A circuit diagram is a diagram representing the structure of a circuit drawn with conventional symbols. You can know the actual circuit situation through the circuit diagram. In this way, when we analyze the circuit, we don't have to turn the real thing over and over and think, but only need to hold a drawing.

1.Classification of electronic circuit diagram

There are schematic diagrams, block diagrams, assembly drawings and printing plates.

1.1 Schematic
Schematic diagram is a kind of circuit diagram used to reflect the working principle of electronic circuits, also known as "electrical schematic diagram". This diagram directly reflects the structure and working principle of electronic circuits, so it is generally used in designing and analyzing circuits. When analyzing the circuit, you can understand the actual working of the circuit by identifying the various circuit component symbols drawn on the drawings and the connection methods between them.

1.2 Block diagram
The block diagram is a circuit diagram showing the working principle and composition of the circuit with boxes and wires. Fundamentally, this is also a schematic. However, in this kind of drawing, there are almost no symbols other than boxes and lines. The main difference between it and the above schematic diagram is that all the components of the circuit and their connection methods are drawn in detail on the schematic diagram, while the block diagram simply divides the circuit installation function into several parts and depicts each part as a box with a simple text description in it, and a connection (sometimes a line with an arrow) between the boxes to explain the relationship between the boxes. Therefore, the block diagram can only be used to reflect the general working principle of the circuit, and in addition to showing the working principle of the circuit in detail, it can also be used as a basis for collecting components and making circuits.

1.3 Assembly drawing
It is a drawing used for circuit assembly, and the symbols on the diagram are often the physical outline of the circuit components. We just need to connect some circuit components in the same way as we did on the picture to complete the circuit assembly. This circuit diagram is generally for beginners.
The assembly drawing is different according to the different assembly templates. The printing plate drawing is the main form of the assembly drawing.
When we started to learn about electronics, in order to gain early access to electronic technology, we chose screw holes as the basic installation template, so the installation diagram became another mode.

1.4 Plate drawing
The full name of the printed board diagram is "printed circuit board diagram" or "printed circuit board diagram". It is actually the same type of circuit diagram as the assembly diagram and is used for assembling the actual circuit.

The printed circuit board is covered with a layer of metal foil on an insulating board, and then the metal foil unnecessary for the circuit is etched away. The remaining metal foil is used as a connection line between circuit components, and then the elements in the circuit. The device is mounted on this insulating board, and the remaining metal foil on the board is used as a conductive connection between the components to complete the circuit connection. Because the metal on one or both sides of this circuit board is copper, printed circuit boards are also called "copper-clad boards". The layout of the components on the printed board is often very different from the schematic. This is mainly because, in the design of printed circuit boards, the distribution and connection of all components are mainly considered, and many factors such as component volume, heat dissipation, anti-interference, and anti-coupling must be considered. The printed circuit board designed by combining these factors is difficult to completely agree with the schematic diagram when viewed from the outside; in fact, it can better implement the function of the circuit.

In the four forms of circuit diagrams introduced above, the electrical schematic diagram is the most commonly used and the most important. If you can understand the schematic diagram, you can basically grasp the principle of the circuit.

2. Composition of the circuit diagram

The circuit diagram is mainly composed of four parts: component symbols, connections, nodes, and notes.

2.1 The component symbol indicates a component in an actual circuit, and its shape is not necessarily similar to the actual component, or even completely different. However, it generally shows the characteristics of the component, and the number of pins is consistent with the actual component.

2.2 The wiring represents the wires in the actual circuit. Although it is a wire in the schematic diagram, it is often not a wire but a copper foil of various shapes in a common printed circuit board, just like the radio schematic diagram. Many of the wires in the printed circuit board are not necessarily linear, but they can also be copper films of a certain shape.

2.3 The node represents the connection relationship between several component pins or several wires. All component pins and wires connected to the node, regardless of the number, are conductive.

2.4 Annotations are very important in the circuit diagram, and all the text in the circuit diagram can be classified into annotation-type. There are comments in various places of the circuit diagram, they are used to describe the type, name and so on of the component.

If you do not know the role of the circuit, you can first analyze the relationship between the input and output signals of the circuit. Such as the signal change law and the relationship between them, the phase problem is in phase, or out of phase. The circuit and composition form are amplifier circuits, oscillator circuits, pulse circuits, or demodulation circuits.

3. Several methods for analyzing common circuits, and the type of circuit and analysis steps suitable for each method.

3.1 AC equivalent circuit analysis method: draw an AC equivalent circuit diagram and analyze the AC state of the circuit, that is, when the circuit has a signal input, whether the voltage and current of each link in the circuit changes according to the law of the input signal. It is also clipped, shaped, and evaluated.

3.2 DC equivalent circuit analysis method: Draw the DC equivalent circuit diagram, analyze the DC system parameters of the circuit, find out the static operating point and bias properties of the transistor, and the inter-stage coupling method. Analyze the status and role of related components in the circuit. For example: the working state of the transistor, such as saturation, amplification, cut-off area, diode is on or off.

3.3 Frequency characteristics analysis method: It mainly depends on whether the frequency of the circuit itself is compatible with the frequency spectrum of the signal it processes. Roughly estimate its center frequency, upper and lower limit frequencies, and frequency band widths, for example: various filtering, notch, resonance, frequency selection and other circuits.

3.4 Time constant analysis method: mainly analyzes the circuit and properties of R, L, C and diode. The time constant is a parameter that reflects the speed of energy accumulation and consumption on the energy storage element. If the time constants are different, although their form and connection are similar, they play different roles. Commonly used are coupling circuits, differential circuits, integrating circuits, decoupling circuits, and peak detection circuits.

Finally, the actual circuit is compared with the basic principle, and according to the role of the component in the circuit, the above method is analyzed step by step.

4. How to understand the circuit diagram

Look at the circuit diagram. First look at the power supply section. Understand what kind of power supply the circuit works, AC or DC, single power supply or multiple power supplies and voltage levels. After clearing the branch circuit, the difference is whether it is a digital circuit or an analog circuit. The analog circuit looks at the signal acquisition. Find out the source of the signal. There are radio frequency, audio, various sensors, instruments and other circuits. DC or pulse is voltage type or current type. Analyze the function of subsequent circuits to find out whether it is demodulation, amplification, shaping or compensation. Finally, look at the output circuit, whether it is modulation or driving. Digital circuits analyze the logic functions and functions of the circuits.

To understand the circuit board, it is best to understand its electrical schematic diagram (ie, circuit diagram), master the marking method of electronic components and its working principle, master the normal parameters of some commonly used components and the normal knowledge of the role played in the circuit, and then analyze the circuit board (called a printed circuit board).

Molecular circuit module, and then find the core components of the sub-circuit, find out the electrical quantity connection between the sub-circuit modules, and finally the output and input or function of the entire circuit.

The whole circuit has certain functions and is composed of unit circuits. The unit circuits form signal processing branches with certain functions, and then these branch circuits form the whole circuit. First of all, it is necessary to find out what kind of circuit the circuit diagram belongs to is audio, video, digital, or mixed circuit, and then use the corresponding unit circuit knowledge to interpret these circuits. At the same time, it must be analyzed from the AC signal level and the DC level. The DC part of the circuit is the basis for the normal operation of the circuit. The AC signal can only be processed after the DC circuit is normal. The circuit cannot work normally without a good DC state. It is also necessary to analyze from the frequency level and the gain level of the amplifier. When the signals of different frequencies are processed by the circuit, due to the non-linear components in the circuit, there will be different processing results for different frequencies. The amplifier also has different signals for different frequencies. Amplification ability, the circuit will purposely process the required frequency signal when designing, so as to meet the needs of the machine function. Then there is the relationship between the unit circuits, and the relationship between the input and output of the unit circuits. What happens to the AC signal after passing through these circuits and so on. After understanding the working principle of each branch, the working principle of the whole machine can be analyzed, and sometimes there is a signal connection between the circuits of each branch.