Summary
Audio amplifier is an indispensable and important component of modern audio systems. It can amplify audio signals so that they can drive speakers to produce louder sounds. Therefore, designing an audio amplifier with stable and reliable performance is of great significance to improve the sound quality and loudness of audio playback.
The main purpose of this design is to design an audio amplifier that drives 8 ohm speakers, with an average output power of 10w and an operating frequency of 20 Hz-18 kHz. To achieve this goal, we adopted the LM358 op amp and achieved a 900x gain using a four-stage op amp. This design solution not only meets the requirements, but also disperses the noise into multiple amplification stages, thereby reducing the noise level of the entire circuit, providing better sound quality and higher loudness.
During the design process, we first conducted circuit design. During the design process, we analyzed and compared some solutions, selected the final solution and conducted simulation tests to ensure the stability and reliability of the circuit.
The result of this design is not only the successful design of an audio amplifier that meets the requirements and is stable and reliable, but also improves the sound quality and loudness of audio playback, reduces the impact of noise on the signal, and provides users with a better listening experience. At the same time, this audio amplifier also has a wide range of application values and can be used in audio speakers, enthusiast DIY audio and other fields. Therefore, this design has important practical significance and application value.
Keywords: audio amplification, multi-stage amplification, LM358, stability, expandability
The composition and working principle of the circuit... 4
1) Circuit design (including scheme comparison, design calculation, etc.)... 4
3) Simulation results and analysis... 11
1 Purpose of design
The main purpose of this design is to design an audio amplifier that drives 8 ohm speakers and outputs an average power of 10w. By adopting a suitable circuit scheme, we can achieve high signal gain, noise reduction processing and power saving while ensuring the stability and reliability of the amplifier, and improving the sound quality and loudness within the operating frequency range of 20 Hz-18 kHz. Provide a better listening experience.
The design of this audio amplifier has important significance and application value. On the one hand, it can master the use of the design software OrCAD PSpise and the basic knowledge of circuit design, improve circuit design, simulation, and analysis capabilities, thereby laying a solid foundation for future study and work. On the other hand, the audio amplifier has a wide range of applications and can be used in audio speakers, enthusiast DIY speakers, etc. to improve the sound quality and loudness of audio playback and provide users with a better listening experience. Therefore, the design of this audio amplifier is of great significance and value at both the academic and practical application levels.
2Design requirements
The goal of this audio amplifier design is to achieve efficient and stable driving of 8 ohm speakers and output an audio signal with an average power of 10W. The basic requirement is to improve the sound quality and loudness of the audio amplifier to achieve a gain of 720 times in the frequency range of 20 Hz - 18 kHz while ensuring stability and reliability. Further improvement requirements are to reduce noise interference and improve the expected signal-to-noise ratio.
When researching a solution, you must first select a suitable circuit design solution and suitable components. This design uses an LM358 operational amplifier, which achieves a gain of 900 times by using a four-stage operational amplifier, and uses an RC circuit for noise reduction to ensure clearer sound quality. On the technical route, the circuit design is first carried out, the required component parameters are calculated, and simulation verification is performed. During the simulation and debugging process, a variety of testing methods are used, such as observing the output waveform through an oscilloscope, testing and analyzing distortion, frequency response characteristics, etc. In particular, this design uses multi-stage operational amplification to reduce the interference of noise on the signal.
The characteristic of this design is that it achieves the perfect combination of high gain and noise reduction processing in one circuit, while ensuring stability and reliability, and achieving high sound quality and loudness requirements. Through this design solution, we can get a universal audio amplifier that can meet the application needs of audio amplifiers in different fields.
3Design content
Circuit composition and working principle
- The average output power is P0 =10w.
- Operating frequency is 20 Hz-18 kHz.
- Load speaker 8Ω
Figure 1 Audio amplifier
The function of the audio power amplifier is to amplify the input audio signal to increase its output power. The working principle of the amplifier can be illustrated by Figure 1. Usually, the preamplifier circuit of the amplifier mainly completes the amplification of small signals, that is, amplifies the voltage of the input small audio signal to obtain the input required by subsequent stages. The subsequent stages mainly perform power amplification on the audio to ensure that it can drive the resistor and output the required audio. When designing an amplifier, first appropriately allocate the gain of the entire circuit [1] according to the technical index requirements, and determine the specific design plan for each level of circuit. In this way, reasonable amplification of the audio signal can be achieved through specific design to obtain the required audio output effect.
Output power P0max=10W, RL=8Ω, calculate U≈9V.
| A=U0Ui |
⑵ |
Assuming that the input signal is 10mV, the gain is 900.
We use four stages of power amplification, and the total gain factor is the product of the gains of each stage. The plan is to gain 5.5 per level.
Option One:
Preamplifier:
Since the signal provided by the microphone is a small signal, very weak, it is necessary to add a preamplifier before the tone control stage. In order to meet the circuit's requirements for frequency response, zero input noise, current and voltage, we selected the LM358 integrated operational amplifier as the preamplifier. The preamplifier circuit is a first-level amplifier circuit composed of an LM358 amplifier, with an amplification factor of 4.5.
| A= 1+RARB |
⑶ |
Among them, RA is the feedback resistor connected to the inverting input terminal NIN and the output, and RB is the resistance connected to the inverting input terminal NIN of the amplifier and ground (GND).
That is, through the calculation of 1+R2/R1=4.5, R2 is 3.5KΩ and R1 is 1KΩ. In addition, the power supply used Vcc is +12V and Vee is -12V. Through the above design, we can achieve effective amplification of the microphone signal to improve its signal strength and further optimize the audio output effect.
Figure 2 Preamplifier
| A=U0Ui |
⑷ |
Av'= = =4.5, we get Ui=45mV, and the input of the next stage is 45mV
Power amplifier design:
We adopted the LM358 monolithic integrated power amplifier circuit [2] as the first solution. This circuit has the characteristics of high rising slope, small transient intermodulation distortion, large output power, simple and easy-to-use peripheral circuits, small size and multiple internal protection circuits, which can ensure the safety and reliability of the circuit's operation.
Based on the above design, we believe that this solution can effectively meet the experimental requirements and bring positive promotion to related experimental research.
From Av= =200, so U0=9V.
| Po=U²RL |
⑸ |
Po≈10W, which meets the requirements of technical indicators.
Figure 3 Power amplifier circuit
Option II
The second-stage power amplifier circuit uses a triode to amplify the signal. In this design, we chose a power amplifier composed of discrete components as the power amplifier circuit. The structure of this circuit is similar to a power amplifier in an integrated circuit, but because it uses discrete components, it is easier to understand and master the structure and characteristics of the circuit. At the same time, the disadvantage of this kind of circuit is that it is complex, difficult to understand and use, and it is easy to damage the device, so it needs to be operated with more caution. During the design and use process, we will fully consider these factors to ensure the stability and reliability of the circuit.
T1 chooses 3DG6 transistor, its amplification factor is 10~30, so it must ensure that the gain of the previous stage is within the range of 30-90. The resistor value is set here so that the gain of the previous stage is 30.
Uo/Ui=30, Uo=9V, calculated Ui=0.3V.
Figure 4 Scheme 2 transistor as power amplifier
Among them, the parameters: R1=10K, R2=390K, R3=1K, R4=10K, R5=10K, R6=47K, RL= 8, C1=14.7uf, C2=100uf
third solution:
A two-stage operational amplifier is used to form a directional proportional device, and the amplification factor
| A=1+R8R7 |
⑹ |
Configure a gain of 30 times for each amplifier stage, and the gain of the final output signal of 30x30 is 900
A=30, then R8/R7=30-1=29, let R8=29k, R7=1k
Figure 5 Scheme 3 two-stage proportional amplifier
compare plan:
There is uncertainty in the amplification factor of the transistor T1 used in Scheme 2, which will directly affect the stability of the output power, which may lead to the inability to meet the requirements of the course design. In addition, the circuit diagram of option 2 is relatively complex, difficult to connect, and difficult to understand. This is because the second option uses discrete components. Compared with integrated circuits, a deeper understanding of the circuit structure and characteristics is required to enable better design and debugging. During the design and use of option 2, we need to operate more carefully to ensure the stability and reliability of the circuit. At the same time, we will also fully consider the complexity and difficulty of the circuit. While understanding and mastering the circuit structure, we will also look for more convenient and efficient connection methods to improve the efficiency and reliability of the circuit.
The circuit of Scheme 1 is more intuitive and concise, and the amplification factor of the NJM386 operational amplifier is also better controlled.
The difference between scheme three and scheme one lies in the difference between power amplifier amplification and voltage amplification.
Here is an analysis and comparison of these two amplification methods:
1. From the perspective of energy conversion, there is no essential difference between power amplifier circuit and voltage amplifier circuit, but the focus of the research problem is different.
2. Voltage amplifier circuits are generally used to amplify small signals. The voltage gain, frequency characteristics, input resistance, output resistance, dynamic range and other indicators of the amplifier circuit are mainly discussed.
3. The power amplifier circuit is mainly used to provide sufficient power to the load. The main features of the power amplifier circuit are:
(1) Have as much output power as possible
(2) The efficiency must be high (the power amplifier circuit has a high working voltage and a large working current. If the working current is large, the circuit loss will be large, so the efficiency must be high)
(3) Nonlinear distortion should be small
(4) Install heat dissipation and protection devices
The NJM386 selected in option 1 has the following advantages:
According to my current knowledge, NJM386 has the following advantages
: wide power supply voltage range: 4v - 12v or 5v - 18v
; large voltage gain amplification: from 20 to 200
; low distortion, as low as 0.2%
•NJM386 has a wide frequency range
Therefore, it is more appropriate to choose option one. A detailed analysis of the circuit of option one is given below.
Figure 6 NJM386 pin diagram
Pin1 (Gain): Gain pin used to adjust the amplifier gain by connecting this IC to an external component capacitor.
Pin 2 (Input -): Non-inverting input terminal, used to provide audio signals.
Pin 3 (Input +): Inverting input terminal, used to provide audio signals.
Pin 4 (GND): Ground pin, connected to the system ground
Pin 5 (Vout): The output pin used to provide amplified output audio, connected to the speaker.
Pin 6 (Vs): Connect to the power supply and accept positive DC voltage.
Pin 7 (Bypass): Bypass pin for connecting decoupling capacitor.
Pin 8 (Gain): Gain setting pin, used to control the gain of the amplifier.
Pin 1 and Pin 8 represent the gain control terminals of the amplifier. These are the terminals where we can adjust the gain by placing a resistor and capacitor or just a capacitor between these terminals.
Pin 2 and Pin 3 represent the audio input signal terminals. These are the terminals where we place the sound we want to amplify, pin 2 is the negative input and pin 3 is the positive input.
Pin 4 is the GND (ground) terminal and is connected to ground in the circuit.
Pin 5 represents the output of the amplifier. The amplified signal is output from this terminal.
Pin 6 receives the positive DC voltage so that the amplifier can receive the power required to amplify the signal.
Pin 7 represents the bypass terminal. This terminal can bypass the 15KΩ resistor. In circuit design, it is usually left open circuit or grounded.
However, for better stability, a capacitor can be added to the circuit to prevent oscillations in the op amp IC.
Figure 7 Complete circuit of solution 1
3 ) Simulation results and analysis
Since this design is essentially the use of amplifiers, AC scanning analysis is used in PSpise[3]
Figure 8 Configuration of simulation parameters
Place voltage probes at the input and output ends, run the simulation, and find that some errors are reported, which are all problems with some schematic links.
Figure 9 Error during simulation run
We modify the connection according to the pin number corresponding to the corresponding component.
Figure 10 Pin numbers of each component in the schematic diagram
When running the simulation, an error was reported and it was found that the NJM386 could not be simulated, so that the capacitive components connected to it reported an error and were not connected. Replace the NJM386 with an LF353 op amp. In order to ensure that the second stage gain is 200. According to the formula
| A= 1+RARB |
⑺ |
RA RB =200-1, that is, the feedback resistance is 199:1. Set R8=199KΩ, R7=1KΩ.
Figure 11 Modified scheme 1 circuit'
Place voltage probes at the input and primary output, label Vin and Vout1, and run the simulation
Figure 12 Input and primary output
 |
A= U 0 Ui =44.998：10=4.4998≈4.5, which is consistent with the 4.5 times gain set by the theoretical value
Place voltage probes at the primary output terminal and the secondary output terminal, mark the text labeled Vout1, Vout2, and run the simulation.
Figure 13 Primary output and secondary output voltage
Vout2/Vout1=8.8916V/44.98mV≈197.67. It is found that there is some gap between the gain of 200 times and the feedback resistor is modified.
Figure 14 The voltage of the two-stage output after modifying the feedback resistor
9.021V/44.998mV≈200.47, which is relatively close to the designed theoretical expected value of 200v
Next, place voltage probes at the input and output ends respectively, mark the text labels Vin and Vout2, and run the simulation.
Figure 15 Input and secondary output voltage simulation results
9.021V/10mV≈900, the gain is in line with the theoretical calculation value. However, it was found that the frequency response characteristics did not meet the design requirements. The voltage gain was unstable and began to decrease after 10KHZ. After analyzing and searching for information, it was found that the load of this circuit is only 8Ω. For this load resistance, a capacitor can be added to the output level circuit. This capacitor value should be sized to match the load impedance to avoid bounce and distortion. The larger the capacitor, the better the low frequency response of the amplifier, which means the slower the response of the circuit. Therefore, an appropriate capacitor value should be chosen to achieve optimal performance.
When adding an output capacitor, it should be placed at the load end of the output circuit and an appropriate capacitance value should be selected. It should be noted that when adding a capacitor, you should ensure that the rated voltage of the capacitor is not less than the peak output voltage of the circuit to avoid overvoltage damage to the capacitor.
Figure 16 Operating frequency
When the frequency is 18KHZ, the output is 8.3955V. Compared with the previous 8.969V, the decrease is only 0.573V, which meets the technical specifications for an operating frequency of 20-18KHZ. It adopts four levels of operational amplification and has the following advantages:
Better stability: Multi-stage amplifiers can divide the entire circuit into multiple amplification stages, and each stage can be optimized and adjusted, thereby improving the stability and reliability of the entire circuit.
Less Noise: Multi-stage amplifiers can spread noise over multiple amplification stages, thereby reducing the noise level of the entire circuit.
More scalable: Multi-stage amplifiers can achieve higher amplification factors by adding amplification stages, thereby improving the performance and scalability of the entire circuit.
But at the same time, due to the large number of operational amplifier stages, the following shortcomings also occur:
Higher cost: Multi-stage amplifiers require the use of more amplifiers and other electronic components, thus increasing the cost of the circuit.
Higher complexity: Multi-stage amplifiers require more circuit design and tuning work, increasing the complexity and difficulty of the circuit.
Slower response: Multi-stage amplifiers need to pass the signal to multiple amplification stages, thereby increasing the signal transmission delay and reducing the response speed.
4 Summary and insights
The design work of this audio amplifier is a process of in-depth study and practice. From circuit design to component selection, from simulation to debugging and modification, I used various methods and angles, consulted many materials, constantly explored and optimized solutions, and finally achieved the expected results. In this process, I not only improved my technical skills, but also enhanced my understanding and understanding of circuit design and components, laying a solid foundation for future study and work.
First of all, in terms of circuit design, we have mastered some basic design methods and techniques, such as the design of four-stage operational amplifiers and the application of RC noise reduction circuits. Through in-depth understanding of circuit theory and exploration of simulations, we have optimized the circuit structure and component parameters to ensure the circuit's high efficiency, stability and high-quality performance.
Secondly, in terms of component selection, we have an in-depth understanding of the usage and characteristics of electronic components, and selected suitable components to meet the needs of the circuit. At the same time, I also realize that the quality and performance of components have a great impact on circuit performance. Therefore, during the component selection process, we need to conduct strict screening and evaluation to ensure the use of components with reliable quality. And in the circuit connection, different operational amplifiers have different power supply parameter requirements. If these requirements are not met, the operational amplifier may not work in an ideal state.
Not only that, we used a variety of testing methods during the simulation, debugging and modification processes to conduct comprehensive testing and analysis of circuit performance. Through continuous debugging and optimization, we achieved the expected results and provided reliable technical support and experimental data, providing a solid basis for future study and work.
I found that any theory must be tested in practice. Although the design of my plan 1 has no problems in theory, some problems that I did not consider arose in the actual simulation. I modified plan 1 to address these problems. Finally, the design of a four-stage gain audio amplifier was completed.
There is still a lot of work I can continue to do though, and a lot of room for improvement. Circuit structure optimization: My design uses a four-stage operational amplifier and RC noise reduction circuit, which is a common audio amplifier circuit structure. However, for different application scenarios and requirements, different circuit structures may be required. Therefore, I can further research and understand other circuit structures, such as dual op amp structure, bridge structure [4], etc., to better meet different application needs.
Optimization of component selection: The components used in my design are of good quality and performance and can meet the technical specifications of the design. However, in actual applications, problems such as quality differences and failures of components may occur, affecting the performance and stability of the circuit. Therefore, I can further research and understand the quality and performance of components, select more reliable components to improve the stability and reliability of the circuit, and design a high-performance audio amplifier [5].
Noise and distortion optimization: In my design, the noise and distortion indicators have reached a certain level. However, for some demanding application scenarios, lower noise and distortion indicators may be needed. Therefore, I can further study and optimize the circuit structure and component parameters to reduce noise and distortion.
Power and efficiency optimization: In my design, the power and efficiency indicators have reached a certain level. However, for some high-power and high-efficiency application scenarios, higher power and efficiency indicators may be required. Therefore, I can further study and optimize the circuit structure and component parameters to improve power and efficiency.
Practical application testing: In my design, I used simulation and experimental testing methods to evaluate and analyze the circuit performance. However, for actual application scenarios, it may be affected by factors such as environment and external interference. Therefore, I can further conduct actual application testing, make PCBs, and solder components for physical testing to verify the performance and stability of the circuit.
To sum up, the design work of this audio amplifier is a successful practice and exploration. Through study and practice, we have improved our technical capabilities, enhanced our knowledge and understanding of circuit design and components, and laid a solid foundation for future study and work. In addition, we also realize that circuit design requires continuous debugging and optimization to achieve the best performance and effects. Therefore, in future study and work, we need to always maintain the spirit of innovation and the courage to try, constantly explore, optimize, and improve circuit design to provide a solid guarantee for achieving better performance and effects.
[1] Fundamentals of Analog Electronic Technology [Fourth Edition], Tong Shi Bai Huachengying, Beijing: Higher Education Press
[2] Huang Ju. Production and debugging of integrated power amplifier circuit based on LM386 [J]. Wireless Internet Technology, 2013, No.35(07):108.
[3] Analog circuit design precautions based on OrCAD Capture and PSpise [Second Edition], Dennis Fitzpatrick Beijing: Machinery Industry Press
[4] Zhang Shilei. Design and implementation of audio power amplifier[D]. Hunan University, 2018.
[5] Hu Yanmuzi. Design and implementation of high-performance audio power amplifier circuit [D]. Xi'an University of Electronic Science and Technology, 2007.