How should I learn analog circuits?

I believe that many science and engineering majors will learn analog and digital circuits, so how should they learn it? Have you just started learning analog circuits? Do you feel that your learning is foggy? Do you feel that the teacher’s lectures are not good? Do you feel that the teaching materials are terrible? Well, stop looking for excuses. If you are not good at learning, you probably haven’t found a method. Let’s share the experiences of three seniors with you. You, look at how the seniors became rookies and became experts.

The first one, senior analog IC design engineer, Zhihu user Yike, has a strong sense of responsibility to teach everyone and solve their doubts, so that everyone who reads this article will avoid detours when learning analog electronics, and there will be more Time to play football, pick up girls, chat and drink...

I know you all have a heavy workload, so let’s get to the point:

When I was in college, I also felt that the course of analog circuits was confusing. Especially after playing a CS, this feeling is even more obvious. Here we must first make sure that the subject is a good and motivated child. I want to learn analog electronics well. When I felt confused, I never thought about learning analog electronics well. What I do is to build a good relationship with the students who study well in the class. In this way, I can successfully borrow the notes at the end of the semester, and it will be easy to score high.

So when it comes to this, the first step is to clarify: what counts as "learning" analog circuits well. If your goal is to get high marks at the end of the semester, then you don’t need to read any further. When you go out, go to the Electronic Science and Technology Bookstore and have exam questions and courseware from previous years. Do what you should do on weekdays. Take Xingye's lines and say, girls dance and dance together. Just wait until the end of the semester to memorize it. If the scores don't tell the story. So what does it take to learn analog circuits well? Ask yourself a question: What can I do if I learn analog circuits?

To design an amplifier with a gain of 5? Many years later, I looked back at my time in college and finally figured out why I always felt that I had not learned analog circuits well. The answer is actually very simple. I feel like I have learned a lot of things, but I don’t know what they are used for. I wonder if those of you who think analog electronics are weird have similar feelings.

What do you want to learn about analog circuits?

I want to answer this question. This is an important question. Many people have questions, now that we are in a digital age, why should I learn analog circuits. There is another question in zhihu: "Will analog circuit designers disappear?"

The answer is: no.

As long as we still need to be in contact with the real world, we will inevitably need analog circuits, so we need lovely analog circuit designers. Let’s use an analogy. Take the CPU as an example. The CPU processes all digital signals. But it has no way to use digital circuitry to monitor its own temperature. This interface will always be a mock interface. The operating voltage requirements required by the CPU are very precise. For example, it is 1V. Think about it, everyone, how to achieve this 1V? There is no way to achieve it using digital circuits. You swiped on the screen of your phone, how does your phone know that you swiped it?

How to realize gravity sensing?

You can't live without analog circuits. Analog circuits are like your eyes, ears, mouth, nose, hands and feet. Digital circuits are like your brain. As long as the future world does not develop into connecting two wires directly to the brain and pumping in some dopamine when needed, the world will need analog circuits to complete the interface between the virtual world and the real world.

Now we can talk about how we complete this interface.

Now suppose we want to build a circuit to sense the temperature of your mobile phone battery to prevent it from getting higher and higher and finally exploding while you are chatting with a girl. The disfigurement was minor, and I had to spend money to buy a new mobile phone. The engineer in charge of peripheral applications thoughtfully gave you a thermistor. The resistance of the resistor will continue to decrease as the temperature rises. One thing he hopes you can do is to send out a digital signal with an amplitude of 3V when the temperature is higher than a certain value so that the system can turn off the battery.

What do we need? First we need a power supply. Without power, nothing works. What does the power supply need to do? It may be a good way to draw power directly from the battery, but the amplitude of the output voltage is limited, what should I do? Now, let’s make a local 3V power supply. What are the power supply requirements? The lower the internal resistance, the better. What kind of circuit can give a low output internal resistance? Voltage-voltage feedback op amp.

So the first required block is an op amp. (Digression: In the era of discrete devices, we could buy an op amp. If you want to do a chip-level design, then we need to choose the appropriate device and build the op amp on the chip.) Okay, op amp There is an amplifier, but there is no reference voltage. How can the op amp output a voltage of exactly 3V?

The second required block is a voltage reference source.

(In the era of discrete devices, we could buy a reference voltage source. If the subject wants to do chip-level design, then we need to make a reference voltage on the chip. At present, almost all reference voltages rely on the energy band of silicon itself. To achieve. So it is called bandgap reference. It is about 1 or 2V. The process of realizing bandgap reference will not be an open-loop implementation, but a closed-loop process. It requires loop analysis, stability analysis, and mismatch analysis. .)

Now, you have obtained the voltage source of 1, 2V, and then made a resistor with a resistance of 1, 2: 1, 8 as a feedback resistor, and used an op amp to obtain a 3V power supply voltage. You use the loop stability analysis method to analyze and know that the loop is stable. Calculate the output resistance of the circuit to know approximately how much current output capability the circuit has and how much load it can carry. Not bad, you think. The analysis method of virtual short circuit and virtual open circuit is quite reliable.

Then comes the really useful part. You need a comparator to compare the thermistor and non-thermistor divided voltages to a reference voltage. Then just use a comparator.

(In the era of discrete devices, you can buy a comparator and want to do it on a single chip, then design a comparator yourself. Comparators designed by yourself are often not so ideal. There is no full voltage output range, and there is no full voltage output. Range. The gain may only be 60 dB. But when you look at the parameter requirements, that’s enough, 60dB is just 60dB, it’s better than nothing.)

alright. You've finished your design.

The above is just a simple example. The actual analog circuit system encountered is much more complex than this small system. Market requirements are also becoming more and more abnormal. Who knows there are so many smart people designing analog circuits? Therefore, people who design analog circuits are working hard. Use your brain to meet all kinds of unreasonable demands and meet many unreasonable standards.

But for the subject, these are all things for later.

If the subject hopes to do work in the field of simulation design, can you tell how many subjects you need to learn based on the small example written above?

1. Circuit analysis

2. Basics of analog circuit design

3. Signals and systems

4. Feedback theory/compensation theory

If you want to do analog IC design, you also need to study the following subjects:

1. Semiconductor process technology

2. Principles of semiconductor devices

3. Knowledge of probability and statistics

4. Analog IC design.

Among them, analog IC design includes:

1. Small signal analysis

2. Linear modeling of amplifiers

3. Baseline design

4. ESD protection

5. Layout design

6. Parasitic effects

6. Failure analysis

7. Noise

8. Oscillator

9. Too many ellipses

I hope the answer will satisfy the questioner or help more young EEs.

The above answers are for analog integrated circuit design. The following Zhihu user Tariel focuses on signal chain design:

(The following content is mainly focused on signal chain design, that is, known application requirements, allocating indicators between interfaces with the real world (sensors/actuators), signal conditioning circuits/actuator drive circuits, ADCs, digital domain devices, and Simulation part of the design process.)

First of all, let me give you some chicken soup for the soul: why not learn analog circuits.

1. Don’t learn analog circuits if you are under financial pressure, especially if you are under pressure to pay off your mortgage in first-tier cities such as Beijing, Shanghai, and Guangzhou. Although analog circuits sound very high-end and are considered by everyone to be black technology, the market in this industry is too small and divided too finely. To expect it to make money is too random; and in this industry, frequent job hopping is actually detrimental to the level of the industry. The improvement is unfavorable. So if you are under financial pressure, please consider turning to the lowest-risk method of getting rich through labor as soon as possible - becoming a code farmer. Of course, if you can afford the entrepreneurial risk, it would be good to participate in the currently popular smart hardware industry.

2. Don’t learn analog circuits if you are not interested. Working on analog circuits requires a lot of time and continuous mood. If you are not interested in this work and feel that you cannot develop interest in it, it is better to change your behavior as soon as possible, because if you are not interested and do not think about it, what will remain in your head will be Things will only become more and more complicated, which will not help improve work.

3. If you don’t know how to do it, or don’t want to do it, don’t learn analog circuits. When working on digital circuits, design engineers can just draw schematics and leave all downstream things (layout, routing and even process matters) to others, but working on analog circuits requires design engineers to build prototypes and diagnose faults by themselves. In this case, simulation alone is not enough. So if you don’t know which end to hold the soldering iron, or you are afraid of holding the soldering iron, or you don’t bother to hold the soldering iron, please change your profession as soon as possible; if the soldered products are the same as tofu dregs, after troubleshooting the soldering iron failure and the use of inferior solder, please also Change careers as soon as possible.

4. If your brain is not easy to use and your logic ability is insufficient; or if your brain tends to go astray, do not learn analog circuits. Of course, if your brain doesn't work well, you won't be able to handle not only analog circuits, but also digital circuits, code writing, and other engineering and technical work. It's better not to be an engineer as soon as possible. As for whether your brain tends to go astray, you can use @ChrisXia's encyclopedia of biases in linguistics and folk science to test yourself - because language is something that everyone has been exposed to every day since childhood. The more your mind wanders in "spontaneous language research", the more likely you are to become a professional. After entering the field, the tendency of the mind to wander becomes greater. If you watched the movie with too many arrows in the knee just now and still insist on your own prejudices after watching it, you can consider eating some walnuts that were caught in the door to replenish your brain.

5. I have a habit of keeping my workbench very neat and tidy, so I am not suitable for working on analog circuits. I won’t go into detail on this point, but here’s a picture (the cover of the book Analog Circuit Design: Art, Science and Personalities edited by Jim Williams):

If you feel uncomfortable facing such a workbench, it is best to change your behavior as soon as possible. Of course, compared with the previous points, this point is easier to overcome. . .

After drinking the spiritual medicine, how many of you are still alive? Congratulations to the students who are alive to see this place. Can I think that you have the consciousness to learn analog circuits well and use it as a hobby or career? Then we will go back to Let’s get to the point—how to learn analog circuits well.

To give an incomplete summary, analog circuit design probably has several characteristics that are different from other engineering design fields, especially digital circuit design:

1. The amount of processing by analog circuits comes from the real world, so the input of analog circuits includes both signals related to the design and signals unrelated to the design. For example, when designing an electrocardiograph, the electrodes collect not only the ECG signal itself that needs to be processed, but also the polarization potential (DC) of the electrode, the 50Hz interference induced from the power supply, etc., and these irrelevant signals are often larger than what is needed. The signal processed is much stronger. Therefore, Siege Lion needs to analyze the characteristics of the signal (such as the different frequency ranges occupied by ECG signals, polarization potentials, and 50Hz interference; 50Hz interference is a common mode signal, while the ECG signal is a differential signal, etc.) and design the corresponding circuit to extract the signals that need to be processed and suppress signals that are irrelevant to the design (such as designing a suitable filter to filter out-of-band interference, using a differential input instrumentation amplifier to eliminate common-mode interference, etc.).

2. Factors ignored during theoretical analysis and simulation may have a great impact, or even a decisive impact, on the actual circuit. The circuit schematic diagram can only reflect the connection between components and is topological; while the actual circuit is physical. This is also the biggest difference between the content in the textbook and the actual circuit. For example, the task requires Siege Lion to design a 220V to 5V switching power supply. OK, many semiconductor manufacturers have controllers for isolating switching power supplies. Just look at the reference design given in the data sheet and change a few feedback components according to the calculations. Is it enough? Most of the things produced in this way can work; but they can only work - in fact, the difference in printed board layout can seriously affect the size of the output ripple. Even in many cases (such as when performing radio frequency design), the distribution parameters of the printed circuit board will also be used as components in the circuit.

3. Analog circuit design is full of various constraints such as technical indicators, power consumption, and cost, and these constraints often cannot be satisfied at the same time, and even conflict with each other. For example, when designing a portable electrocardiograph, considering the strict restrictions on power consumption and the convenience of directly using the power supply provided by the system, you will tend to use a low-voltage single power supply; taking into account the common mode rejection ratio and dynamic range requirements, you will also Prefer dual power supplies. Compromises and compromises and trade-offs are made between these constraints throughout the design.

So what do you need to do if you want to engage in analog circuit design? The following is an incomplete list (details to be added):

1. Be familiar with circuit analysis methods and master the use of at least one simulation software.

2. Master the method of reading component data sheets.

3. Do more hands-on experiments.

4. Organize the results you obtain in a timely manner, especially the negative results.

5. Master the skills of power supply design.

6. Understand the content of thermal design.

7. Regarding troubleshooting skills.

If the above two are not enough, here is another one. This Zhihu user Li Xuan recommends a very useful free software for you: strong interest will be your eternal motivation in learning analog circuits. Amateur radio? Audio amplifier? Instrument and tool enthusiast?…Many masters fell in love with analog design from their childhood hobbies.

Analog circuit design = system design (the essence is feedback) + circuit analysis (graphical understanding of mathematical methods) + utilization of various characteristics of active/passive devices (understanding the actual characteristics of the device)

Specific to learning:

1. The skillful application of graphical methods such as UI, combined with the characteristic curves of various devices, is of great help to the understanding of the circuit and promotes intuitive understanding.

2. Try to use more methods such as superposition principle and Thevenin equivalent.

(The above two items actually require a good foundation of circuit analysis)

3. Don’t memorize too many formulas, pay attention to reasonable approximation and intuitive understanding of the meaning of the formulas.

4. Feedback is the essence of analog design. Learn it thoroughly and accumulate skills. It is recommended to use pure transistors to make AGC circuits and regulated power supplies to deepen your understanding.

5. This is an engineering discipline that requires a lot of practice to correct biases in your understanding.

Strongly recommended: If the experimental conditions are insufficient, it is recommended to use LTspice simulation. It is a compact free software, easy to operate, and you will love it! The above are some recommended learning methods. I hope it will be helpful for everyone to learn analog electronics.