Field effect tube characteristics and applications

The FET not only combines the advantages of ordinary transistors and tubes, but also has the advantages that both lack. The FET has bidirectional symmetry, that is, the source and drain of the FET are interchangeable (undamped). It is not easy to do this with a typical transistor, which is simply impossible to achieve. The so-called two-way symmetry, for ordinary transistors, is the exchange of emitter and collector, for the electron tube, the cathode and anode are interchanged.

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The principle of the FET controlling the operating current is completely different from that of the ordinary transistor. It is much simpler than the ordinary transistor. The FET simply uses the applied input signal to change the resistance of the semiconductor. In fact, it changes the channel size of the working current. The transistor uses the signal voltage applied to the emitter junction to change the junction current flowing through the emitter junction, and also includes a very complicated process of minority carriers crossing the base region and entering the collector region. The unique and simple principle of action of the FET gives the FET a number of excellent properties that give the user an enticing brilliance.

First, the characteristics of the FET tube FET has the advantages of high input impedance, small noise figure, good thermal stability and large dynamic range compared with ordinary transistors. It is a voltage-controlled device that has similar transmission characteristics as a tube, and thus has been widely used in high-fidelity audio equipment and integrated circuits, and has the following features.

1. The high input impedance is easy to drive, and the input impedance varies little with frequency. The input junction capacitance is small (feedback capacitor), the change of the output load has little effect on the input end, the driving load capacity is strong, and the power utilization rate is high.
2. The noise of the FET is very low, and the noise figure can be less than 1dB. Now most of the FETs have a noise figure of about 0.5dB, which is difficult to achieve in general transistors and tubes.
3. The FET has better thermal stability and a larger dynamic range.
4. The output of the FET is the input of the power function of 2, the distortion is lower than the transistor, slightly larger than the bile duct. The distortion of the FET is mostly even harmonic distortion, the sense of hearing is good, the energy distribution of high and low frequency is appropriate, the sound has density, the low frequency is deeper, the sound field is stable, the transparency is moderate, the layering, resolution and positioning The senses are better, with good sound field space rendering ability, and good performance of music details.
5. When the normal transistor is in operation, since the input terminal (emitter junction) is forward biased, the input resistance is very low, and the input terminal (between the gate and the source) of the FET can be applied with a negative bias. That is, the reverse bias can also be applied with a forward bias, thus increasing the flexibility and diversity of the circuit design. Usually, when the reverse bias is applied, its input resistance is higher, up to 100MΩ. This feature of the FET makes up for the shortcomings of some transistors and tubes in some aspects.
6. The radiation resistance of FETs is about 10 times higher than that of ordinary transistors.
7. The conversion rate is fast and the high frequency characteristics are good.
8. The voltage and current characteristics of the FET are very similar to the output curve of the five-pole tube.

There are many types of FETs, which can be roughly divided into junction FETs and insulated gate FETs, and both have N-type channels (current channels) and P-type channels, each with There are four types of enhanced and depleted types.
Insulated gate field effect transistor is also called metal (M) oxide (O) semiconductor (S) field effect transistor, referred to as MOS tube. According to its internal structure, it can be divided into two types: general MOS tube and VMOS tube, and each has two types: N-channel and P-channel, enhanced and depleted.
The VMOS field effect transistor, which is called a V-channel MOS field effect transistor, is a new type of high-efficiency power switching device developed on the basis of a general MOS field effect transistor. It not only inherits the high input impedance of MOS FET (more than 100MΩ), the drive current is small (about 0.1uA), but also has high withstand voltage (up to 1200V), large operating current (1.5~100A), and high output power (1~). 250W), good cross-wire performance, 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 up to several thousand times), RF power amplifier, switching power supply and inverter. Because it combines the advantages of tubes and transistors, the high-fidelity audio amplifiers made with it have warm and sweet sound without losing strength, and are favored by Philharmonic people, so they have broad application prospects in the audio field. Like the general MOS tube, the VMOS tube can be divided into two types: N-type channel and P-type channel, enhanced type and depletion type. The classification characteristics are the same as those of a general MOS tube. The VMOS FET also has the following features.

1. The input impedance is high. Since the SiO2 layer is between the gate and source, the DC resistance between the gate and source is basically SiO2 insulation resistance, generally about 100MΩ, and the AC input impedance is basically the capacitive reactance of the input capacitor.
2. The drive current is small. Due to the high input impedance, the VMOS transistor is a voltage-controlled device that can be driven with a voltage, and the required drive current is extremely small.
3. The linearity of the transconductance is better. It has a large linear amplification area, which is very similar to the transmission characteristics of the tube. Better linearity means lower distortion, especially with a negative current temperature coefficient (ie, the on current will decrease as the tube temperature increases with the voltage between the gate and source unchanged). ), so there is no tube damage caused by secondary breakdown. Therefore, the parallel connection of VMOS tubes has been widely used.
4. The junction capacitance has no variability effect. The junction capacitance of the VMOS transistor does not change with the junction voltage, and there is no variability effect of the general transistor junction capacitance, which avoids the distortion caused by the varactor effect.
5. Good frequency characteristics. The majority carrier motion of the VMOS FET is a drift motion, and the drift distance is only 1 to 1.5 um, which is not limited by the transition time of the minority carrier base such as a transistor. Therefore, the power gain varies little with frequency and the frequency characteristics are good. .
6. The switching speed is fast. Since there is no storage delay time for minority carriers, the switching speed of the VMOS FET is fast, and it can turn on or off several tens of A current in 20 ns.

Second, the main parameters of the field effect tube and selection In order to correctly and safely use the FET, to prevent static electricity, misuse or improper storage and damage to the FET, the main parameters of the FET must be understood and mastered. There are dozens of parameters of the FET, and the main parameters and meanings are listed in Table 1 for reference.

1. The parameters of the field effect tube ID are selected according to the circuit requirements, which 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, and the higher the high frequency response and distortion of the circuit. For a tube with ID 2A, CGS is about 80pF; tube with ID is 10A, CGS is about 1000pF. The reliability of the use can be guaranteed by a reasonable heat dissipation design.
2. The source-drain withstand voltage BVDSS of the VMOS tube should not be too high and can meet the requirements. Because the BVDSS large tube has a large saturation pressure drop, it will affect the efficiency. The junction FETs should be as high as possible because they are not high, generally 30 to 50V for BVDSS and 20V for BVGSS.
3. The BVGSS of the VMOS tube is as high as possible, because the gate of the VMOS tube is very delicate, it is easy to be broken down, and the storage or operation should be cautious to prevent the electrostatically charged object from contacting the pin. In the storage, the lead pin should be short-circuited and shielded by a metal box to prevent the external induced potential from penetrating the gate. In particular, the tube cannot be placed in a plastic box or a plastic bag. In order to prevent gate inductive breakdown, all instrumentation, soldering iron, circuit board and human body must have good grounding effect during installation and debugging. All the pins of the tube must be shorted before the tube is connected to the circuit. State, the shorting material can be removed after welding.
4. The matching tube requires the same batch number of the same factory, so the parameters are consistent. Try to use the twinning tube to make the pinch-off voltage and transconductance of the tube as consistent as possible, so that the pairing error is less than 3% and 5% respectively.
5. As far as possible, the audio special tube is used, which is more suitable for the requirements of the audio amplifier circuit.
6. When installing the FET, the position should be kept close to the heating element. In order to prevent the tube from vibrating, the tube should be fastened. When the lead wire is bent, it should be bent at a distance of more than 5 mm from the root to prevent the tube from being broken or causing air leakage and damage the tube. The pipe must have good heat dissipation conditions, and sufficient heat sink must be provided to ensure that the pipe temperature does not exceed the rated value to ensure long-term stable and reliable operation.

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