"Designing Power Amplifier Tutorial Dir, Power Amplifier Diy Process
Keywords: power amplifier, amplifier design, amplifier circuit, amplifier production
I. Power amplifier basic circuit characteristics Complementary symmetrical OTL power amplifier basic circuit is shown in Figure 1. Where: C1 is a signal input coupling element, it is necessary to pay attention to the potential condition of polarity in the actual circuit. R1 and R2 constitute a bias circuit of BG1, providing a static operating point to BG1 while also serving as a DC negative feedback effect throughout the circuit. The base current is required to be greater than the base current of the BG1 at least 5 times, according to β is 100, IC1 is 2 mA, and R1 should be no more than 6K, so it is given to 5.1k; C1 therefore also gives 22 μ, it is 20 Hz The restricted impedance is 362Ω; R2 needs to be determined according to the specific voltage employed, about 120K according to the 32V voltage value, and the exact value is used to make the BG1 collector voltage 15.4V is obtained. The C2 and R3 constitute a bootstrap circuit, requiring R3 × C2> 1/10, (R3 + R4) × IC1 = E / 2-1.2, because R4 is the AC load resistance of BG1, should be taken as much as possible, R3 is generally taken in 1K within. According to the 32V supply voltage value and IC1 2 mA, R3 and R4 are 7.2k, and R3 is given to 820 Ω, R4 is 6.8K, and IC1 is 1.94 mA; C2 can therefore be taken to 220μ. R5 and D are bias circuit components of BG2, BG3 complementary tubes, together with BG2, BG3, provide a suitable static operating point, and minimally take a small value in the case of eliminating cross distortion, generally taken at 3mA to 4 mA according to experimental results; Changing the R5 resistance can cause the voltage drop of the BG2 and the BG3 to achieve the adjustment of its static operation, and D in series with R5 is to compensate for the change of BG2, BG3 emission finitectural voltage with temperature, preferably two The diode is connected in series to compensate for the change of the complementary tube and the temperature occurring, so that the static operating point of the complementary tube is stabilized. A diode is omitted in the simplified circuit. C4 in parallel between BG2, BG3 base, can reduce ΔuAb during dynamic operation, generally taken from 47 μ; C3 is to prevent BG1 from producing high-frequency self-exciting AC negative feedback capacitance, generally taken from 47p to 200p. The BG1 is actuated in the circuit, referred to in the circuit as an excitation stage, requiring buceo> E, ICEO ≤ic1 / 400 = 5 μA, β = 100 to 200, so the small power low noise triode should be selected. BG2 and BG3 are mutual replenishment current amplifiers, and the BG4, BG5 constitutes a composite tube to enlarge the output current, requiring buceo> E, ICEO ≤ pro2 / 100 = 30 μA, β = 100 to 200. In the case of BG4, BG5 uses a normal high-power three-stage tube, which is not internal, the BG2 and BG3 need to provide a peak drive current that exceeds 100 mA of the primary power tertiary tube, which should be Use a medium power three-level tube. BG4 and BG5 are responsible for amplifying large power tubes of the output current, and the static working current can be taken at 10 mA to 30 mA, requiring buceo> E, ICEO ≤ 15/100 = 0.1 mA, β = 50 to 100. The maximum ultimate current IMAX of BG4 and BG5 should be more than 1 time more than the output current, and the maximum amplitude of the output current is β> 10. R6 and R7 are BG4 and BG5 static work points to adjust shunt resistors, and the splitting effects during dynamic operation can be ignored. When UBE4 and UBE 5 are equal to 0.6V standard parameters, the static operating current amplified by the complementary current amplifier is taken at 3 mA to 4 mA, and R6 and R7 should be calculated to be 220Ω. In fact, the high-power tertiary tube UBE may differ, the UBE of BG4 and BG5 needs to be paired by measuring, and the total current magnification of the half-composite tube operated by the self-raised circuit should be more than the help of self-raising circuits. Another half of the composite tube is small. R8 and R9 are to prevent current-limit resistors from BG4 and BG overcurrent, generally between 0.2 Ω to 0.5 Ω. The two ends of the enameled wire of 3 mm are long and the diameter of φ0.08 will be welded at both ends of 1K or higher, and the folded painting wiring can be wound around the resistor. It is equivalent to the role of fucking the fuse, belonging to the simplest non-intelligent current limiting blow protection method. C5 and C6 are signal output capacitors, using a small capacitance capacitance and a large capacitance in parallel, eliminating a larger inductance of a larger inductance on a high frequency signal internally. Note that it actually plays a moderate floating power supply, so that the capacity is not calculated in accordance with the impedance of the AC signal at the lower end of the belt, but is calculated according to the output power required to consume. Severe clipping distortion occurs when the output signal is output as the output current is fluctuated. Depending on the energy stored in the capacitance and the voltage square, the output capacitance of the midpoint floating power supply should be 4 times that of the total storage capacitance of the total power supply. C9 and R10 are AC negative feedback networks, together with R2, R1, constitute a voltage parallel and negative. The direct current consisting of R2 and R1 allows the total voltage amplification ratio equal to R2 divided by 1.2k (equal to the transmitting node dynamic resistance of R1 and BG1), as shown in Figure 1 design parameter is about 100 times, add C9 and R10 After the AC negative feedback network, the total voltage amplification ratio is about equal to R2 and R10, the parallel resistance is 1.2K, about 18 times. Practice has proved that the voltage to operate in this way is not good.
Second, improvements to the basic circuit of the power amplifier in Figure 1, the complementary symmetric OTL power amplifier of Figure 1, the internal resistance of the signal input excitation is only 1K, and the impedance transformation is required to be in most parts, high-resistance signal sources. match. The signal input excitation is directly changed to the composite tube is the simplest manner. There are a variety of concurrent circuits of the composite tube, and the preferred solution is the next method shown in Fig. 2. The newly added front level is actually equivalent to a simple voltage control current type operational amplifier, and the base and emitter of the BG0 correspond to the positive input and negative input end of the operational amplifier. The dynamic resistance of the positive input terminal has increased to 10k or more. At the same time, the ratio of the power amplifier output is received to the negative input emitter negative feedback resistor R10 and the sampling resistor R11 determine the total voltage amplification magnification.
The circuit debugging point is also the first to adjust the R5 to a short circuit 0 resistance to cause the BG2 to BG5 at an off state, and two 1k / 2W resistance receive the midpoint voltage from the total power supply at both ends of the total power supply. Use a 200K potentiometer instead of R1 or R2 on the board, and the C1 inputs the C1 input and the ground path with the wire. Turn on the power, measure the voltage drop of the BG1 to the emitter of the emitter, adjust the 200K potentiometer to make the UCE equal to E / 2-0.6; when the total supply voltage is 32V, the static UCE of BG1 should be equal to 15.4 V ± 0.1V. Then measuring the resistance value of the 200K potentiometer actually, replaced with the same resistance value fixed resistance replacement potentiometer, and then measured that the BG1 static UCE should be within 15.4V ± 0.2V. After determining the static UCE of BG1, the static operating current of BG4 and BG5 is 15 mA from a small to large regulation R5. For the sake of insurance, R8 and R9 can be converted into a 100 Ω / 2W resistor, and the quantitative voltage of R8 and R9 is measured to be 1.5V. Disconnect the power, measure the resistance value of the R5 adjustable resistor, replace the R5 to the fixed resistance of the same resistance, and remove the 1K / 2W voltage resistor from the output from the output terminals to the power supply. The power is turned on, and the static voltage of R8 and R9 is measured between 1.2V to 1.8V. The measuring output in the output should also be between 16V ± 0.5V. The input of the C1 input capacitor signal is suspended from the ground, and the voltage drop of R8 and R9 is measured, and the voltage drop in R8 and R9 when the C1 input end is used to remove the voltage drop. The R8 is then replaced with 0.3Ω resistor, and the speaker is listened. When the power is turned on due to C0, the point voltage in the output needs to rise from zero, so it only produces a slight impact. After 2 seconds, the speaker will issue a "" communication when you touch the C1 input. Short the C1 input and the ground (power supply negative), the speaker should not make a sound, actually make a slight background white noise or a small emotion. Figure 2 The complementary symmetrical OTL power amplifier improved circuit, there is a significant disadvantage that the signal input DC level is 2V ~ 3V in the middle of the point voltage than the output, and there is no op amp use and the price of the triode component in the public. In the early 1980s, it is already a very good power amplifier utility circuit that can use a single power source. In the mid-1980s, op amp IC began to launch, people began to use op amp to serve as pre-pole and excitation. The typical circuit is shown in Figure 3, which is not necessary to adjust the static operating point due to the op amp IC, as long as R5 is adjusted, the static working current of the BG2 to BG5 can be adjusted to 20mA. Note that although the op amp IC does not need to adjust the static operating point, when the BG2 to BG5 is at an off state, the R8, R9 and BG3, and the BG5 transmitted junction positive conduction will be placed at a high level, transport Put the IC output low, which is set to a low level of the op amp negative input by the BG3 emission end, and the op amp output is flipped to high, the result is in an unprofit low frequency oscillation state, and cannot provide stable Refer to the medium point level. Adjusting the static operating current of BG2 ~ BG5 under this condition, the op amp is adjusted to adjust the R5 when the IC output is high; when the op amp output is 0, the BG5 cannot be turned on, and the adjustment R5 can only make BG2, BG3 The BG4 enters the work area, and the BG2 actually only plays the role of the diode. The current passes through the current of BG4 and BG2, and the input of the IC output is performed, and the BG2 and op amp IC are damaged by overcurrent! (I used to damage all the domestic op amp ICs and more than ten medium power three-level tubes at that time, and I have not adjusted static work points.) Must first use the wire to put the IC's negative input Continuous with the output, no negative feedback resistor R6, allowing the op amp to output a stable reference midpoint level in follower, in this state, adjusting R5 to make the static operating current of BG2 to BG5 to 15mA, will R5 After replacing the fixed resistance of the same resistance, confirm that the static working current of the BG2 to BG5 is between 10 mA to 20 mA, and then the negative input end of the op amp Ic is open to the output terminal, and the feedback resistor R6 is connected to the circuit.
After using the op amp IC, it is preferable to modify the static operating current bias of BG2 to BG5 to a voltage regulator composed of a triode and a partial voltage resistance, which can change at a large change in the power supply voltage. Keep almost the same static working current. Fig. 4 is a modified circuit, and the BG1 emission finitectuation voltage is changed with the temperature changes with the temperature change, it can function as a temperature compensation. In order to reduce the static working current of the op amp output, the shunt resistor R10 to the ground is given to the opinion IC output. With this shunt resistor, adjust the static operating current of the BG2 to BG5, which can be adjusted to the op amp IC before, directly from the R7, R8, and R10 to the approximate midpoint reference level. First, from 0 to large regulation R5, the static operating current of BG2 ~ BG5 is between 10 mA to 20mA, and then access the op amp, the circuit can work. Further, one 1K current limiting resistor R15 is connected in series in the op amp output terminal, and the BG5 does not enter the shutoff state when the op amp output is at 0 level.
The maximum advantage is that the output-only DC level is strict and the signal input is strict, and the phase difference is not more than ± 0.05V. This makes it possible to produce a BTL power amplifier composed of two OTL power amplifiers, while the two OTL power amplifiers are positive and reverse output terminals of the output DC level and the signal input DC level. DC level tends to differ by more than 0.5 V, significantly affects the horn 's work balance position. The positive and inverting output DC level DC levels of the BTL power amplifier must be less than 0.1V. The working balance position of the speaker does not have a significant deviation of free balancing position. When the horn's work balance position is obviously deviated from the free balance position, the mechanical vibration amplitude of the forward and reverse direction is asymmetrical, and the sound waves emitted will result in distortion. In addition, the DC level of the output is strictly consistent with the direct flow level of the signal input, so that the OCL power amplifier powered by the positive and negative dual power supply is a reality. Otherwise, due to the difference between the DC level and the power source, the point of power is large, it will cause the speaker to work properly. Since most of the operating voltages of most op amp are not high, the high-voltage high-voltage transportation is less variety, high prices, and people can also use the same differential amplifier circuit as the prior grade of op amp. To achieve the same purpose. Fig. 5 is a typical circuit that uses a differential amplification method to do a precursor to the complementary symmetrical OTL power amplifier shown in Fig. 1, the basic circuit is multi-use, which is more than the characteristic, which is more than the modified complement of the drawing 2. Symmetric OTL power amplifier utility circuit uses one triode. Say that it is not that people don't know how to design power amplifiers, but are limited by the selection of devices, which can only use the corresponding design circuit during different historical periods. In the late 1980s, people began to match the triodes that were easily found in the characteristics for pairing. The static current of the differential amplifier can be accurately given by the circuit design parameters, without adjusting the static current of the differential magnifier. In the case of the 32V power supply using a 32V supply, the quiescent current of the front differential amplifier is 0.51 mA to 0.52 mA, and as long as R12 is adjusted to reduce the voltage of the BG1 to 15.4V, then adjust the R5 to make BG2 ~ The static working current of the BG5 can be between 10 mA to 20 mA.
When the quiescent current of the BG1 is adjusted, the R5 should be adjusted to a short circuit 0 resistance to cause the BG2 to BG5 in an off state, and the negative feedback resistor R10 is not accessed, and the base of the BG6 and BG0 is shorted by the wire. When the power is turned on, the R12 is adjusted to reduce the voltage of the BG1 collector to the ground to 15.4 V ± 0.2V, and the R5 is adjusted to cause the static operating current of the BG2 to BG5 to 15 mA. For the sake of insurance, the R8 and R9 are first converted into 100 Ω / 2W resistance, and the quantitative voltage of R8 and R9 is reduced to 1.5V. Disconnect the power, measure the resistance value of the R5 and the R12 adjustable resistor, and replace them to the fixed resistance of the same resistance. Turn on the power, measure the static voltage of R8 and R9 to remain between 1.2V ~ 1.8V. MeasurementThe mid-point level should be between 16V ± 0.3V. Disconnect the power, remove the base between the BG6 and BG0, and the negative feedback resistor R10 is connected to the circuit. The power is turned on, and the static voltage of R8 and R9 is measured between 1.2V to 1.8V. The measuring output midpoint level should be between 16V ± 0.2V, the larger the difference between the differential tube current amplifier, the smaller the difference between the output DC level and the signal input DC level. The voltage drop on R8 and R9 is significantly changed when using the sub-input terminal. The R8 is then replaced with 0.3Ω resistor, and the speaker is listened. The output of the output terminal needs to rise slowly from zero when turning on the power supply, and thus only the slight impact is generated. After 2 seconds, the input of the input terminal with the hand will issue "". Short the C1 input and the ground (power supply negative), the speaker should not make a sound, actually make a slight background white noise or a small emotion. Third, the power amplifier for the utility circuit of the power amplifier uses the power amplifier designed for the bootstrap circuit, although the circuit is relatively simple, but there is a lower limit operating frequency cutoff. Introducing the bootstrap circuit is to avoid having enough current to provide a complementary tube when the upper half wave is enlarged, and the constant current source can be used to ensure that the upper half waves are enlarged when the upper half wave is enlarged. The current is supplied to the complementary tube. At the same time, the differential amplifier is also designed to provide a working current from the constant current source, which can greatly improve the suppression ratio of the co-noise and the relaxation of the power supply voltage. Figure 6 is a typical circuit using a power amplifier using a constant current source, wherein: BG3 and BG4 constitute a standard constant current source, the former provides 1MA constant total current, 2 differential bits BG1, and BG2 each get 0.5mA Static working current; the latter provides 2 mA constant current, equal to 2mA of the static operating current of the excitation pole BG5, so that the static midpoint voltage of the amplifier output Q is completely determined by the same voltage divided by the R14 partial pressure, not Large deviations from E / 2. D1 in series in the lower R14 is to compensate for the lower complementary pipe dumps lower than the upper composite tube, and the midpoint voltage is determined by the same R13 and R14 partial pressure of the resistance value. more acurrate. The static working current of the excitation pole BG5 has been determined to be 2 mA via 1V voltage drop and R12 resistance 200Ω on R4, nor adjustment. Therefore, the BG4 and BG5 do not access the BG4 and BG5 before adjusting the static operating current of the BG7 ~ BG10, and directly in the R13 and the R14 are divided into the point reference voltage and provide a bias current of 0.4 mA to 1.1 mA to the BG6 operating conditions. The start regulation of the minimum zero resistance starts R10 to make the static operating current of the BG7 to BG10 are 15 mA. The R10 is then replaced with a fixed resistor, and the BG4 and BG5 are connected to the board, and the amplifier is immediately working. Although the actual static working current of the element parameter may make the actual static working current of the BG5 excitation and the BG4 constant current source current thereof, the differential amplifier is also automatically changed from the moderate status of the output terminal Q from the midpoint condition. The working current makes the actual static working current of the BG5 are completely equal to the BG4 constant current source current. Of course, after automatic adjustment of the actual static working current of BG5, the static working current of the differential amplifier is not allowed to be significantly reduced. As long as the component parameters in Figure 6, the static working current of BG5 can be changed to 1 mA as long as it changed 0.1 mA, sufficient to achieve static working current adjustment to BG5.
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