Field-effect tube characteristics and design of single-ended Class A power amplifier FETs not only have the advantages of ordinary transistors and electron tubes, but also have the advantages that both lack. The field effect tube has bidirectional symmetry, that is, the source and drain of the field effect tube are interchangeable (no damping). It is not easy for general transistors to do this, and it is impossible for the electron tube to achieve this point. The so-called bidirectional symmetry means that for ordinary transistors, the emitter and collector are interchanged, and for electron tubes, the cathode and anode are interchanged. The principle of the field effect tube to control the working current is completely different from that of ordinary transistors, and it is much simpler than ordinary transistors. The field effect tube simply uses the external input signal to change the resistance of the semiconductor, in fact, it changes the channel size of the working current flow. However, the transistor uses the signal voltage applied to the emitter junction to change the junction current flowing through the emitter junction. It also includes extremely complicated processes such as minority carriers crossing the base region and entering the collector region. The unique and simple working principle of the field effect tube gives the field effect tube many excellent performances, and it radiates attractive light to the user. 1. The characteristics of the field effect tube Compared with the ordinary transistor, the field effect tube has the advantages of high input impedance, small noise coefficient, good thermal stability, and large dynamic range. It is a voltage-controlled device with transmission characteristics similar to electron tubes, so it has been widely used in high-fidelity audio equipment and integrated circuits, and its characteristics are as follows. There are many varieties of field effect transistors, which can be roughly divided into two types: junction field effect transistors and insulated gate field effect transistors, and there are two types of N-type channels (current channels) and P-type channels, each of which has There are four types of enhanced and depleted types. Second, the main parameters and selection of the field effect tube In order to correctly and safely use the field effect tube to prevent static electricity, misuse or improper storage damage to the field effect tube, the main parameters of the field effect tube must be understood and mastered. There are dozens of parameters for the field effect tube. The main parameters and meanings are listed in Table 1 for reference.
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Insulated gate field effect tube is also called metal (M) oxide (O) semiconductor (S) field effect tube, or MOS tube for short. According to its internal structure, it can be divided into two kinds of general MOS tube and VMOS tube, each of which has two types of N-type channel and P-type channel, enhancement type and depletion type.
VMOS field effect tube, which is called V-groove MOS field effect tube, is a new high-efficiency power switching device developed on the basis of general MOS field effect tube. It not only inherits the high input impedance (more than 100MΩ) and small drive current (about 0.1uA) of the MOS field effect tube, but also has high withstand voltage (up to 1200V), large operating current (1.5 ~ 100A), and high output power (1 ~ 250W), good cross-conductivity, fast switching speed and other excellent characteristics. At present, it has been widely used in circuits such as high-speed switching, voltage amplification (voltage amplification can reach thousands of times), RF power amplifiers, switching power supplies and inverters. Because it has the advantages of both electron tubes and transistors, the high-fidelity audio amplifiers made with it have a warm and sweet sound quality without loss of strength, which is favored by Philharmonic people and has broad application prospects in the audio field. VMOS tubes and general MOS tubes can also be divided into two types of N-channel and P-channel, enhancement type and depletion type, the classification characteristics are the same as general MOS tubes. VMOS field effect transistors also have the following characteristics. Table 1 Main parameters and meanings of FET
The selection of FET should pay attention to the following points. The parameters of the ID of the field effect tube are selected according to the circuit requirements. It can meet the power consumption requirements and have a slight margin. Do not think that the larger the better, the larger the ID, the larger the CGS, which is unfavorable for the high frequency response and distortion of the circuit For example, for a tube with ID 2A, the CGS is about 80pF; for a tube with ID 10A, the CGS is about 1000pF. The reliability of use can be guaranteed by a reasonable heat dissipation design. The source-drain withstand voltage BVDSS of the VMOS tube should not be too high, as long as it can meet the requirements. Because the saturation pressure drop of a tube with a large BVDSS is also large, it will affect the efficiency. The junction field effect tube should be as high as possible, because they are not high, generally BVDSS is 30 ~ 50V, BVGSS is 20V. The BVGSS of the VMOS tube should be as high as possible, because the gate of the VMOS tube is very delicate, it is easy to be broken down, storage or operation should be cautious, to prevent static objects from contacting the pin. During storage, the lead-out pin should be short-circuited, and the metal box should be used to shield the package to prevent the external induced potential from puncturing the grid. In particular, be careful not to put the tube in a plastic box or plastic bag. In order to prevent grid induced breakdown, all instruments and meters, electric iron, circuit board and human body must have a good grounding effect during installation and debugging. Before the tube is connected to the circuit, all pins of the tube must be kept short. Status, after the welding is completed, the shorting material can be removed. Paired tubes require the same batch number from the same factory, so the parameter consistency is good. Try to choose twin paired tubes to keep the pinch-off voltage and transconductance of the tubes as consistent as possible, so that the pairing errors are less than 3% and 5%, respectively. Use audio tubes as much as possible, so as to be more suitable for the requirements of audio amplifier circuits. When installing the field effect tube, the position should avoid being close to the heating element. In order to prevent the tube from vibrating, the tube should be fastened. When the lead of the pin is bent, it should be bent at a distance greater than 5mm from the root to prevent the pin from being broken or leaking to damage the tube during bending. The tube must have good heat dissipation conditions, and must be equipped with sufficient radiators to ensure that the temperature of the tube does not exceed the rated value and ensure long-term stable and reliable operation.
symbol name meaning BVGSS Gate source withstand voltage The SiO2 layer between the gate and the source is very thin, and the withstand voltage is generally only 30-40V BVDSS Source drain voltage VGS = 0, VDS value when the source-drain reverse leakage current reaches 10uA VP Pinch off voltage When the source is grounded, in order to make the drain-source current output zero, the gate-source voltage VT Turn on voltage When IDS reaches 1mA, the voltage between gate and source IGss Leakage current The reverse current under the reverse voltage applied to the gate-channel junction, the junction tube is nA level, and the MOS tube is pA level IDss Saturation drain-source current Leakage current when zero bias VGS = 0 RGS Input resistance Gate-source insulation resistance, the resistance of the gate-channel under reverse bias, the junction tube is 100M Ω, the MOS tube is more than 10000MΩ RDS Output resistance The derivative of the slope of the drain characteristic curve, ie 1 / RDS = △ ID / △ VDS gm Transconductance Represents the ability of the gate voltage to control the drain current IDs Source leakage current PD Power dissipation NF Noise Figure The noise is caused by the irregular movement of carriers in the tube. The field effect tube is much smaller than the transistor. CGS Gate source capacitance Input capacitance, the smaller the better, reduce distortion, and improve frequency characteristics CDS Drain-source capacitance The output capacitor, the smaller the better, reduce distortion, and improve the frequency characteristics CGD Gate-drain capacitance Feedback capacitor, the smaller the better, reduce distortion, and improve the frequency characteristics