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　　Interesting Analog Circuit Course_It turns out that analog circuits are still very important

　　Among sub-majors, analog circuits is a very important course, and one that many people find difficult. Here I will talk about my understanding of the analog circuit course, I hope it can be helpful to everyone.

　　1. Engineering thinking

　　When it comes to test scores, my test scores are average, not high scores; but when it comes to the understanding and application of analog circuits, I have made some things with analog circuits and participated in some competitions. Analog circuit is an engineering course. The focus of learning it is to master the engineering ideas, and it is best to use it in practice, not just for exams.

　　What is engineering thinking? Baidu Encyclopedia’s explanation is this:

　　Engineering is a certain application of science and mathematics. Through this application, the properties of matter and energy in nature can be made through various structures, machines, products, systems and processes in the shortest time and with the smallest amount of manpower. Something that is efficient, reliable and useful to humans. So the concept of engineering came into being, and it gradually developed into an independent discipline and skill.

　　For example, in analog circuits, there is a very important approximation of engineering ideas. In middle school physics classes, many of the circuits we learn are ideal circuits. The wire resistance is always 0, the efficiency of the transformer is 100%, the internal resistance of an ideal voltmeter is infinite, the internal resistance of an ideal ammeter is 0, etc. You can find that many times calculations in analog circuits often omit one or two relatively small terms, and use the equal sign directly instead of approximating the equal sign.

　　Why use approximation? To put it bluntly, human science's understanding of nature is not comprehensive enough and cannot describe natural phenomena absolutely accurately; or human understanding is limited and accurate description is too costly. Through approximate means, not only does it have no obvious impact on solving the problem, but it also greatly simplifies the steps and saves time and energy. Using this kind of thinking, human science has achieved many results and its reliability has been fully proved.

　　2. Summary

　　Analog electronics itself is a very complex subject, and analog electronics courses are only the most basic of them. Analog circuit (AnalogCircuit) means an electronic circuit that processes analog signals. Most signals in nature are analog signals, which have continuous amplitude values, such as the sound signal when speaking. Analog circuits can directly process such signals (of course they need to be converted into electrical signals first). For example, power amplifiers can amplify sound signals, and radio stations can send analog sound signals and image signals. It can even be considered that the basis of all circuits is analog circuits (even digital circuits, their underlying principles are based on analog circuits). Its importance is self-evident.

　　Due to the rapid development of digital circuits and programmable devices, many superior characteristics have been demonstrated. Many electronic devices are slowly becoming digital, but they are still inseparable from analog circuits.

　　Currently, the most important devices in analog circuits are semiconductor devices. The most basic and commonly used semiconductor devices are diodes, transistors, field effect transistors and operational amplifiers.

　　Diodes have many functions. For example, ordinary diodes can be used for rectification, light-emitting diodes can be used for indicators and lighting, voltage regulator tubes can stabilize voltage, varactor diodes can be used for signal modulation, etc. In analog electronics courses, the part involving diodes is relatively simple, and many characteristics of field effect transistors are similar to triodes, so triodes or op amps are often used as the main subject for explanations.

　　3. Transistors and amplifiers

　　The basic function of the triode is amplification. Through this characteristic, the triode forms various circuits and embodies many engineering ideas.

　　The basic circuit of a triode is an amplifier. For example, a power amplifier is an amplifier. The input sound is very small, but the output sound is very loud. The ratio of the amplifier's output to the input voltage (or current) is called amplification, also called gain.

　　For a voltage, if time is used as the horizontal axis and voltage is used as the vertical axis, the graph is the waveform of the voltage.

　　If an amplifier with an amplification factor of 5 inputs a constant voltage of 1V (the waveform is as shown in the left picture), the output should always be 5V (the waveform is as shown in the middle picture), which will neither change with time nor with temperature. The output The shape is exactly the same as the input voltage. However, if the magnification is unstable and constantly changing, the original input signal will be deformed (as shown on the right below), and the signal may change from a horizontal straight line to a curve. This change in waveform is called distortion.

　　An ideal amplifier would have a constant amplification factor. If the amplification factor of the power amplifier is unstable, the sound will be louder and softer, and the change in waveform will also cause the sound to change, that is, distortion.

　　Reality always contradicts ideals. Unfortunately, the characteristics of the triode are not ideal. When it works in an amplification circuit, the amplification factor is not only affected by the input voltage and power supply voltage, but also the temperature change caused by its own heating will also affect its amplification factor. This is really a headache for many engineers. If no effective method can be found to reduce the impact of this characteristic, it will be difficult to apply the transistor in practice.

　　4. Negative feedback

　　basic concept

　　So some very powerful people found a good way: negative feedback. What is negative feedback?

　　Feedback refers to returning the output of the system to the input end to affect the input, thereby affecting the overall output of the system. Feedback can be divided into positive feedback and negative feedback. Negative feedback causes the output to have the opposite effect to the input and stabilizes the system output.

　　The above explanation is not easy to understand, so I will give you two examples.

　　1. When playing the inverted pendulum, we support an inverted wooden stick with our hands. When the wooden stick tilts in a certain direction, we offset this change by moving our hands to the direction of the tilt of the wooden stick, so that the wooden stick can move in the direction of the inverted pendulum. Balance on hand.

　　2. When I was in high school, I often took monthly exams. I found that some students had this habit: when they did poorly on one exam, they would start studying hard, and then their grades would go up next time; The next month I will relax again, so my grades will drop again, and so on.

　　Both examples fully illustrate that negative feedback can make the system more stable.

　　negative feedback amplifier

　　We ignore the specific circuit and only draw a simple block diagram to illustrate how the triode amplifier circuit uses negative feedback.

　　The triangle below represents an amplifier composed of a triode. The amplification factor is A and the input is I. The output is O=A*I. Since the amplification factor A is unstable, the output waveform will be distorted.

　　Some devices have been added to the circuit as follows.

　　The purple circle is the adder, combined with the purple + and - symbols, indicating that its output Y=(+I)+(-X)=I-X, which can be realized with resistors in the actual circuit;

　　Box F is the feedback device, which means taking the signal from the output O and multiplying it with F to get X, so X=0*F, where F<1 (this part can be implemented with resistors in the actual circuit);

　　Amplifier A represented by a triangle is mainly composed of triodes and satisfies O=A*Y, and the amplification factor of A is unstable and easily affected by interference.

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