Deep Understanding of Analog Electronics - Analog Electronics Tutorial

In electronics, analog circuit course is very important, and many people find it difficult. Here I would like to talk about understanding course of analog circuits, I hope to be useful to everyone.

01 Engineering

In terms of understanding and applying analog circuits, I've done some work with analog circuits and competed in some competitions. Analog circuit is an engineering course, purpose of studying it is to master engineering ideas contained in it, at same time, it is best to use it in practice, and not just in exams.

What is engineering thinking?

Baidu Encyclopedia explains:

Engineering is application of science and mathematics by which properties of matter and energy in nature can flow through structures, machines, products, systems and processes with least amount of time and effort. People create effective, reliable and useful for people. This is how concept of engineering was born, which gradually turned into an independent discipline and skill.

For example, in analog circuits there is a very important engineering idea - approximation.

In high school physics class, many of circuits we learn are ideal circuits, resistance of a wire is always 0, efficiency of a transformer is 100%, internal resistance of an ideal voltmeter is infinite, internal resistance of an ideal ammeter is 0, etc. < /p>

You may find that in many cases analog circuit calculations often omit one or two relatively small items and directly use equals sign instead of approximate equals sign.

Why use zoom? Simply put, human science's understanding of nature is not comprehensive enough to describe natural phenomena with absolute accuracy, or human understanding is limited and price of accurate description is too high.

Because of approximation, not only does this not have an obvious impact on solution of problem, but it also greatly simplifies steps and saves time and energy. Using this type of thinking, human science has made many advances, and its reliability has been fully proven.

02 Summary

Model electronics is a very complex subject in itself, and analog electronics course is only most basic. The analog circuit value is electronic circuit that processes analog signals.

The vast majority of signals in nature are analog signals that have continuous amplitude values, such as beep of a conversation. Analog circuits can process such signals directly (of course, they must first be converted to electrical signals), for example, power amplifiers can amplify audio signals, and radio stations can send analog audio and video signals.

You could even think of all circuits as being based on analog circuits (even if it's a digital circuit, it's aboutthe basic principle is also based on analog circuits). Its importance is obvious.

Due to rapid development of digital circuits and programmable devices, it has demonstrated many excellent features. Many electronic devices are gradually being digitized, but analog circuitry is still inseparable.

The most important device in modern analog circuitry is semiconductor device. The simplest and most widely used semiconductor devices are diodes, transistors, field-effect transistors, and operational amplifiers.

Diodes have many functions. For example, conventional diodes can be used for rectification, LEDs for indicator lights and lighting, voltage regulators for voltage regulation, and varactor diodes for signal. modulation. In an analog electronics course, diode part is relatively simple and many characteristics of FETs are similar to triodes, so triodes or op amps are often used as main explanation.

Transistors and Amplifiers

The main function of a triode is amplification. Due to this characteristic, triode forms various circuits that embody many engineering ideas.

The main circuit of a triode is an amplifier. For example, a power amplifier is an amplifier. The input sound is very quiet and output sound is very loud. The ratio of an amplifier's output signal to its input voltage (or current) is called gain, also known as gain.

For voltage, if time is used as horizontal axis and voltage is used as vertical axis for plotting, plot will be a voltage waveform.

If a 1V DC voltage is applied to input of a 5x amplifier (waveform shown in left figure below), output should always be 5V (waveform shown in middle figure), which will not change over time, nor change with temperature change, output voltage waveform is exactly same as input voltage.

However, if gain is not stable and constantly changing, original input signal will be deformed (as shown in right figure below), and signal may change from a horizontal straight line to a curve. This change in waveform is called distortion.

We hope that in an ideal amplifier its gain is constant. If power amplifier gain is unstable, sound will fluctuate, and waveform change will also cause sound to change, i.e., distortion.

Reality always contradicts ideals. Unfortunately, characteristics of triode are not ideal, when working in an amplifier circuit, not only input voltage and supply voltage affect increase, but temperature change caused by self-heating will also affect its increase.

This is indeed a headache for many engineers. If no effective method is found to reduce influence of this characteristic, application of triode in practice will be difficult.

Negative Feedback

Negative feedback. What is negative feedback?

Feedback refers to returning output of system to input in order to influence input, thereby affecting overall output of system. Feedback can be divided into positive feedback and negative feedback. Negative feedback is to force output to play opposite role of input, so that output of system tends to be stable.

The above explanation is not easy to understand, let me give you two examples.

1. When playing inverted pendulum, we support inverted wooden stick with our hands. When wooden stick leans in a certain direction, we counteract this change by moving our hands in direction that wooden stick leans, making wooden stick stick can be balanced in hand.

2. When I was in high school, I often took monthly exams. I found that some students had a habit: when they got a bad mark on one exam, they began to study hard, and then their marks went up. next time; The next month will be quiet again, so ratings will drop again, and so on.

These two examples fully demonstrate that negative feedback can make a system more stable.

Negative feedback amplifier, we are ignoring specific circuit, just draw a simple block diagram to illustrate how a triode amplifier circuit uses negative feedback.

The triangle below represents an amplifier consisting of a triode, gain is A, input is I, then output is O=A*I. Since gain A is not stable, output waveform will be distorted. .

Several components are added to schematic as follows.

The purple circle is adder, combined with purple "+" and "-" symbols, this means that its output is Y=(+I)+(-X)=I-X, which can be realized with a resistor in a real scheme ;

The F box is a feedback device, which means that signal is taken from output O and multiplied by F to get X, so X=0*F where F<1 (this part can be implemented with a resistor in a real chains );

Amp A, represented by a triangle, is mostly triodes, which satisfies O=A*Y condition, and A's gain is unstable and easily broken.

Equations can be listed:

Get an enlargement of the entire scheme:

If circuit is designed so that increase in A is very large and F is not too small, then

The symbol ">>" means much more than

As a rough idea, an enlargement of entire scheme above:

Because feedback device can be realized with resistors, resistance of ordinary resistors is not easily disturbed by outside world, so F value is very stable, so increase of whole circuit is very stable. We have successfully solved problem of triode amplification stability due to negative feedback.

It can be seen that feedback part and gain part form a loop here, so gain of whole circuit is called loop gain or feedback gain; and gain of circuit A before addition of feedback is called open-loop gain. Due to negative feedback, although circuit gain stability is improved, there is a cost:Because

So A>>1/F

That is, open-loop gain is much larger than closed-loop gain, that is, gain of amplifier is significantly reduced. But in general, stability is worth it.

03 Operational amplifier

In above circuit, to actually build a high-gain A open-loop amplifier, a series connection of multi-stage triode amplifier circuits is often used.

Because demand for this type of high gain amplifier is very common, some people in history have made it into a finished PCB module. When needed, it can be used directly as a component. which is very convenient.

This is original op amp, 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, so integrated operational amplifier is very commonly used today.

The operational amplifier got its name from fact that it was originally used to perform mathematical operations on analog computers. Although widely used digital computers no longer use op-amps for computational operations, name has remained.

Today, op-amps play a very important role in analog circuits and have become one of main areas of operation for analog circuits.

04 Op-Amp Virtual Short and Virtual Open Characteristics

Normally, an op-amp has two input terminals U+ and U- and one output terminal Uo, which satisfies

The gain A of an open-loop op-amp often reaches hundreds of thousands to several millions, but output voltage of an op-amp is limited by power supply voltage and cannot exceed power supply voltage. . Thus, input-output ratio of an op-amp is similar to shape shown in figure below.

Horizontal axis in figure , vertical U-axis.

In rectilinear region in middle, op-amp is in a normal gain state called linear region, which satisfies

Uo=A*(U+−U−)

When absolute value of input is slightly larger, output will be limited by a power supply that no longer satisfies above relationship, and Uo value is usually slightly less than voltage range of power supply. (note that an op amp can use two power supplies, i.e. power supply voltage range can be between negative and positive), which is called non-linear region.

The output of a rail-to-rail op-amp can reach supply voltage. If you are interested, you can search and study on Internet for yourself.

When op-amp operates in linear region, value of Uo is very limited, but A is very large, so U+-U-=Uo/A≈0

U+≈U–

At this time, voltage at positive and negative input terminals of op-amp is almost same, as in a short circuit, which is called a virtual short circuit. Therefore, only when op amp is operating in gain region will it have "virtual short circuit" characteristics rather than inherent properties of op amp itself.

On other hand, due to inherent structural characteristics of an op-amp, its input impedance is very high.

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

High input impedance means that op-amp requires very little input current to operate properly.

Therefore, op amps can be used to detect some weak currents such as brain waves and muscle waves of human body. The maximum voltage value is only a few mV, and current value is also very small.

This characteristic of an op-amp is called a virtual break, that is, input terminal is like an open circuit and almost no current flows.

Unlike a virtual short, a virtual circuit breaker is an intrinsic property of op amp itself and does not change when circuit changes.

05 Non-ideal op-amp characteristics

Op-amps are made up of triodes, which, like triodes, have many undesirable characteristics. The above are characteristics of ideal operational amplifiers.

As for actual op amp,then it does not fully satisfy virtual short circuit characteristics. During normal operation, input terminal needs current to flow, and this current is called input bias current. Similarly, an op amp also has non-ideal parameters such as input bias voltage, input bias voltage, and input bias current.

These non-ideal characteristics, such as input bias current, are small, but sometimes they have a strong effect on circuit, causing it to fail.

Therefore, it is necessary to somehow reduce influence of these factors. In practical applications, non-ideal characteristics of operational amplifiers are a very important problem. There are many ways to eliminate non-ideal characteristics of op-amps, which are not covered here.

Other content

The basis of analog circuit course is a triode and an operational amplifier. Various schemes are explained around these devices, including:

● Calculation and analysis of amplifier circuits, multi-stage amplifier circuits, frequency response of amplifiers, feedback ideas;

● Power amplifier circuit

● Comparators, generators, integrators, differentiators, signal generators, etc.;

● signal processing;

● filter;

● Built-in variable power supply circuit, etc.

OpAmp vs. Transistor Comparison

In real circuits, op amps are used more often than triodes. Because many characteristics of an op-amp are better than those of a triode, circuit design is simple and cost of an op-amp is often low.

In many cases, same effect can be achieved using a triode and an op-amp, but cost of using an op-amp is actually lower. Because an op amp contains a large number of transistors, average manufacturing cost per transistor is very low.

For example, for a general audio preamplifier, a general-purpose op-amp can cost 0.2 yuan, and if a triode is used, 10 or more triodes may be needed to achieve same effect. , and cost can be 0.5 yuan, and design labor is much higher than op-amp solution.

Of course, triode has its advantages. In some very simple circuits, gain stability is not strictly required and one or two triodes can do job, and triodes are often used for cost savings.

In addition, under certain extreme conditions such as workin high frequency and high power environments (such as RF signal transmission circuits), performance of a well-designed triode circuit will be much better than that of an op-amp, or cost is much lower, and even in some cases it can only be done using transistors directly. Currently, triodes must be used to build circuits.