Field Effect Transistor Characteristics and Design of Single-ended Class A Power Amplifier

Field Effect Transistor Characteristics and Design of Single-ended Class A Power Amplifier

Field effect tubes not only have the advantages of ordinary transistors and electron tubes, but also have advantages that both lack. Field effect tubes have bidirectional symmetry, that is, the source and drain of the field effect tube are interchangeable (no damping), which is not easy for ordinary transistors and impossible for electron tubes. The so-called bidirectional symmetry, for ordinary transistors, is the interchange of emitter and collector, and for electron tubes, is the interchange of cathode and anode.

The principle of controlling the working current of the field effect tube is completely different from that of ordinary transistors. It is much simpler than ordinary transistors. The field effect tube simply uses the external input signal to change the resistance of the semiconductor, which actually changes the size of the channel through which the working current flows. The transistor uses the signal voltage added to the emitter junction to change the junction current flowing through the emitter junction, and also includes extremely complex 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 properties, and it emits an attractive glow to the user.

1. Characteristics of field effect tubes
Compared with ordinary transistors, field effect tubes have the advantages of high input impedance, low noise factor, good thermal stability, and large dynamic range. It is a voltage-controlled device with transmission characteristics similar to those of electron tubes, so it has been widely used in high-fidelity audio equipment and integrated circuits. Its characteristics are as follows.

1. High input impedance is easy to drive, and the input impedance changes little with frequency. The input junction capacitance is small (feedback capacitance), the change of the output load has little effect on the input, the load driving capacity is strong, and the power supply utilization rate is high.
2. The noise of field effect tubes is very low, and the noise factor can be below 1dB. Now the noise factor of most field effect tubes is around 0.5dB, which is difficult to achieve with ordinary transistors and electron tubes.
3. Field effect tubes have better thermal stability and a larger dynamic range.
4. The output of the field effect tube is a square function of the input, with a lower distortion than the transistor and slightly larger than the tube. The distortion of the field effect tube is mostly even harmonic distortion, with good listening experience, proper energy distribution of high, medium and low frequencies, a sense of density in the sound, a deeper low frequency, a more stable sound field, moderate transparency, good performance in layering, resolution and positioning, good ability to depict the sound field space, and good performance of music details.
5. When ordinary transistors are working, because the input end (emitter junction) is forward biased, the input resistance is very low. The input end (between the gate and the source) of the field effect tube can be negatively biased, that is, reverse biased, or forward biased, which increases the flexibility and diversity of circuit design. Usually, when reverse biased, its input resistance is higher, up to 100MΩ or more. This feature of the field effect tube makes up for the shortcomings of ordinary transistors and electron tubes in some aspects.
6. The radiation protection capability of field effect transistors is about 10 times higher than that of ordinary transistors.
7. Fast conversion rate and good high frequency characteristics.
8. The voltage and current characteristic curve of the field effect tube is very similar to the output characteristic curve of the pentode tube.

There are many varieties of field effect transistors, which can be roughly divided into two categories: junction field effect transistors and insulated gate field effect transistors. Both have two types: N-type channel (current channel) and P-type channel, each of which has four types: enhancement type and depletion type.
Insulated gate field effect transistors are also called metal (M) oxide (O) semiconductor (S) field effect transistors, referred to as MOS tubes. According to their internal structure, they can be divided into general MOS tubes and VMOS tubes, each of which has two types: N-type channel and P-type channel, enhancement type and depletion type.
VMOS field effect transistors, whose full name is V-groove MOS field effect transistors, are new high-efficiency power switching devices developed on the basis of general MOS field effect transistors. It not only inherits the high input impedance (greater than 100MΩ) and small drive current (about 0.1uA) of MOS field effect transistors, but also has excellent characteristics such as high withstand voltage (up to 1200V), large working current (1.5~100A), high output power (1~250W), good cross-conductivity, and fast switching speed. At present, it has been widely used in circuits such as high-speed switching, voltage amplification (voltage amplification can reach thousands of times), radio frequency amplifiers, switching power supplies and inverters. Because it has the advantages of both electron tubes and transistors, the high-fidelity audio amplifiers made of it have warm and sweet sound quality without losing power, which is favored by music lovers, so it has broad application prospects in the field of audio. VMOS tubes, like general MOS tubes, can also be divided into four categories: N-type channel and P-type channel, enhancement type and depletion type. The classification characteristics are the same as general MOS tubes. VMOS field effect tubes also have the following characteristics.

1. High input impedance. Since there is a SiO2 layer between the gate and source, the DC resistance between the gate and source is basically the SiO2 insulation resistance, which is generally around 100MΩ. The AC input impedance is basically the capacitive reactance of the input capacitor.
2. The driving current is small. Due to the high input impedance, the VMOS tube is a voltage-controlled device, which can generally be driven by voltage and requires very small driving current.
3. The linearity of transconductance is good. It has a large linear amplification area, which is very similar to the transmission characteristics of electron tubes. Better linearity means lower distortion, especially with a negative current temperature coefficient (that is, when the voltage between the gate and the source remains unchanged, the on-current will decrease as the tube temperature increases), 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 varactor effect. The junction capacitance of the VMOS tube does not change with the junction voltage, and does not have the varactor effect of the junction capacitance of a general transistor, thus avoiding the distortion caused by the varactor effect.
5. Good frequency characteristics. The majority carrier motion of the VMOS field effect tube belongs to drift motion, and the drift distance is only 1 to 1.5um, which is not limited by the transition time of the minority carrier base region like the transistor. Therefore, the power gain changes very little with the frequency, and the frequency characteristics are good.
6. Fast switching speed. Since there is no storage delay time for minority carriers, the switching speed of VMOS field effect tubes is fast, and tens of A of current can be turned on or off within 20ns.

2. Main parameters and selection of field effect tubes
In order to use field effect tubes correctly and safely and prevent damage to field effect tubes due to static electricity, misoperation or improper storage, it is necessary to understand and master the main parameters of field effect tubes. There are dozens of parameters for field effect tubes. The main parameters and their meanings are listed in Table 1 for reference.

1. The parameters of the field effect tube ID are selected according to the circuit requirements. They can meet the power consumption requirements with a slight margin. Don't think that the larger the ID, the better. The larger the ID, the larger the CGS, which is not good for the high-frequency response and distortion of the circuit. For example, for a tube with an ID of 2A, the CGS is about 80pF; for a tube with an ID of 10A, the CGS is about 1000pF. The reliability of use can be guaranteed by a reasonable heat dissipation design.
2. The source-drain withstand voltage BVDSS of the selected VMOS tube should not be too high, just enough to meet the requirements. Because the tube with a large BVDSS has a large saturation voltage drop, which will affect the efficiency. The junction field effect tube should be as high as possible, because they are not high to begin with, generally BVDSS is 30~50V, and BVGSS is 20V.
3. The BVGSS of the VMOS tube should be as high as possible, because the gate of the VMOS tube is very delicate and can be easily broken down. It is necessary to be extremely careful when storing or operating it to prevent static objects from contacting the pins. During storage, the lead pins should be short-circuited and the tube should be shielded and packaged with a metal box to prevent the gate from being broken down by external induced potential. In particular, the tube should not be placed in a plastic box or plastic bag. In order to prevent gate induction breakdown, all instruments, soldering 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 in a short-circuited state, and the short-circuited material can be removed after welding is completed.
4. Paired tubes are required to be from the same factory and the same batch number, so that the parameters are consistent. 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 error is less than 3% and 5% respectively.
5. Try to use audio-specific tubes as much as possible, as they will be more suitable for the requirements of audio amplifier circuits.
6. When installing the field effect tube, avoid placing it near the heating element. To prevent the tube from vibrating, tighten the tube. When bending the tube pin lead, bend it at a distance greater than 5mm from the root to prevent the tube pin from being broken or causing leakage and damage to the tube. The tube must have good heat dissipation conditions and must be equipped with sufficient radiators to ensure that the tube temperature does not exceed the rated value and ensure long-term stable and reliable operation.