Field-effect tube characteristics and the whole

by:Gisen     2020-06-05
The principle of the field effect tube to control the working current is completely different from that of ordinary transistors, which 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 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 operating principle of the field effect tube gives the field effect tube many excellent performances, which exudes an attractive glow to the user. 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 a two-way 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. 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. 1. Characteristics of FET Compared with ordinary transistors, FETs have the advantages of high input impedance, small noise figure, 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. Its characteristics are as follows. High input impedance is easy to drive, and the change of input impedance with frequency is relatively small. The input junction capacitance is small (feedback capacitance), and the change of the load at the output has little effect on the input, the driving load is strong, and the power utilization is high. The noise of the field effect tube is very low, and the noise figure can be less than 1dB. Now the noise factor of most field effect tubes is about 0.5dB, which is difficult for general transistors and electronic tubes to achieve. FET has better thermal stability and larger dynamic range. The output of the field effect tube is a power function of the input, and the degree of distortion is lower than that of the transistor, which is slightly larger than the bile tube. The distortion of the field effect tube is mostly even harmonic distortion, good listening, high and low frequency energy distribution is appropriate, the sound has a sense of density, the low frequency is deeper, the sound field is more stable, the transparency is moderate, the sense of hierarchy, resolution and positioning The sense has a good performance, has a good ability to describe the sound field space, and has a good performance on the music details. When the ordinary transistor is working, the input terminal (emitter junction) is forward biased, so the input resistance is very low, and the input terminal (between the gate and the source) of the field effect tube can be applied with a negative bias. That is, the reverse bias voltage can also be added forward bias voltage, thus increasing the flexibility and diversity of circuit design. Usually when the reverse bias is added, its input resistance is higher, up to 100MΩ or more. This characteristic of the field effect tube makes up for the lack of application of ordinary transistors and electronic tubes in certain aspects. The field-effect tube's radiation protection ability is about 10 times higher than that of ordinary transistors. Fast conversion rate and good high frequency characteristics. The voltage and current characteristic curve of the field effect tube is very similar to the output characteristic curve of the five-pole tube. 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. 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), small drive current (about 0.1uA) of MOS field effect tube, but also has high withstand voltage (up to 1200V), large operating current (1.5 ~ 100A), 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 warm and sweet sound quality without losing strength, and are highly favored by Philharmonic people, so it 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. The input impedance is high. Because the SiO2 layer is between the gate and source, the DC resistance between the gate and source is basically the SiO2 insulation resistance, which is generally about 100MΩ, and the AC input impedance is basically the capacitive reactance of the input capacitor. The drive current is small. Due to the high input impedance, the VMOS tube is a voltage-controlled device, which can generally be driven with voltage and requires very little drive current. The linearity of the transconductance is better. With a large linear amplification area, it is very similar to the transmission characteristics of the electron tube. Better linearity means lower distortion, especially with a negative current temperature coefficient (that is, when the voltage between the gate and source is constant, the on-current will decrease as the tube temperature increases ), So there is no pipe damage caused by secondary breakdown. Therefore, the parallel connection of VMOS tubes has been widely used. Junction capacitance has no varactor effect. The junction capacitance of the VMOS tube does not change with the junction voltage, and there is no varactor effect of the general transistor junction capacitance, which can avoid the distortion caused by the varactor effect. Good frequency characteristics. Most carrier motions of VMOS field effect transistors are drift motions, and the drift distance is only 1 ~ 1.5um, which is not limited by the transition time of the minority carrier base region like transistors, so the power gain changes very little with frequency and the frequency characteristics are good . Fast switching speed. Because there is no storage delay time for minority carriers, the switching speed of the VMOS field effect transistor is fast, and it can turn on or off dozens of A current within 20ns. Second, the main parameters and selection of field effect tube In order to use the field effect tube correctly and safely, to prevent damage to the field effect tube due to static electricity, mishandling or improper storage, it is necessary to understand and master the main parameters of the field effect tube. There are dozens of parameters for the field effect tube. The main parameters and meanings are listed in Table 1 for reference. The selection of field effect tube should pay attention to the following points. The parameters of the ID of the FET 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. Select the source and drain voltage BVDSS of the VMOS tube not to 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 careful, to prevent static objects from contacting the pins. During storage, the lead-out pin should be short-circuited and shielded with a metal box 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, electric irons, circuit boards and human bodies 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. In this condition, the short-circuited material can only be removed after welding. Paired tubes require the same batch number from the same factory, so the parameter consistency is good. Try to use 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. Select the special tube for audio as much as possible, so as to be more suitable for the requirements of the audio amplifier circuit. When installing the field effect tube, avoid 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. Third, the artistic charm and evaluation of audio amplifiers Audio amplifiers can be divided into tube amplifiers, transistor amplifiers, integrated circuit amplifiers, field effect tube amplifiers and hybrid amplifiers composed of two or more of the above-mentioned devices according to the amplifier components used. Various types of amplifier circuits and components used are also diverse. Since it is ever-changing, the sound quality of the replay of the sound source has its own characteristics. It is difficult to say which type of amplifier can become a universal amplifier with a comprehensive overview and technical skills. Due to the time lag effect of space charge transmission, the replay tone is warm and soft, especially the string vocals, which are mellow and intriguing. Transistors and integrated circuit amplifiers have sharp analytical power, wide frequency response and strong dynamics, and have the energetic and energetic appeal. Field-effect tube amplifiers and hybrid device amplifiers strive to integrate the audio characteristics of tubes and transistors to create splendid colors, make the music more vivid, and make the sound more perfect. In recent years, with the continuous development of electronic computer technology, various electronic synthesizers, various audio effects and bile effect software and virtual speaker technology have emerged in an endless stream. This makes the development and popularization of audio amplifier hardware far behind the speed of software, and the hardware is often unable to keep up with the software in terms of accuracy. For example, the computer simulation of 3D effect fidelity greatly exceeds the real 3D effect, and it is not affected by the listening room space and sound source synthesis. Restrictions, while also saving on hardware costs. Green audio, dual-material fever-Computer audio is likely to become the mainstream of future audio, hardware is not available, software and hardware are implemented, and the function is powerful. It embodies the characteristics of high efficiency, convenience, magic and economy. For example, if you set up a virtual CD-ROM drive in your computer, you do n’t need to start the physical CD-ROM drive every time you play music. This not only reduces the waiting time of the track and the wear of the physical CD-ROM drive, but also eliminates the noise of the physical CD-ROM drive and realizes high-fidelity playback. Another example is that the bile duct amplifier is soft and durable, and the production cost is not thin, and there are many requirements for obtaining beautiful sounds. Through the bile sound effect software, we can create a 'soft gall' in the computer for simulation. The sound of the amplifier. At present, computer multimedia audio is in an advanced stage, and it has also established a bridge of communication with TV. Its prospect is very bright and attractive! Computer and audio enthusiasts are a special aspect of the pursuit of sound quality, regardless of time and energy. They will continue to take a responsibility for philharmonic music. If there is a sweet song in life, there will be less bitter disputes. Whether it is a general audio or a computer multimedia audio, the power amplifier is still an indispensable terminal for the expansion of audio energy to drive the speaker. All types of amplifiers can better achieve this function. However, modern people are demanding for sound (mainly technical factors, such as frequency response, distortion, signal-to-noise ratio, etc.) and music (mainly artistic charm, such as whether the sound bottom is mellow, whether the hall sounds are rich, and whether the sense of hearing is good, etc.) As they get higher and higher, many 'gold ears' can hear the singer's teeth, verbal, and immersive, on-the-spot feeling, so they also place greater demands on the sound reproduction of the audio amplifier, and strive to use distinctive audio Create a charming music atmosphere. Various types of audio amplifiers have their own advantages and attributes, but also have their own shortcomings. The mainstream of field effect tube amplifiers has the advantages of both transistors and tubes, and also has the advantages that both do not. In the circuit program, a lot of practice has proved that the single-ended Class A amplifier is a model of efficiency for sound quality, with unparalleled musical charm. Starting from pursuing sound quality, many audiophiles repeatedly make power amplifiers, compare listening to music repeatedly, and are finally moved by category A. It seems that music without category A is like lonely music. Fourth, single-ended Class A amplifier performance Amplifiers can be generally divided into three categories according to different working conditions: ① Class A amplifier, also known as Class A amplifier; ② Class AB amplifier, also known as Class A and Class B amplifier; ③ Class B amplifier, also known as Class B amplifier. Among these three types of amplifiers, the best linearity and the most beautiful sound are the Class A amplifiers. The single-ended Class A amplifier and the push-pull amplifier are different in design. They use one amplifying device to amplify the entire music waveform. The push-pull design uses two amplifier devices to amplify the positive and negative half cycles of the signal, including some push-pull class A amplifiers. A significant difference between single-ended Class A amplification and push-pull amplification is that the amplified music waveform is a complete waveform that is very similar to the input waveform. There is no crossover distortion of the positive and negative waveforms of push-pull amplification, although push-pull amplification uses matching The twin tubes with an accuracy of up to 2% error and even smaller errors, but this is only a one-sided digital description. In fact, the positive and negative waveforms cannot be well connected, and the phase shift caused by the nonlinearity of circuit components exists, and the crossover distortion will be further Increase, of course, distortion and timbre are not in a certain degree of opposition, it depends on the purpose and purpose of the design of the amplifier, not push-pull amplification to stop here, and the push-pull amplifier, due to the existence of multiple harmonics, although the original positive and negative waveforms The handover is not good, but the harmonic handover cannot be denied, but it is difficult to contend with single-ended waveforms. The statement that push-pull amplified harmonics, especially even-order harmonics, will cancel each other out. The author does not fully agree that only harmonic components such as phase shift distortion of 180 ° or 360 ° will cancel each other. For example, the AC ripple in the DC high voltage in the push-pull power amplifier is divided into two circuits by the center tap of the push-pull transformer. Due to the opposite polarity of the coils of the two arms, the phase difference is 180 °, and the AC ripple is almost completely canceled. Single-ended Class A amplifiers have the most natural musicality, and their asymmetry is similar to the characteristics of compressed and expanded air. Because the most composed of air is non-polar molecular nitrogen (N2), accounting for about 78%, air is a 'single-ended electrodeless' medium whose pressure energy becomes very high, making single-ended A-class music sound most expressive and timbre most mellow. Fifth, the production of VMOS field effect tube single-ended Class A power amplifier There are two basic principles for designing an amplifier: one is simple, and the other is linear. And the simplest amplifier circuit that can be achieved is single-ended Class A, and simplicity is not the only reason for single-ended Class A amplification, because single-ended Class A has the most charming sense of music. In class A, B, and AB line programs, the best linearity is class A, but the disadvantage is that the efficiency is the lowest, about 20%, which is a model of efficiency for sound quality. Amplifier devices used in single-ended Class A amplifier circuits are also particular. The transistor has an input impedance that is too low, the input impedance of the tube is very high,
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