Useful Information | Everyone says analog electronics is difficult, but what exactly is so difficult about it? This expert explains it thoroughly

In terms of test scores, my test scores were average, not very high; but in terms of my understanding and application of analog circuits, I have done some things with analog circuits and participated in some competitions. Analog circuits is an engineering course, and the key to learning it is to master the engineering ideas, and it is best to apply them in practice, not just for exams.

What is engineering thinking? Baidu Encyclopedia explains it this way:

Engineering is a certain application of science and mathematics, through which the properties of natural materials and energy can be used through various structures, machines, products, systems and processes to make efficient, reliable and useful things for mankind in the shortest time and with the least manpower. Thus, the concept of engineering was born, and it gradually developed into an independent discipline and skill.

For example, in analog circuits, there is a very important engineering concept - approximation. In high school physics class, many circuits we learned are ideal circuits, the wire resistance is always 0, the efficiency of the transformer is 100%, the internal resistance of the ideal voltmeter is infinite, the internal resistance of the ideal ammeter is 0, etc. You can find that in many cases, the calculations in analog circuits often omit one or two relatively small terms, and directly use the equal sign instead of the approximately equal sign.

Why do we need to use approximation? To put it bluntly, human science does not have a comprehensive understanding of nature and cannot describe natural phenomena with absolute accuracy; or human understanding is limited and the cost of accurate description is too high. Using approximation not only has no obvious impact on solving the problem, but also greatly simplifies the steps and saves time and energy. Using this idea, human science has achieved many results and fully proved its reliability.

Analog electronics itself is a very complex subject, and analog electronics courses are only the most basic of them. Analog circuits refer to electronic circuits that process analog signals. Most signals in nature are analog signals, and they have continuous amplitude values, such as sound signals when speaking. Analog circuits can process such signals directly (of course, they need to be converted into electrical signals first), such as power amplifiers that can amplify sound signals, and radio stations that can send analog sound signals and image signals. It can even be said 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 features have been demonstrated. Many electronic devices are gradually becoming digital, but they still rely on analog circuits.

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

Diodes have many functions, such as ordinary diodes can be used for rectification, light-emitting diodes can be used for indicator lights and lighting, voltage regulators can be used for voltage stabilization, and variable capacitance diodes can be used for signal modulation. In analog electronics courses, the part involving diodes is relatively simple, and many characteristics of field-effect transistors are similar to those of triodes, so triodes or op amps are often used as the main body for explanation.

The basic function of the transistor is amplification. Through this characteristic, the transistor forms various circuits and reflects 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 output and input voltage (or current) of the amplifier is called the amplification factor, also known as gain.

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

If an amplifier with a gain of 5 is fed with a constant 1V voltage (waveform as shown in the left figure below), the output should always be 5V (waveform as shown in the middle figure below), and it will not change with time or temperature, and the output and input voltage shapes are exactly the same. However, if the gain is unstable and constantly changes, the original input signal will be deformed (as shown in the right figure below), and the signal may change from a horizontal straight line to a curve. This waveform change is called distortion.

An ideal amplifier would have a constant gain. If the gain of the amplifier is unstable, the sound will fluctuate, and the waveform change will also cause the sound to change, that is, distortion.

Reality is always contrary to ideal. Unfortunately, the characteristics of the triode are not ideal. When it works in the amplification circuit, the amplification factor is not only affected by the input voltage and power supply voltage, but also by the temperature change caused by its own heat, which will also affect its amplification factor. This is really a headache for many engineers. If we can't find an effective way to reduce the impact of this characteristic, it will be difficult to apply the triode in practice.

basic concept

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

Feedback refers to the process of returning the output of a system to the input to affect the input, thereby affecting the overall output of the system. Feedback can be divided into positive feedback and negative feedback. Negative feedback is when the output has the opposite effect to the input, making the system output tend to be stable.

The above explanation is difficult to understand, so I will give two examples.

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

2. There were frequent monthly exams in high school, and I found that some students had this habit: when they did poorly on an exam, they would start to study hard, and then their grades would go up the next time; but when they did well on the exam, they would slack off in the next month, and then their grades would go down again, and this cycle would repeat itself.

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

Negative Feedback Amplifier

We ignore the specific circuit and just 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 O=A*I. Since the amplification factor A is unstable, the output waveform will be distorted.

Some components are added to the circuit as follows.

The purple circle is an adder, and combined with the purple "+" and "-" symbols, it means that its output is Y=(+I)+(-X)=IX, which can be realized with resistors in actual circuits;

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

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

The system of equations can be listed:

Solve for the amplification factor of the entire circuit:

If the circuit is designed so that the amplification factor A is very large and F is not too small, then

The symbol ">>" indicates that it is much greater than

According to the approximate idea, the amplification factor of the above entire circuit is:

Since the feedback device can be realized by a resistor, the resistance value of an ordinary resistor is not easily affected by external interference, so the value of F is very stable, and the amplification factor of the entire circuit is very stable. We have successfully solved the problem of the stability of the amplification factor of the triode through negative feedback.

It can be seen that the feedback part and the amplification part here form a loop, so the amplification factor of the entire circuit is called loop gain, or closed-loop gain; and the amplification factor A of the circuit before adding feedback is called open-loop gain. Because it is negative feedback, although the circuit gain stability is improved, there is also a price:
due to

then

A>>1/F

That is, the open-loop gain is much greater than the closed-loop gain, which means that the amplifier gain is greatly reduced. But in general, for the sake of stability, it is worth doing so.

In the circuit above, in order to actually create an amplifier with a large open-loop gain A, it is often designed in a way that multiple triode amplifier circuits are connected in series. Since the demand for such high-gain amplifiers is very common, some people in history made them into a finished circuit board module, which can be used directly as a component when needed, which is very convenient. This is the original operational amplifier, or op amp for short.

The development of integrated circuits has made it possible to integrate a large number of transistor components on a small chip, thus giving rise to the integrated operational amplifiers that are very commonly used today.

The name "operational amplifier" was derived from the fact that it was originally used to perform mathematical operations on analog computers. Although digital computers, which are now widely used, no longer use op amps for calculations, the name has been retained. Today, op amps play a very important role in analog circuits and have become one of the focuses of analog electronics courses.

Usually the op amp has two input terminals U+ and U− and one output terminal Uo.

The open-loop gain A of the op amp is often as high as hundreds of thousands to millions, but the output voltage of the op amp is limited by the power supply voltage and cannot exceed the power supply voltage. Therefore, the input-output relationship of the op amp is similar to the shape shown in the figure below.

In the figure, the horizontal axis is , and the vertical axis is U.

In the middle straight line area, the op amp is in normal amplification state, called the linear region, satisfying

Uo=A∗(U+−U−)

When the absolute value of the input is slightly larger, the output will be limited by the power supply and will no longer satisfy the above relationship. The value of Uo is usually slightly smaller than the power supply voltage range (note that the op amp can use dual power supplies, that is, the power supply voltage range can be between a negative value and a positive value), which is called the nonlinear region.

The output of the rail-to-rail op amp can reach the power supply voltage. If you are interested, you can search and learn about it online.

When the op amp works in the linear region, the value of Uo is very limited, but A is very large, so U+−U−=UoA≈0

That is, U+≈U−

At this time, the voltages at the positive and negative input terminals of the op amp are almost equal, just like a short circuit, which is called a virtual short. Therefore, the "virtual short" characteristic only occurs when the op amp works in the amplification area, rather than being an inherent property of the op amp itself.

On the other hand, due to the internal structural characteristics of the op amp, its input impedance is very large.

Input impedance can be simply understood as input impedance = input voltage / input current

The large input impedance means that the op amp input only needs a small current to work properly. Because of this, the op amp can be used to detect some weak currents, such as the human brain waves and muscle waves, whose maximum voltage value is only a few mV and the current value is also very small.

This characteristic of the op amp is called virtual open circuit, which means that the input end is like an open circuit and almost no current flows into it.

Unlike virtual short, virtual open is an inherent property of the op amp and does not change with different circuits.

The operational amplifier is composed of triodes, and like triodes, it obviously has many non-ideal characteristics. The characteristics of ideal operational amplifiers are discussed above. However, the actual operational amplifier will not completely meet the virtual short and virtual open characteristics. When working normally, the input terminal needs current to flow in. This current is called input bias current. Similarly, the operational amplifier also has non-ideal parameters such as input bias voltage, input offset voltage, and input offset current.

These non-ideal characteristics, such as input bias current, although small, can sometimes have a great impact on the circuit, causing the circuit to not work. Therefore, some means are needed to reduce the impact of these factors. In practical applications, the non-ideal characteristics of op amps are a very important issue. There are many ways to eliminate the non-ideal characteristics of op amps, which will not be introduced here.

The core of the analog electronics course is transistors and operational amplifiers. Focusing on these devices, various circuits are explained, including:

●Calculation and analysis of amplifier circuits, multi-stage amplifier circuits, frequency characteristics of amplifiers, and the idea of ​​feedback;

●Power amplifier circuit;

●Comparator, oscillator, integrator, differentiator, waveform generator, etc.;

●Signal operation processing;

●Filter;

●Integrated voltage-stabilized power supply circuit, etc.

In actual circuit design, op amps are used more often than triodes. This is because op amps have many better characteristics than triodes, the circuit design is simpler, and the cost of op amps is often not high. In many cases, the cost of using op amps is lower when using triodes and op amps to achieve the same effect. This is because op amps integrate a large number of transistors into one piece, and the average manufacturing cost of each transistor is very low.

For example, a conventional audio preamplifier can be made with a general-purpose op amp, and the cost may be 0.2 yuan. However, to achieve the same effect with transistors, 10 or even more transistors may be needed, and the cost may be 0.5 yuan. In addition, the labor cost spent on the design is much higher than that of the op amp solution.

Of course, transistors also have their advantages. In some very simple circuits, the stability of the amplification factor is not strictly required, and one or two transistors can complete the task, so transistors are often used to save costs. In addition, under some more extreme conditions, such as working in a high-frequency, high-power environment (such as RF signal transmission circuits), the performance of a well-designed transistor circuit will be much better than that of an op amp, or the cost will be much lower. In some cases, it can only be completed directly using transistors. At this time, it is necessary to use transistors to build the circuit.

This is the end of the introduction to analog electronics. But what I want to say is that analog circuits are a very complex subject, and the knowledge involved is far more than what is in the books. The books are generally introduced according to the working principles, simplifying many problems that are difficult to understand but must be considered in practice, so the gap between the actual circuit and the book is very large. For example, the triangle wave generator built with op amps in the analog electronics book will not work in most cases when used in actual circuits. However, the main principles of the actual circuit are consistent with those described in the book. Therefore, designing analog circuits often requires a lot of experience, and many things are even difficult to explain and cannot be calculated.

Source: Internet compilation. If copyright is involved, please contact us to delete.

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