Model learning experience

First, we should understand the research object of this course. In fact, this course can be said to be an extension of circuit theory. The analysis method that needs to be applied to the circuit theory is different in that many complex electrical components have been added.
Speaking of components, the first thing that comes into contact with is the second, the transistor. Regardless of the version of the textbook, the features of the pn will be introduced at the beginning. I personally think it is okay not to care too much about the structure, but the characteristic equations must be remembered. Then, the diode is relatively simple, that is, a single PN junction. Different models can be used in different situations in the circuit (ideal model, constant pressure drop model, and small signal model, the first two are used for DC analysis, and the last one is used for AC analysis ). The transistor is relatively more complicated. I don't want to talk about the things in the book here, but I just want to emphasize the notes in my study:
1. For a transistor, there are three working states in total. When it is put in a circuit, the first thing we need to do is to judge its working status under the given parameter. (In the model-electric exercise, unless the question is specifically about your transistor status, it will all work in the amplification area, because only in this way can the pipe play the role we want it to have. However, in digital power, we rely on the switch of different states of the pipe to implement the control switch)
2. Since the tube is basically in the amplification area, its DC characteristics are as follows: Be junction voltage is 0.7 V (silicon tube, GE pipe is 0.2 V ), the emission current is approximately equal to the current of the collector and equal to Beta times of the base current. Through these known relationships, we can calculate the static operating point of the pipe-the so-called static working point is the voltage between Ce and the current of the three poles.
3. Why do we have to calculate the static work point first? This requires us to figure out the relationship between DC and AC: In a mode-electric system, all of our research objects are amplification circuits, and the amplification is both AC signals and Weak AC signals. As we all know, the work of the transistor requires certain bias conditions, and the AC signal is small and negative, so we cannot directly enlarge the AC signal. Here we use the following method: a dc offset is provided for the tube to work in the amplification area, and an AC signal is superimposed on the DC (that is, the voltage fluctuates, but it does not fluctuate around 0 like a single sine wave, but around the DC voltage fluctuation you added), and then the nature of the transistor can generate a magnified AC signal.
4. Analysis circuit: From the above description, we can see that the analysis circuit is divided into two parts: DC analysis and AC analysis. The circuit diagram varies with different analyses, because the components have different characteristics under different quantities. (For example, a capacitor is equivalent to an open circuit while a short circuit can be used in an AC ). The transistor has an equivalent model in the AC, that is, to equivalent a resistance between the be and the CE to a controlled current source, and its current value is Beta times of the current between the be. In this way, the analysis can proceed very well.
5. Note: In a model electric system, we analyze all engineering circuits. in engineering, the accuracy requirements are not very high. Therefore, we should ignore the factors that can be ignored during analysis, for example, in addition and subtraction, if there is a ten-fold difference between items, then that small item can be ignored.
(2)
Then there is the Fet.
There are many types of FET, And the nature is more complex than the transistor, but the principle is the same. So I think if your transistor will be analyzed, it will not be a problem. Compared with a transistor, the field effect tube requires you to remember its DC characteristics (which is to write the current ID as a quadratic equation about vgs). When communicating, pay attention to the concept of cross-channel, books are written.
Next we will talk about the high-frequency and low-frequency model of the transistor.
In the beginning, the communication analysis is conducted in the intermediate frequency. In the intermediate frequency, the coupling capacitor can be regarded as short circuits, and the inter-pole capacitor can be regarded as open circuits -- and in the low frequency, the coupling capacitor cannot be used as a short circuit. at high frequencies, the pole capacitor cannot be used as an open circuit. This results in the influence of the frequency of the AC signal on the circuit amplification characteristics (the equivalent model of the entire circuit has changed? ^_^)
Here, we write the magnification into a frequency function, so that we can get a curve. We can use the 20log | A | link to draw a porter diagram. I don't want to go into details about the Potter diagram. I just want to emphasize the concept of low frequency cutoff frequency and high frequency cutoff frequency, pay attention to the slope of the phase frequency and amplitude frequency curves of several-level amplification circuits that change with the frequency.
That is to say, a major application of the transistor-integrated amplification circuit
The size of the integrated circuits is as small as possible, so we can no longer use large capacitors, so all the circuits adopt direct coupling. In this way, work at all levels will affect each other. Moreover, because the characteristics of the transistor are very sensitive to temperature, we must take measures to suppress the noise caused by temperature changes.
Almost all methods are mirroring: Using symmetric circuits to suppress the effects of temperature or other noise. I will not talk about it much more. However, this part of content is based on the analysis of the transistor, but the pipe is getting more and the circuit structure is becoming more clever!
After learning the structure of the integrated amplifier circuit, the content is relatively simple, because at this time we are no longer using a single transistor to form a circuit, but using an integrated amplifier to form a circuit. For the integrated amplifier, we must have learned it in the course of circuit theory, but note that in circuit theory, we only emphasize the nature of its "virtual short" and "virtual broken", but never consider its same-phase and reverse-phase connection problems. In fact, because the open-loop output phase is directly related to the port connection method, we have to consider it here. Then there is feedback, the signal processing circuit and the signal generation circuit.
(3)
The next step is negative feedback. I think this part of content is the most difficult and important. The main content is: feedback type judgment, feedback introducing methods, feedback impact on the performance of the amplification circuit, feedback amplification factor calculation and self-excitation fluctuation (not required ).
First, the method used for determining the type of feedback is the instantaneous pole method. I will not go into details here, but I would like to say that when considering the impact of the output through the feedback loop on the input, the input should be treated as zero, then, the overlay principle is used to check the function of the feedback loop to determine the type of feedback. Then, both the feedback network and the enlarged network can be regarded as a two-port network. Therefore, we can abstract the network during analysis without considering its structure, different feedback types have different network connection structures. When calculating the amplification of the in-depth feedback amplification circuit, the magnification is the reciprocal of the feedback network magnification. Therefore, we only need to abstract the feedback network and then calculate it! I will not go into details about the impact of the feedback network on the amplification circuit. In fact, the feedback output affects the input, which naturally affects some characteristics of the amplifier circuit.
Why do I say feedback is important? Because the content in the last two chapters must use Integrated Op-ops, and the open-loop Performance of Op-ops is very poor, we usually introduce feedback to make it work in a closed loop. For the signal processing circuit, we usually introduce negative feedback, while for the signal generation circuit, we introduce positive feedback.
I think there is nothing to say about the signal processing circuit first. In fact, it is essentially a special case of negative feedback, but in order to implement different functions, we must introduce different negative feedback. In addition, deep negative feedback is introduced. Therefore, when analyzing the performance of the entire circuit, it is mainly to grasp the nature of the feedback circuit!
Besides, the signal generation circuit is different from the signal processing circuit. In addition to providing the operating voltage of the integrated op amp, there is no input. In this case, in order to obtain the desired signal, positive feedback must be introduced. In this case, positive feedback can enlarge the accidental noise source and then output it. Of course, we don't want noise. We just use noise to obtain the signal we need, therefore, we need a network of signal selection in our circuit to filter out unwanted signals. in general, the signal generation circuit is divided into three parts: amplification circuit, feedback network, and frequency selection network. Of course, the feedback network and amplification network must meet certain parameter conditions. For details, refer to the document!
(4)
For the rest, add the content of the DC power supply:
The DC power supply consists of three parts: Rectification, filtering, and voltage regulation. This part is relatively simple. I will not talk about some things in the book, but I will talk about some things that inspire everyone (mainly looking at the whole process from the perspective of signals ). First, we have a single frequency sine AC signal. At this time, we first make a rectification. The book says that the purpose of the rectification is to convert the AC signal into a single DC signal, here, I think a better understanding is to use the unidirectional conductivity of the diode to convert a single signal with a single frequency into a signal with a rich frequency (the original signal spectrum is completely concentrated on its angle frequency, the rectified signal has a spectrum distribution at a frequency of 0 (that is, DC), twice the original angle frequency, and an even multiple of the original angle frequency, the maximum power at zero frequency), and what we want is a DC signal (that is, a signal with zero frequency ). At this time, we can use an ideal low-pass filter to extract the DC signal and filter the AC signal at other frequencies, but in fact the ideal low-pass does not exist, we only need to extract the low-frequency part with an RC Filter with poor performance. At this time, the signal we get is similar to the DC signal, but there are still a few AC signals. Next, we will use the regulator feature of the regulator to filter out small AC signals!

 

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