A quick understanding of analog electricity

introduction

Don't worry, iron sons , This article will start from the most basic , Suitable for children's shoes that just touch Electronics , Even lying in bed , After reading this article for more than ten minutes, you will have a painless and comprehensive understanding of analog circuits ！
Analog electricity is a hard bone , I will elaborate on the very basic concept , I will try to explain some advanced concepts in words , So I will try to include all the knowledge points involved in analog circuits , It aims to let you see the whole picture of analog circuit first , Lay a foundation for future study ！
The following figure shows the analog circuit Knowledge framework , I'll talk about Brown in detail , You can master it immediately , I will try to make the knowledge of blue clear in language . Hopefully that helped .

 

Text

Why should I learn analog electricity ？ I personally think , Mode electricity , And counting electricity （ digital circuit ）, It is the two cornerstones of Electronics , And digital electricity is developed from analog electricity , I strongly recommend that college majors involve children's shoes of electricity , Learn these two courses well , If you are interested in logarithmic inductance, please go to Kangkang for another article A quick understanding of digital . Understand analog electricity, digital electricity , Only then can you know the basic principle of all the electrical appliances around you , You can have the ability to understand the beauty of this electronic world .
OK, The following contents are closely linked , This matter should not be delayed , Let's do something about it ！
 

Semiconductor devices

The first lesson of analog electricity must be to understand semiconductor devices . Semiconductors are neither conductors , Nor is it an insulator , In a very awkward position , But it is such a cross feature , Gives it great flexibility , Led to changes in the times , Nowadays, all electrical appliances can't do without the credit of semiconductors . silicon and germanium Is now commonly used semiconductor materials , Silicon can be extracted from sand , So now the crystallization of human wisdom “ chip ”, It comes from sand, haha .

PN junction ： use doping technology , Add more electrons to semiconductors , formation P Type semiconductor ; Add more holes , formation N Type semiconductor .（ This is to make it easier to understand , In fact, it is mixed 5 Valence phosphorus forms P Type semiconductor , Incorporation 3 Valence boron element forms N Type semiconductor ）（ Holes are the positions left by electrons after they leave , In fact, there is nothing , Just to be opposite to negatively charged electrons , We artificially call this position a hole , It is defined as positively charged ） These two go in both directions , Together , A region is formed in the middle , Its name is PN junction . In theory, a N Type a semiconductors and P Type semiconductor forms diode , Positive conduction , Reverse cut-off .

Bipolar transistor BJT： Also called triode , That's the two one. PN junction , Sandwiched together like a sandwich , Yes NPN, Also have PNP.


Field effect transistor MOSFET： Two, too PN junction , But with BJT Dissimilarity , Now more use is MOSFET, Because of its low power consumption and easy integration , The chip is full of mosfet.


A book can be written about the concept of semiconductor devices , It's hard for me to understand in a few words , It's recommended here Two great b Stop video BV12x41187RN,BV1Ly4y1178Q, After watching these two videos, you are right PN junction 、BJT and MOSFET Have a good understanding .
 

Zoom in

Personally, I think amplification is the most important concept in Analog Electronics ！ After modeling electricity, basically all talk about amplification . What is your understanding of amplification ？
Do you think there is such a device , Only two mouths , An input , An output , Input a small signal , The output will have a big signal ？ It's impossible , Otherwise, the perpetual motion machine would have been made . The reality is that you have to plug it in ！ It has the energy to magnify .
So the essence of amplification , Namely Control of power supply energy ！ It can also be understood as Energy conversion , It converts the energy of the power supply into output .

The figure above illustrates this point figuratively , The energy of the power supply can be regarded as the water in the bucket , At the end of the triode b Pole input small signal , It can be regarded as the control of the valve , That is, the control of water flow , stay e The so-called amplified signal received by the pole is essentially b The energy of the pole controlled power supply ！ If b There is no current at all , That is, the valve is closed , be e There will be no current at the pole ;b The current of the pole slowly increases , The valve slowly opens ,e The current received by the pole naturally increases , therefore $I_{B}And{I}_E$ In direct proportion to each other , We call it $I_E$ It's enlarged $I_B$, But now you should understand it control The smell of it .

 

Single tube amplification

Now enter the contents of single tube amplification , A single transistor is a circuit with only one BJT or MOSFET.
For any amplification circuit , The three areas we are most concerned about are ： Input resistance 、 Output resistance 、 gain
Want to understand the input and output resistance , Let's start with a concept , It's called With load capacity . in real life , Both stanzas are marked 3v The battery A and B, Give ideal 1Ω Resistance power supply , Then what is the voltage on the resistor ？ You may say right away ,3V！ The answer is wrong , It is impossible to get 3V, Because the battery will have internal resistance , Suppose the battery A The internal resistance of is 1Ω, The battery B The internal resistance of is 2Ω, Then the voltages of the resistors are 1.5V and 1V, You see , Both sections say 3V The effect is different when the battery is connected to the load , We call A battery with small internal resistance A Strong load carrying capacity .

Input resistance It is the equivalent resistance seen from the input , From the battery example we gave above , See from the input , The input resistance is 1Ω The load resistance of , The bigger it is , The more voltage you grab , therefore The higher the input resistance, the better ！
Output resistance It is the equivalent resistance seen from the output , From the battery example we gave above , See from the output , The output resistance is the internal resistance of the battery , The smaller it is , The more voltage the load grabs , therefore The smaller the output resistance, the better ！
So for gain , That's the magnification , Do you think the bigger the better ？ We talked about , The essence of amplification is the control of power supply energy , My power supply is only 5V, No matter how much magnification , Even for you 10000 times , It's also magnified to 5V, In reality 4.8v All right , Instead, the magnification is too large , Give a small input , Just zoom in to saturation , This circuit loses its meaning , therefore Proper magnification is best ！

So is it for a single tube , Add power to it , It can enlarge as we wish ？ Not yet ！ Just like before the exam, we should eat well and sleep well , Adjust yourself , We also need to adjust the amplifier first , Professionally, it's called Set the static working point ！
Because the general input is a small AC signal , And the power supply is DC , So generally, we will draw it first for the analysis of amplification circuit DC path , Do static analysis , Analyze static work points , Ensure that the small signal of communication, whether positive or negative, can be amplified perfectly （ Forward bias of emitter junction in triode , Collector reverse bias ）; Then draw its Communication channels , Identify the circuit configuration , Do dynamic analysis , This circuit is basically done by you .

Take a look at the following classic single transistor amplifier circuit , Common emission means that the emitter of the triode is grounded , It is also grounded in the AC path .

$V_{CC}$ and $V_{BB}$ For power supply ,$V_s$ Small signal for control , The bigger it is , The larger the output . resistance $R_b$ and $R_c$ It is used to adjust the static working point .
Empathy , There are also... In the triode Collective （ Common collector ） and Common base （ Common base ）, Each has its own characteristics ;MOSFET Yes Common source 、 Common leakage and Common grid , It is similar to triode .
 

Multi tube amplifier circuit

Of course, we need to learn more if we can do it alone , That's more than one. BJT or MOSFET Together . Here's the picture ：

Multiple tubes connected together will increase the magnification , Reduce the front stage drive current , That is, when the input signal is very small , It may be necessary to connect multiple amplification tubes together to increase the magnification . It is usually placed in the of multistage circuits Intermediate stage Amplification gain .
Expand the , The connection between triodes is called coupling , Directly connected together like this is called Direct coupling ,, Good low frequency characteristics , Conducive to integration ; If they are connected by capacitance , Call Resistance capacitance coupling , It's good for analysis ; There are many other ways of coupling , Each has its own characteristics .
 

Differential amplifier circuit

When the amplifier circuit was first used , People find that there is a very annoying parameter that will cause output fluctuations , It's temperature . Even if there's no input , Because of the change of temperature , Sometimes the output will fluctuate up and down from the fixed value , This is it. Zero drift , This is what we don't want to see , Just when people have a headache , An elder brother proposed Difference Thought , Very, very wonderful ！ Is to introduce Two inputs , We make a difference between these two inputs ！ Because the two inputs are very close , So the effect of temperature on them is the same , Make a difference , It was eliminated ！ This signal is called Common mode signal . You may soon ask , Then the signal we used to have is also eliminated by you , What's the point ？ Change your mind , We invert the useful signals , Input , This will not disappear , And it doubled , wonderful ！ This signal is called Differential mode signal .
So for differential amplifier circuit , Its core idea is Suppress common mode , Amplified differential mode , It is usually placed in the of multistage circuits first stage When the input circuit , It can effectively eliminate interference .

 

Power amplifier circuit

As the name suggests, it is amplification power , For relatively large loads , It can drive directly , It can effectively amplify the current , For normal operation under heavy load . It is usually placed in the of multistage circuits The last level When the output circuit .
Yes Class A , class B , Class A and class C . Don't feel dizzy yet , Just have an impression , Class A and class B power amplifiers Is the most important , It overcomes Cross over distortion , It takes into account the problems of distortion and efficiency , Most high-quality sound systems use class A and class B power amplifiers . The following figure shows the classic class A and class B power amplification circuit （ The red part overcomes the crossover distortion problem of class B power amplifier ）.

 

Current source

If you have learned circuit analysis , I should be familiar with current source , If you don't know much, you can go to Kangkang in my article 《 A quick understanding of circuit analysis 》. In circuit analysis, current source is a very important power source , But in analog electricity , The current source mainly acts as a load ！ I heard you right , Use current source as load , This load is called Active load , Because the current source can provide static and stable current , So the gain of the whole circuit will be greatly improved , Yes Proportional current source , Mirror current source wait .
The following figure T1 and T2 The tube forms a current source .

 

Integrated operational amplifier （ Operational release ）

high energy alert ！！！ We put the differential amplifier circuit that can eliminate interference in the first stage , The multi transistor amplifier is placed in the intermediate stage to improve the gain , The power amplifier is placed at the end to drive the load , In addition, the current source acts as an active load , An integrated operational amplifier was born ！！！

Such a complicated circuit is nothing more than that , We can simply draw it as ：（V+ It is in phase ,V- It is the inverting end , These two are the inputs of the first stage differential amplifier ,Vs For power ,Vout For export ）

Personally, I think the operational amplifier is analog Lead ！ Its function is very, very powerful , Input can be implemented Add, subtract, multiply and divide 、 differential 、 integral 、 Index 、 logarithm 、 wave filtering 、 rectification 、 plastic … I think the best thing is that it is Pure simulator . Generally speaking, analog signals come in when processing signals , After analog-to-digital conversion AD Change to numbers 0 and 1, Send it to digital chip for processing , After digital to analog conversion DA hold 0 and 1 Into an analog signal , Send to output . The op amp is simulated , Simulation processing , Simulate out , There's nothing wrong with no digital signal , Very primitive , A powerful circuit .
This is its formula ！
$$V_{out}=A_{od}\ast (V_{+}-V_{-})$$ Look at its parameters , Input resistance is infinite , The output resistance is 0, The gain is infinite , So it's the perfect player .
Wait a minute , Its gain is so great , Didn't we say that the gain is too large and easy to saturate ？ Right , So we need to add an important concept ： feedback ！

feedback

Lead part of the output of the signal back to the input , Together with the original input, it affects the next output .
The positive feedback ： This exam was very good , Give me a big boost in confidence , Do better next time ;
Negative feedback ： This exam was very bad , It greatly reduced my confidence , I'll do worse next time ;
Through feedback （ Deep negative feedback ）, We can add resistance Freely control the gain of amplification ！ At this time, the operational amplifier can really be said to be perfect .

Deficiency

Look back at its formula , because $A_{od}$ For infinity , Except for the past left , A number divided by infinity is 0, therefore ：
$$V_{+}-V_{-}=0$$

Virtual break

Because the input resistance of the op amp is infinite , So the input current is approximately 0！, namely ：
$$I_{+}=I_{-}=0$$

In phase proportional operation

utilize Virtual break , Break the input , recycling Deficiency ,$U_N$ The potential of is equal to $U_I$ . That's easy , about R, Yes $i_R=\frac{U_N}{R}$ , about Rf, Yes $i_F=\frac{U_0-U_N}{R_f}$ , Because they have the same current , therefore
$$\frac{U_I}{R}=\frac{U_0-U_I}{R_f}$$ Change the formula in the figure ！

Inverse proportional operation

It is the same as the proportion in phase , use Virtual short and virtual break You can find the formula in the figure .

Advanced knowledge

Operational amplifiers can form many functional circuits , Add, subtract, multiply and divide 、 differential 、 integral 、 Index 、 logarithm 、 wave filtering 、 rectification 、 plastic wait , At this time, you can study the input of different frequencies , How will these circuits respond . Use active components to form a filter , It's called Active filter , The effect will be much better than the passive filter .
Then the op amp can form Voltage comparator . about Waveform generation and conversion , For example, square wave becomes triangular wave , You can also study .
This knowledge is hard , You need to calm down and read a book to understand .
 

END

Finally, let's take a look at the knowledge framework of all knowledge of Analog Electronics , I hope it can help you form a general understanding of analog electricity .

Attach links to two other articles 《 A quick understanding of digital 》,《 A quick understanding of circuit analysis 》.