Comparison of Two Implementation Methods of Power Amplifier
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Comparison of Two Implementation Methods of Power Amplifier Abstract: Power amplifier circuits are usually composed of integrated power amplifier devices or discrete component amplifier circuits, both of which have their own advantages and disadvantages. This paper analyzes and compares how to correctly use the two different circuit principles, debugging, performance, and results.Keywords: complementary symmetric OCL circuit; output transformerless power amplifier BTL; Class A and B; discrete components The power amplifier circuit is an energy conversion circuit that requires to efficiently provide the load with as much power as possible within the permissible range of distortion. The working current and voltage of the power amplifier tube vary greatly, so the triode often works in a large signal state or close to the limit application state, and there are various working modes such as Class A, Class B, and Class AB. In order to improve efficiency, the amplifier circuit is made into a push-pull circuit, and the working state of the power amplifier tube is set to Class AB to reduce crossover distortion. Common audio power amplifier circuits are mainly connected in the form of dual-power complementary push-pull power amplifier OCL (without output capacitor), single-power complementary push-pull power amplifier OTL (without output transformer), balanced (bridge) transformerless power amplifier BTL, etc. Because the power amplifier tube bears large current and high voltage, the protection and heat dissipation of the power amplifier tube must be taken seriously. The power amplifier can be composed of discrete components or implemented by integrated circuits. ? 1 Discrete components make up the power amplifier Figure 1 is a DC-based complementary symmetrical OCL circuit composed of discrete components. The circuit consists of a differential amplifier stage, a voltage driver stage, and a composite output stage. This circuit introduces a DC negative feedback circuit. In general, the low-frequency response is limited by the presence of feedback capacitors in power amplifiers. In order to eliminate this adverse effect, the feedback capacitor can only be increased, but a large capacitor will make the circuit unstable. This circuit cancels the feedback capacitor and completely solves this contradiction. At the same time, the negative feedback of this stage is introduced through the emitter resistor, which significantly improves the performance of this stage and simplifies the circuit. The output stage works in Class A and Class B state, which not only takes into account the efficiency, but also ensures the linear working state of the components. The differential tube gain is equal to 200, and the difference between the two tubes must be very small. The voltage driver tube gain is equal to 80.
Under this condition, a high-performance regulated power supply is added to provide energy and bias, and finally the entire circuit is debugged. It can be measured that when the pre-input is 20 mV, the output power is greater than 12 W. This circuit should pay attention to the reduction of nonlinear distortion and noise, and the final debugging is more complicated. But the circuit only has a basic amplification circuit, so there is a lot of room for function expansion. 2 Integrated power amplifier circuit There are many integrated amplifier chips with excellent performance on the market, such as dual op amp NE5532, TDA2040, LM1875, TDA1514, etc. Among them, TDA2040 has a small power margin and TDA1514 has a complex peripheral circuit, so LM1875 is used in the integrated amplifier design.
Figure 2 shows an integrated power amplifier composed of LM1875, with an open-loop gain of 26 dB, that is, the amplification factor A=20. Among them, 20 kΩ and 1 kΩ resistors form a negative feedback network, two diodes are protection diodes, the output resistor and capacitor form an anti-high-frequency self-excitation circuit, and the capacitors at both ends of the positive and negative power supplies are power supply decoupling capacitors. If the power on the output resistor load is greater than 10 W, plus the voltage drop of 2 V on the power tube, it can be calculated that the output efficiency is 66?2%, the maximum undistorted voltage peak-to-peak value is 25?3 V, and the input signal voltage peak-to-peak value is 2?53 V. The circuit still needs to improve the preamplifier circuit, waveform conversion circuit, voltage regulator and protection circuit. The preamplifier circuit can use the 2-level NE5532 typical application circuit; the waveform conversion is completed by the high-precision operational amplifier OP07; the voltage regulator is composed of LM317 and LM337; the protection circuit is composed of a relay plus a triode circuit. This circuit is mainly based on analog amplification, and the circuit boards should be connected with double-core shielded wires to prevent self-excitation interference. The power amplifier parameters of this circuit are reasonable and relatively simple.  
There are many other forms of power amplifier circuits, such as multi-stage direct-coupled OCL amplifier circuits. Regardless of the type of power amplifier circuit, attention should be paid to the design of the pre-voltage amplifier stage and the power supply circuit. Finally, attention should also be paid to the operating point drift caused by the operating temperature rise, and appropriate compensation should be made to reduce the nonlinear distortion of the circuit and improve the stability.
3 Conclusion A power amplifier composed of discrete components can perform well if the circuit is well selected, the parameters are appropriately selected, the components are of good performance, and the design and debugging are good. Many high-quality power amplifiers are discrete power amplifiers. However, as long as there is a problem in one of the links, the performance will be lower than that of general integrated amplifiers. In addition, in order to prevent overload, overcurrent, overheating and other damage to the components, complex protection circuits are required. The core circuit composed of transistors, diodes, resistors, capacitors and other devices in the discrete component amplifier provides room for free adjustment. The integrated amplifier circuit is mature, the low-frequency performance is good, and the internal design has a composite protection circuit, which can increase its working reliability. In particular, the integrated thick film device parameters are stable and do not need to be adjusted. The signal-to-noise ratio is small, and the circuit layout is reasonable, the peripheral circuit is simple, the protection function is complete, and an external heat sink can be added to solve the heat dissipation problem. References ［1］Qian Cong. Analysis and Design of Electronic Circuits［M］. Xi’an: Shaanxi People’s Publishing House, 2002
［2］Jiang Xiaoan. Analog Electronic Technology［M］. Xi’an: Xi’an University of Electronic Science and Technology Press, 2002
［3］Su Liping. Fundamentals of Electronic Technology［M］. Xi’an: Xi’an University of Electronic Science and Technology Press, 2002
［4］Li Yaxuan. Analog Electronic Technology［M］. Xi’an: Xi’an University of Electronic Science and Technology Press, 2000
［5］Tan Wenxin. Principles and Applications of Analog Integrated Circuits［M］. Xi’an: Xi’an Jiaotong University Press, 1995
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