Universal triode equivalent model! Help you solve various parameter analysis

　　1 Commonly used amplifier circuits in analog circuits

　　1.1 Transistor amplifier circuit

　　The main components of the transistor amplifier circuit are pnp type, npn type silicon transistors and several resistors. It mainly uses the characteristics of the transistor to amplify the current in the circuit. Through the analysis of the input and output characteristics of the circuit, it is concluded that the amplifier circuit contains three working states, namely the saturation region, the amplification region and the cut-off region. Under normal circumstances, the amplifier circuit should work in the amplification region. The common circuit form is shown in Figure 1.

　　1.2 Field Effect Transistor Amplifier Circuit

　　Field effect tube is also a basic amplifier circuit. Its main components are field effect tube and several resistors. Its working principle and circuit form are the same as transistor amplifier circuit. Compared with transistor amplifier circuit, it has the advantages of high input impedance, low noise and good thermal stability. The common circuit form is shown in Figure 2. 　　1.3 Differential amplifier circuit

　　The differential amplifier circuit is mainly composed of transistors, field effect transistors and several resistors. Its main feature is that it adopts a completely symmetrical circuit structure to suppress the zero-point drift produced by the basic amplifier circuit and make the operating point more stable. The common circuit form is shown in Figure 3.

　　1.4 Active load amplifier circuit

　　The main components of the active load amplifier circuit are transistors, field effect tubes and several resistors. This amplifier circuit uses a current source circuit as an active load. In this way, when the power supply voltage remains unchanged, it can obtain a suitable static current and a larger equivalent resistance for AC signals, thereby increasing the voltage gain. The common circuit form is shown in Figure 4.

　　2 Application examples of equivalent circuit method in amplification circuit analysis

　　In order to study the application of equivalent circuit method in the analysis of actual amplifier circuits, the analysis of transistor amplifier circuits is taken as an example below.

　　2.1 Specific application process of equivalent circuit method

　　First, treat the transistor as a two-port network, as shown in Figure 5.

　　With BE as the input port and CE as the output port, the relationship between the terminal voltage and current outside the network is the input characteristic and output characteristic of the transistor, as shown in Figures 6 and 7.

　　According to the characteristic curve, the input characteristics and output characteristics can be written as a relational expression:

　　The subscript e of the h parameter indicates the common emitter connection, i indicates input, r indicates reverse transmission, f indicates forward transmission, and o indicates output.

　　From the h parameter equation, we can see that the voltage vBE consists of two parts. The first term indicates that a voltage is generated by iB, so hie is a resistor; the second term vCE generates a voltage, so hre is dimensionless; so BE can be equivalent to a resistor in series with a controlled voltage source.

　　The current iC also consists of two parts. The first term indicates that a current is generated by iB, so hfe is dimensionless; the second term indicates that a current is generated by vCE, so hoe is the conductivity; so CE can be equivalent to a controlled current source connected in parallel with a resistor. The equivalent model of the transistor obtained in this way is shown in Figure 8. Since the dimensions of the four h parameters in the h parameter equation are different, it is called the h parameter equivalent model.

　　2.2 Simplified model of equivalent circuit method

　　Input loop: From the previous analysis of transistor characteristics, we know that when the transistor works in the amplification region, the voltage between CE has little effect on the input characteristic curve, that is, the internal feedback of the tube can be ignored, and any characteristic curve with vCE>VBE can be used to replace all characteristic curves with vCE>VBE. Therefore, if hre=0, the input loop of the transistor is equivalent to only a dynamic resistance rBE(hie).

　　Output loop: When the transistor works in the amplification region, the change of the voltage between CE has little effect on iC, that is, in the amplification region, the output characteristic curve is almost a parallel line to the horizontal axis, and the dynamic resistance between CE 1/hoe can be considered infinite. Therefore, hoe is approximately 0, and the output loop of the transistor is only equivalent to a current source βiB (hfeiB) controlled by a current iB. The simplified h parameter equivalent model is shown in Figure 9.

　　3 Conclusion

　　The above analysis of the transistor equivalent model, the analysis conclusion can be directly used to analyze the various dynamic parameters of the amplifier circuit composed of transistors, the conclusion can be fully applicable to the field effect tube and the amplifier circuit composed of it, and other types of amplifier circuits such as differential amplifier circuits, power amplifier circuits, and integrated operational amplifier circuits composed of many basic circuits, because their basic structure is the same as that of transistor and field effect tube amplifier circuits, so they can also be analyzed by the equivalent circuit method.