Field Effect Transistor Characteristics and Single-Ended Class A Amplifier Production

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 brilliance to the user.

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.

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 similar transmission characteristics 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 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 utilization rate is high.

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.

Field effect tubes have better thermal stability and a larger dynamic range.

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.

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.

The radiation protection capability of field effect transistors 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 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.

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.

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.

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.

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.

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.

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 of field effect tubes. The main parameters and their meanings are listed in Table 1 for reference.

The following points should be noted when selecting field effect transistors.

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.

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.

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.

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.

Try to use audio-specific tubes as much as possible, as they will be more suitable for the requirements of audio amplifier circuits.

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.

3. 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 devices according to the amplifier devices used. The various amplifier circuits and components used are also varied and ever-changing, so the sound quality of the sound source is also unique. It is difficult to say which amplifier can be generalized and become a universal amplifier.
Due to the transmission time lag of space charge, the tube amplifier reproduces warm and soft sound, especially string music and human voice, which is mellow and intriguing. Transistors and integrated circuit amplifiers have sharp analytical power, wide frequency response and strong dynamics, and have a vigorous and inspiring appeal. Field effect tube amplifiers and hybrid device amplifiers strive to integrate the audio characteristics of tubes and transistors to create a unique color, 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 tube sound effects 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 often cannot catch up with the software in terms of accuracy. For example, the fidelity of computer simulated 3D effects is much higher than that of real 3D effects, which is not limited by the space of the listening room and the synthesis of sound sources, and also saves the cost of hardware investment.
Green audio, double fever - computer audio is likely to become the mainstream of audio in the future. If the hardware is not good, the software will come. It implements both hardware and software, has powerful functions, and embodies the characteristics of high efficiency, convenience, magic and economy. For example, if a virtual optical drive is set in the computer, there is no need to start the physical optical drive every time a song is played. This not only reduces the waiting time for the song and the wear of the physical optical drive, but more importantly, eliminates the noise of the physical optical drive and achieves high-fidelity playback. For example, the sound of a tube amplifier is soft and pleasant to listen to, but the production cost is not low, and there are many elements to obtain beautiful sound. Through the tube sound effect software, we can create a "soft tube" in the computer to simulate the sound of a tube amplifier. At present, computer multimedia audio is in an advanced period, and it has also built a bridge of communication with television. Its prospects are very bright and attractive! Computer and audio enthusiasts are a special level that spares time and energy to actively explore and pursue sound quality. They will continue to shoulder a responsibility of loving music. One more sweet song in life means one less bitter dispute. Whether it is ordinary audio or computer multimedia audio, the power amplifier is still an indispensable terminal for expanding audio energy and driving the speaker to sound. All kinds of amplifiers can achieve this function well. However, modern people have higher and higher requirements 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 is mellow, whether the hall sound is rich, whether the listening experience is pleasant, etc.). Many "golden ears" can hear the singer's dental sounds, corners of the mouth, and the feeling of being on the scene, so they also place greater demands on the audio amplifier to reproduce the sound, and strive to create a charming music atmosphere with characteristic sound.
Various types of audio amplifiers have their own advantages and properties, and each has its own shortcomings. The mainstream field effect tube amplifier has the advantages of both transistors and tubes, and also has advantages that neither of them has. In terms of circuit programming, a lot of practice has proved that single-ended Class A amplifiers are a model of efficiency in exchange for sound quality, and have unparalleled musical charm. Many audiophiles start from the simple pursuit of sound quality, repeatedly make amplifiers, repeatedly compare and listen, and finally are moved by Class A, and seem to feel that music without Class A is like lonely music.

4. Brief discussion on the performance of single-ended Class A amplifiers
Amplifiers can generally be divided into three categories according to their working conditions: ① Class A amplifiers, also known as Class A amplifiers; ② Class AB amplifiers, also known as Class AB amplifiers; ③ Class B amplifiers, also known as Class B amplifiers. Among these three types of amplifiers, Class A amplifiers have the best linearity and the most beautiful tone. The difference between single-ended Class A amplifiers and push-pull amplifiers in design is that one amplifier device is used 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 respectively, 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, without the crossover distortion of the positive and negative waveforms of push-pull amplification. Although push-pull amplification uses twin tubes with a matching accuracy of up to 2% error or even smaller error, this is only a one-sided digital description. In fact, the positive and negative waveforms cannot be well interleaved. In addition, the phase shift caused by the nonlinearity of circuit components will further increase the crossover distortion. Of course, distortion and timbre are not opposite to a certain extent. This depends on the purpose and goal of the amplifier design. Push-pull amplification does not stop there. Moreover, in push-pull amplifiers, due to the presence of multiple harmonics, although the original positive and negative waveforms are not well interleaved, the harmonic interleaving cannot be denied, but it is difficult to compete with the single-ended waveform.
Regarding the statement that the harmonics of push-pull amplification, especially the even harmonics, will cancel each other out, I do not fully agree. Only harmonic components with phase shift distortion of 180° or 360° will cancel each other out. For example, the AC ripple in the DC high voltage of the push-pull amplifier is evenly divided into two paths through the center tap of the push-pull transformer. Since the polarity of the two arm coils is opposite and the difference is 180°, the AC ripple is almost completely offset.
The single-ended Class A amplifier has the most natural musicality, and its asymmetry is similar to the characteristics of air being compressed and expanded. Since the largest component of air is non-polar molecular nitrogen (N2), accounting for about 78%, air is a "single-ended non-polar" medium whose pressure can become very high, making the single-ended Class A music the most vivid and the timbre the most mellow.

5. The production and
design of VMOS field effect tube single-ended Class A power amplifier There are two basic principles for designing amplifiers: one is simplicity, and the other is linearity. The simplest amplifier circuit is single-ended Class A. Simplicity is not the only reason for using single-ended Class A amplifiers, because single-ended Class A has the most charming sense of music. Among the Class A, Class B, and Class AB circuits, Class A has the best linearity, but the disadvantage is that the efficiency is the lowest, about 20%, which is a model of efficiency in exchange for sound quality. The
amplifier components used in the single-ended Class A amplifier circuit are also particular. Transistors have too low input impedance, and the input impedance of electron tubes is very high, but their output impedance is also relatively high. In principle, electron tubes are not suitable for power amplifier output tubes, so the only choice is field effect tubes. Field effect tubes have high input impedance and transconductance, and can also output large currents, which are very suitable for use in single-ended Class A amplifiers. Among the many field effect tubes, single-ended Class A amplifiers made with VMOS field effect tubes are more popular and unique in charm. The high-end titanium diaphragm sound, the full, delicate and smooth magnetic sound of the mid-frequency, and the elastic and shocking low-frequency bombing sound have a unique domineering momentum.

In general designs, the advantages of field-effect tubes are not fully utilized, and some people even think that the sound is cold and dark. In fact, this is not the reason for the field-effect tube. The reason why the sound is not good is that people use it to directly replace transistors, and the circuit of the transistor cannot play the characteristics of the field-effect tube; on the other hand, these circuits usually use class AB bias. According to the transfer characteristics of the field-effect tube, it has serious nonlinearity at low bias, which brings serious distortion. The solution is to let it work in class A state, especially single-ended class A, which has excellent transient characteristics, pure sound quality, rich even harmonics, pleasant sound, and more mellow sound of the electron tube.

1. Circuit Principle

 

There are many kinds of single-ended Class A field effect tube power amplifier circuits, each with its own characteristics. The circuit of this machine is shown in the attached figure. In order to obtain beautiful sound quality, the principle of simplicity is adopted. One more component means one more distortion, and one more line means one more distortion. Now let's briefly describe the circuit principle to stimulate discussion. Its main features are as follows.
(1) In order to avoid the transmission distortion, non-steady-state contact resistance, friction noise and fatigue of ordinary volume potentiometers, this machine uses an audio-type ultra-low noise VMOS field effect tube IRFD113 as a touch volume control. Compared with the key-controlled volume circuit, it reduces some components and shields them, so that the noise coefficient of the volume control part is less than 1dB (the noise coefficient of the VMOS field effect tube is about 0.5dB), which dares to compete with high-end vacuum stepping potentiometers or passive transformer potentiometers, and the feel is more appropriate and humanized. The
VMOS field effect tube has a high internal resistance and is a voltage control device. A charging capacitor is connected between the gate and the source. Since the gate leakage current is extremely small, the capacitor voltage can remain basically unchanged for a long time. When the tube works in the adjustable resistance area, its drain-source resistance will be controlled by the gate-source voltage, that is, the voltage of the capacitor. At this time, the tube is equivalent to a voltage-controlled variable resistor. When the finger-touch (conducting according to the finger resistance) switch S1 is closed, the capacitor is charged. When the finger-touch switch S2 is closed, the capacitor is discharged, thereby achieving the purpose of controlling the drain-source resistance with voltage. Press it into the audio equipment to adjust the volume. S1 and S2 can be made of thin silver or thin copper sheets, with a spacing of about 2mm. After debugging, the volume increase and decrease is set at about ±2dB.
(2) IRF510 is used for voltage amplification. The amplified audio voltage is directly coupled to the upper arm tube IRF150 for current expansion and source output. The lower arm tube IRF150 constitutes a constant current source. DC is the path and AC is the open circuit, so that the AC signal drives the speaker through the output capacitor.
(3) Since the VMOS field effect tube has a negative current temperature coefficient, that is, when the voltage between the gate and the source remains unchanged, the conduction current will decrease as the tube temperature increases, thereby avoiding secondary breakdown of the tube. However, the change of tube temperature is far different from the change rate of current. In order to prevent the negative temperature coefficient inertia delay from affecting the working state, this machine has a positive temperature coefficient compensation resistor (100Ω/2W) with appropriate resistance in series with the IRF510 cathode to play a buffering role. The principle is that when there is no cathode resistor, the gate-source voltage of IRF510 is a constant fixed bias voltage, which is independent of the change of tube current. After adding the cathode resistor, when the tube current decreases, the source potential also decreases. Compared with the gate, the gate potential is increased, so the gate-source voltage increases, and the tube current increases at this time, thereby offsetting the current steep slope phenomenon caused by the negative temperature coefficient. The value of the cathode resistor determines the size of this effect, thereby playing an appropriate buffering role. This resistor is not a current negative feedback resistor.
(4) After consideration, this machine does not use OCL, that is, no output capacitor circuit, one is for the safety of the speaker, and the other is to consider that the zero-point offset voltage, especially in dynamic conditions, will produce DC bias magnetic displacement on the speaker voice coil, which directly affects the speaker performance and deteriorates the sound quality. Since most of the large-capacity output capacitors are electrolytic capacitors, it is generally believed that the noise is large. In fact, this is a problem of signal-to-noise ratio. The key is what circuit it is used in. For example,
it is not suitable to use electrolytic capacitors in the dynamic cartridge amplifier circuit. The dynamic cartridge signal is only about 2mV, which requires the amplifier circuit to have a high signal-to-noise ratio. The signal-to-noise ratio is low when using electrolytic capacitors. However, the situation is different when electrolytic capacitors are used for the final output of the power amplifier. The signal-to-noise ratio will be greatly improved relative to the low-level circuit. Another point is that it is best to power on and age the electrolytic capacitors before use, and then fully burn them in after they are put on the machine, which can reduce the noise coefficient. There is no component without noise. The key is to use it reasonably and take measures to achieve the necessary purpose. In order to reduce the influence of the output electrolytic capacitor on the high frequency due to the inductive reactance, this machine uses 3 electrolytic capacitors in parallel to reduce the inductive reactance, and connects the negative pole of the speaker to the negative pole of the electrolytic capacitor to clamp the voice coil bias displacement caused by the leakage current of the electrolytic capacitor.
(5) The bias voltage of the field effect tube is provided by the power module LM7812. The power supply of the power amplifier is not supplied by a regulated power supply to avoid limiting the low-frequency strength and dynamics of the music, that is, reducing voltage in exchange for current, and reducing power in exchange for sound quality.

2. Production and debugging
     When making this machine, the two channels should be powered by independent power supplies to improve separation, reduce interference, and enhance the working stability of each channel. Since the rear stage of this machine adopts a direct-coupled circuit, the working points will be mutually restrained, and it takes several repeated debuggings to complete. The working current of IRF510 is about 20mA, and the working current of the upper and lower tubes IRF150 (paired) is about 1.5A. The gate-source voltage is about 3.8V. Repeatedly adjust the two-stage bias resistors to make the midpoint voltage l8V. Tubes from different origins and different batches will have some differences. The data is for reference only. It is best to use an oscilloscope to adjust it to the best working state of Class A. Otherwise, due to the discreteness of the tube, even if the working point is adjusted according to the parameters given in the manual or characteristic curve, it may not work in the best Class A state. There are many field-effect tubes that can be replaced by this machine, and the parameters, characteristics and timbre of different tubes are also different. Table 2 lists several commonly used tube parameters for reference. For the selection of other components of this machine, please refer to relevant information, which will not be repeated here.

    Single-ended circuits consume a lot of power. The heat loss of a single output tube of this machine is about 30W. Increasing the working voltage can also increase the output power, but the heat loss also increases accordingly. Therefore, the tube must be installed on a heat sink with a thermal resistance not greater than 1kΩ/W, and the specifications must be not less than 200mm×200mm×6mm. The tube should be coated with silicone grease and fastened in an appropriate position.

3. Parameter indicators
The measured technical indicators are shown in Table 3.

4. Test and audition
The following equipment was used for the test and audition of this unit:
(1) Philips LHH-500 top-level CD player;
(2) Homemade direct-heated tube 3A5 preamplifier;
(3) Italian Aubac speakers;
(4) American Music Ribbon Super Flatine Cable speaker cable;
(5) Ortofon AC-5000 8N oxygen-free copper signal cable;
(6) Hitachi 4N single copper 3×3.5mm silicone rubber power cable;
(7) G&W TW-05D audio-specific power purifier.
    When the genuine Hugo CD was used for audition, the sound field was wide and the dynamics were particularly good. The sound and image positioning and resolution were good, the low frequency was strong, and the control was good. The sound quality was very pure and unforgettable!