to bear. Clearly, if certain performance requirements are mandatory, then worst-case 键入文字2values must be used. In many cases, however, the de
13、sirability of a certain defined performance will be a compromise between ease of implementation, degree of importance, and economic considerations. In the end, we are all controlled by cost, and it is really pointless taking a sledgehammer to crack a nut, Simplicity is of the essence since the low p
14、arts count implementation is invariably cheaper and more reliable. As an example of this judgment about worst-case design, consider a low-gain DC transducer amplifier required to amplify 10 mV from a voltage source to produce an output of .l V with an accuracy of 1% over a temperature range of 070C.
15、 Notice that the specification calls for an accuracy of 1%. This implies that the output should be 1 V 10 mV from 0 70C. The first step is, of course, to consider our list above, and decide which of the many parameters are relevant. Two of the most important to this (very limited) specification are
16、offset voltage drift and gain stability with temperature. We will assume that all initial errors are negligible (rarely the case in practice). The experienced designer would know that most op amps have a very large open-loop gain, usually very much greater than 10000. A closed-loop gain change of 1% implies that the loop gain (as explained later) should change by less than 100% for a closed-loop gain of 100. This is clear
