Characteristics of field effect transistors and the entire production process of single-ended Class A power amplifiers

The principle of controlling the operating current of a field effect transistor is completely different from that of an ordinary transistor. It is much simpler than an ordinary transistor. A field effect transistor simply uses an external input signal to change the resistance of the semiconductor. In fact, it Changing the size of the channel through which the working current flows, and the transistor uses the signal voltage applied to the emitter junction to change the junction current flowing through the emitter junction, and also includes extremely complex effects such as minority carriers crossing the base region and then entering the collector region. process. The unique and simple principle of action of the field effect tube gives the field effect tube many excellent properties, and it exudes an attractive brilliance to the user.

Field effect transistors not only have the advantages of ordinary transistors and electron tubes, but also have the advantages that are lacking in both. The field effect transistor has bidirectional symmetry, that is, the source and drain of the field effect transistor are interchangeable (without damping). It is not easy for ordinary transistors to achieve this, and it is simply impossible for electron tubes to achieve this. The so-called two-way symmetry means that for ordinary transistors, it means that the emitter and collector are interchanged, and for electron tubes, it means that the cathode and anode are interchanged.

1. Characteristics of field effect transistors

Compared with ordinary transistors, field effect transistors have 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 that of an electron tube. Therefore, 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 (feedback capacitance) is small, changes in the output load have little impact on the input, the drive load capability is strong, and the power utilization rate is high.

The noise of field effect transistors is very low, and the noise coefficient can be less than 1dB. The noise coefficient of most field effect transistors now is about 0.5dB, which is difficult to achieve by ordinary transistors and electron tubes.

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

The output of the field effect tube is a 2-power function of the input, and the distortion is lower than that of the transistor and slightly larger than that of the bile duct. The distortion of field effect tubes is mostly even-order harmonic distortion. It has a good listening experience, appropriate energy distribution of high, middle and low frequencies, a sense of density in the sound, deeper low frequencies, a stable sound field, moderate transparency, layering, resolution and positioning. It has good performance in all senses, has good sound field spatial delineation ability, and has good performance in music details.

When an ordinary transistor is working, since the input terminal (emitter junction) is forward biased, the input resistance is very low. The input terminal of the field effect transistor (between the gate and the source) During operation, a negative bias voltage, that is, a reverse bias voltage, or a forward bias voltage can be applied, thus increasing the flexibility and diversity of the circuit design. Usually when reverse bias is applied, its input resistance is higher, up to more than 100MΩ. This characteristic of the field effect transistor makes up for the shortcomings of ordinary transistors and electron tubes in some applications.

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 curves of field effect tubes are very similar to the output characteristic curves of pentode tubes.

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 of which have N-type channel (current channel) and P-type channel. Two types, each of which has four types: enhanced type and depleting type.

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 tubes and VMOS tubes. Each type has two types: N-type channel and P-type channel, enhancement type and depletion type.

VMOS field effect transistor, whose full name is V-groove MOS field effect tube, is a new type of high-efficiency power switching device developed on the basis of general MOS field effect tube.

It not only inherits the high input impedance (greater than 100MΩ) and small driving current (about 0.1uA) of the MOS field effect tube, but also has the characteristics of high voltage resistance (up to 1200V), large operating current (1.5~100A), and high output power (1~100A). 250W), good transconductance linearity, fast switching speed and other excellent characteristics. It has been widely used in circuits such as high-speed switching, voltage amplification (voltage amplification can reach thousands of times), radio frequency power amplifiers, switching power supplies and inverters. Because it has the advantages of both tubes and transistors, the high-fidelity audio amplifier made with it has warm, sweet and powerful sound quality, which is favored by music lovers, so it has broad application prospects in the audio field. VMOS tubes, like general MOS tubes, can also be divided into four categories: 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. Since there is a SiO2 layer between the gate and the source, the DC resistance between the gate and the source is basically the SiO2 insulation resistance, which is generally about 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 and can generally be driven with voltage, and the required driving current is extremely small.

The linearity of the transconductance is better. It has a large linear amplification area and is very similar to the transmission characteristics of an 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 remains unchanged, the conduction 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 VMOS tubes does not change with the junction voltage, and there is no varactor effect of the junction capacitance of ordinary transistors, which can avoid distortion caused by the varactor effect.

Good frequency characteristics. The movement of majority carriers in VMOS field effect transistors is drift movement, and the drift distance is only 1~1.5um. It is not limited by the transition time of the minority carrier base area like transistors, so the power gain changes very little with frequency and has good frequency characteristics. .

The switching speed is fast. Since there is no storage delay time for minority carriers, VMOS field effect transistors have fast switching speeds and can turn on or off dozens of A currents in 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 caused by static electricity, misoperation or improper storage, field effect tubes must be Understand and master the main parameters of the tube. There are dozens of parameters for field effect transistors. 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 ID parameters of the field effect transistor are selected according to the circuit requirements. It can meet the power consumption requirements with a slight margin. Don’t think that bigger is better. The larger the ID, the larger the CGS. It will have a negative impact on the circuit. The high frequency response and distortion are unfavorable. For example, a tube with an ID of 2A has a CGS of about 80pF; a tube with an ID of 10A has a CGS of about 1000pF. The reliability of use can be ensured through reasonable heat dissipation design.

The source-drain voltage BVDSS of the VMOS tube should not be too high, as long as it meets the requirements. Because tubes with large BVDSS also have large saturation pressure drops, which will affect efficiency. Junction field effect transistors 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. Be careful when storing or handling it to prevent statically charged objects from contacting the pins. During storage, the lead pins should be short-circuited and shielded and packaged in a metal box to prevent external induced potential from breakdown of the grid. Particular attention should be paid to not placing the tubes in plastic boxes or bags.

In order to prevent grid induced breakdown, all instruments, soldering irons, circuit boards and human bodies must have good grounding effects during installation and debugging. Before the tube is connected to the circuit, all pins of the tube must be kept short-circuited. status, the short-circuiting material can be removed after welding is completed.

Paired tubes are required to be from the same manufacturer and 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 errors are less than 3% and 5% respectively.

Try to use audio-specific tubes as much as possible, which are more suitable for the requirements of audio amplification circuits.

When installing field effect tubes, avoid placing them close to heating elements. In order to prevent the pipe from vibrating, the pipe must be fastened. When bending, the pin leads should be bent at a distance greater than 5 mm from the root to prevent the pipe from being broken or causing air leakage during bending. The pipe must have good heat dissipation conditions and must be equipped with enough radiators to ensure that the temperature of the pipe does not exceed the rated value and ensure long-term stable and reliable operation.

3. 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 the above-mentioned devices according to the amplifier components used. Mixed amplifiers composed of two or more types, various amplifier circuits and components used are also diverse and ever-changing. As a result, the playback sound quality of the sound source has its own characteristics. It is difficult to say which amplifier can be generalized and technical. Yi Qunfang became a universal amplifier.

Due to the transmission time lag of space charge, the reproduction sound of the tube amplifier is warm and soft, especially the string vocals, which are mellow, clear and intriguing. Transistor and integrated circuit amplifiers have sharp analytical power, wide frequency response and strong dynamics, and have a vibrant and inspiring charisma. Field-effect transistor amplifiers and hybrid device amplifiers strive to combine the audio characteristics of tubes and transistors to create unique brilliance, make the music more vivid, and make the timbre more perfect.

In recent years, with the continuous development of electronic computer technology, various electronic synthesizers, various audio effectors and bile sound effector software, as well as virtual speaker technology, have emerged in endlessly. This makes the development and popularization of audio amplifier hardware far behind the speed of software. Hardware often cannot keep up with software in terms of accuracy. For example, the fidelity of computer-simulated 3D effects greatly exceeds the real 3D effects, and is not affected by the space of the listening room and the synthesis of sound sources. limitations, while also saving hardware costs.

Green audio, dual-material fever - computer audio is likely to become the mainstream of audio in the future. Hardware cannot be replaced by software. It implements both software and hardware, and has powerful functions, embodying the characteristics of efficiency, convenience, magic and economy. If you set up a virtual optical drive in your computer, you don't have to start the physical optical drive every time you play music. This not only reduces the waiting time for tracks and the wear and tear of the physical optical drive, but more importantly, eliminates the noise of the physical optical drive and achieves high-fidelity playback. For another example, the bile tube amplifier has a soft and durable sound, but the production cost is not low, and there are many requirements for obtaining beautiful sound. The bile tube effector software can create a "soft bile" in the computer for us to simulate. Bring out the tone of the tube amplifier. At present, computer multimedia audio is in an advanced stage and has also established a bridge of communication with TV. Its prospects are very bright and attractive! Computer and audio enthusiasts are people who spare no time and energy to actively explore and pursue the special aspect of sound quality. They will continue to take on the responsibility of music. One more sweet song in life and one less scene of bitter disputes. Whether it is ordinary audio or computer multimedia audio, the power amplifier is still an indispensable terminal for amplifying audio energy and promoting speaker sound. All types of amplifiers can achieve this function well. However, modern people have demanding requirements for audio (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 tone is rich, whether the listening experience is pleasant, etc.) Increasingly high, many "golden ears" can hear the singer's sibilance, quarrels, and the immersive and live feeling. Therefore, they also place greater demands on the audio amplifier's playback tone, and strive to use characteristic sound. Create a charming musical atmosphere.

Various types of audio amplifiers have their own advantages and attributes, as well as their shortcomings. The mainstream field effect tube amplifier has the advantages of both transistors and electron tubes, and also has features that neither of them has. Advantages. In terms of circuit design, a large amount of practice has proven that single-ended Class A power amplifiers are a model of exchanging efficiency for sound quality and have unparalleled musical charm. Many audiophiles start from the pure pursuit of sound quality, repeatedly build amplifiers, and repeatedly compare and listen to sounds. In the end, they are moved by Class A. They seem to feel that music without Class A is like lonely music.

IV. A brief discussion on the performance of single-ended Class A amplifiers

Amplifiers can generally be divided into three categories according to different working conditions: ① Class A amplifiers, also known as Class A amplifiers; ② Class AB amplifiers Amplifiers are also called Class A and B amplifiers; ③ Class B amplifiers are also called Class B amplifiers. Among these three types of amplifiers, the Class A amplifier has the best linearity and the most beautiful sound. One difference in design between single-ended Class A amplifiers and push-pull amplifiers is that they use one amplifier 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 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 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 pairing Twin tubes with an accuracy of up to 2% and even smaller errors, but this is only a one-sided digital description. In fact, the positive and negative waveforms cannot be well connected. In addition, there is a phase shift caused by the nonlinearity of circuit components, and the crossover distortion will further Increase, of course, distortion and timbre are not opposed to a certain extent. This depends on the purpose and goal of designing the amplifier. 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 The handover is not good, but the harmonic handover cannot be denied, but it is difficult to compete with the single-ended waveform.

Regarding the statement that push-pull amplification harmonics, especially even harmonics, will cancel each other out, the author does not fully agree. Only harmonic components such as phase shift distortion reaching 180° or 360° will cancel each other out. . For example, the AC ripple in the DC high voltage in a push-pull power amplifier is evenly divided into two paths through the center tap of the push-pull transformer. Since the polarities of the two arm coils are opposite and 180° apart, the AC ripple is almost completely offset.

Single-ended Class A amplifiers have the most natural musicality, and their asymmetry is similar to the compression and expansion characteristics of air. Since the largest content of air is non-polar molecule nitrogen (N2), accounting for about 78%, air is a "single-ended non-polar" medium whose pressure can become very high, making single-ended Class A music the most expressive and timbre. mellow.

5. Production of VMOS FET 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 the single-ended Class A amplifier. Simplicity is not the only reason for using single-ended Class A amplifiers. It is because single-ended Class A amplifiers have the most charming sense of music. Among the circuit formulas of Class A, Class B, and Class AB, Class A has the best linearity, but the disadvantage is that the efficiency is the lowest, about 20%, which is a model of exchanging efficiency for sound quality.

The amplifier components used in single-ended Class A amplifier circuits are also very particular. Transistors have too low input impedance. 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 use as power amplifier output tubes, so the only option is field effect tubes. Field effect transistors have high input impedance and transconductance, and can also output large currents, making them 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 have unique charm. The high-end titanium diaphragm sound, the full, delicate and smooth magnetic sound in the mid-range, and the elastic and shocking low-frequency bombing sound have a domineering momentum.

In general designs, the advantages of field effect tubes are not fully utilized, and the sound is even considered to be cold and dark. In fact, this is not the reason for the field effect tubes. Its sound is not good. On the one hand, people use it to directly replace transistors, and the transistor circuit cannot bring out the characteristics of field effect transistors. On the other hand, these circuits usually use Class AB bias.

According to the transfer characteristics of the field effect transistor, it has serious nonlinearity at low bias, causing serious distortion. The solution is to make it work in a class A state, especially single-ended class A, which has excellent transient characteristics and good sound quality. Pure beauty, rich even harmonics, pleasant sound, and the mellow sound of a vacuum tube.

1. Circuit principle

10W single-ended Class A field effect transistor power amplifier circuit

There are various 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, the principle of simplicity first is adopted. One more component adds one point of distortion, and one more line adds one point of distortion. Now I will give a brief description of the circuit principle to introduce some ideas. Its main features are as follows.

(1) In order to avoid the transmission distortion of ordinary volume potentiometers, unstable contact resistance, friction noise and susceptibility to fatigue during operation, this unit uses the audio-type extremely low-noise VMOS field effect transistor IRFD113 as the indicator. Touch the volume control. Compared with the keyed volume circuit, it reduces some components and shields them, so that the noise coefficient of the volume control part reaches below 1dB (the noise coefficient of the VMOS field effect tube is about 0.5dB), which dares to compete with high-end vacuum stepper potentiometers or without The source transformer potentiometer is used to counterbalance, and the feel is more user-friendly.

The VMOS field effect transistor has 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 basically remain constant for a long time. Change. 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 touches (conducts electricity according to the finger resistance) switch S1 When it is closed, it charges the capacitor. When the finger touch switch S2 is closed, it discharges the capacitor, thereby achieving the purpose of controlling the drain-source resistance with voltage. Press it into the audio device to adjust the volume. S1 and S2 can be made of thin silver sheets or thin copper sheets, with a spacing of about 2mm, which will be determined after debugging, and the volume increase or decrease is set to about ±2dB.

(2) IRF510 is used for voltage amplification. The amplified audio voltage is directly coupled to the upper arm tube IRF150 for current amplification and source output. The lower arm tube IRF150 forms a constant current source, with DC as the path and AC as the path. It is an open circuit that allows the AC signal to push the speaker through the output capacitor.

(3) Since the VMOS field effect transistor 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 rate of the tube temperature is far different from the change rate of the 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 of appropriate resistance (100Ω/2W) on the IRF510 cathode string. ) to act as a buffer. The principle is that when there is no cathode resistance, the IRF510 gate-source voltage is a constant fixed bias voltage, which has nothing to do with changes in the tube current. After adding the cathode resistance, when the tube flow decreases, the source potential also decreases. Compared with the gate, the gate potential increases, so the gate-source voltage increases. At this time, the tube current increases, thus appropriately offsetting the current steep slope phenomenon caused by the negative temperature coefficient. The size of the cathode resistor determines the magnitude 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, there is no output capacitor circuit. One is for the safety of the speaker, and the other is to consider the zero-point offset voltage, especially the DC bias magnetic displacement of the speaker voice coil when it is dynamic. , directly affects the speaker performance, thereby deteriorating the sound quality. Since most large-capacity output capacitors are electrolytic capacitors, the noise is generally considered to be larger. In fact, this is a signal-to-noise ratio problem. The key is what circuit it is used in. For example, if electrolytic capacitors are used in a moving coil phono amplifier circuit, it will not Suitable, the moving coil cartridge signal is only about 2mV, which requires the amplification circuit to have a high signal-to-noise ratio, and the signal-to-noise ratio will be low if an electrolytic capacitor is used. When electrolytic capacitors are used for the final output of a power amplifier, the situation is different. The signal-to-noise ratio will be greatly improved compared to low-level circuits.

Another point is that electrolytic capacitors should be energized and aged before use, and should be selected and used, and then fully burned in after being put on the machine, which can reduce the noise coefficient. There is no component without noise. The key is to use it rationally and take measures to achieve the necessary purpose. In order to reduce the impact of the output electrolytic capacitor on high frequency due to inductive reactance, this machine uses three electrolytic capacitors in parallel to reduce the inductive reactance, and connects the negative electrode of the speaker to the negative electrode of the electrolytic capacitor to clamp the sound generated by the leakage current of the electrolytic capacitor. Cyclic bias magnetic displacement.

(5) The bias voltage of this field effect transistor is provided by the power module LM7812. The power amplifier does not use a regulated power supply to avoid limiting the low-frequency strength and dynamics of Roxy, that is, reducing the voltage in exchange for current and reducing the power. Change the sound quality.

2. Production and debugging

When making this unit, the two channels must 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 uses a direct coupling circuit, the working points will restrain each other, and it needs to be debugged several times to complete. The working current of IRF510 is about 20mA, and the working current of the upper and lower IRF150 tubes (paired) is about 1.5A. The gate-source voltage is about is 3.8V, repeatedly adjust the two-stage bias resistors so that the midpoint voltage is l8V. Tubes from different origins and batches will have differences. The data is for reference only. It is best to use an oscilloscope to adjust it to the best working condition of Class A. Otherwise, due to the discrete nature 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. This machine can replace many field effect tubes, and different tubes have different parameters, characteristics and timbres. Table 2 lists several commonly used pipe parameters for reference. For the selection of other components of this machine, please refer to relevant information and will not be repeated here.

Table 2 Main parameters of several commonly used field effect transistors

Single-ended circuits are large power consumers. The heat loss of a single output tube of this machine is about 30W. Increasing the operating voltage can also increase the output. power, but heat losses also increase accordingly. Therefore, the tube must be installed on a radiator with a thermal resistance not greater than 1kΩ/W and a specification of not less than 200mm × 200mm × 6mm. Apply silicone grease to the tube and tighten it in place.

3. Parameter indicators

The measured technical indicators are shown in Table 3.

Table 3 Measured technical indicators

4. Evaluation and listening test

The equipment used for the evaluation and listening test of this machine is as follows:

(1) Philips ( Philips) LHH-500 top CD player; (2) Self-made direct heat pipe 3A5 preamp;

(3) Italian Aoba Cass speaker;

(4) American music ribbon Super Flatine Cable speaker cable; (5) Ortofon AC-5000 8N oxygen-free copper signal cable; (6) Hitachi 4N single product copper 3×3.5mm silicone rubber power cable; (7) G&W TW-05D audio dedicated power supply Purifier.

Parameters of field effect transistor STD45N10F7

Manufacturer: STMicroelectronics

Product type: MOSFET

RoHS:? Detailed information

Technology: Si

Installation style: SMD/SMT

Package/Box: TO-252-3

Number of channels: 1 Channel

Transistor polarity: N-Channel

Vds-drain-source breakdown voltage: 100 V

Id-continuous drain current: 45 A

　Rds On-drain-source on-resistance: 18 mOhms