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 field-effect transistor's principle of controlling the working current is completely different from that of ordinary transistors. It is much simpler than the ordinary transistor. The field-effect transistor simply uses the external input signal to change the resistance of the semiconductor. 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.

The high input impedance is easy to drive, and the change in input impedance with frequency is relatively small. The input junction capacitance is small (feedback capacitance), the change of the load at the output end has little effect on the input end, the driving load capacity is strong, and the power supply utilization rate 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. The field effect tube has better thermal stability and larger dynamic range. The output of the field effect tube is a function of the second power of the input, and the degree of distortion is lower than that of the transistor and 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 can also be forward biased, 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 radiation resistance of the field effect tube is about 10 times higher than that of ordinary transistors. The conversion rate is fast and the high frequency characteristics are good. 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) 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.

The input impedance is high. Because the SiO2 layer is between the gate and the 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 that 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 the 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. The frequency characteristics are good. 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 . The switching speed is fast. 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 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.

Table 1 Main parameters and meanings of FET
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 is applied to the gate-channel junction.
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 = â–³ ID / â–³ 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. The smaller the NF, the smaller the tube noise.
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
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. 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.

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