Research on the design of high efficiency audio power amplifier with Class D amplification

Traditional audio power amplifiers mainly include Class A, Class B, and AB. Class A power amplifiers have current flowing continuously through the power amplifier device during the entire input signal cycle. Its advantage is that the distortion of the output signal is relatively small, and its disadvantage is that the dynamic range of the output signal is small and the efficiency is low. Ideally, its maximum efficiency is 50%. Class B power amplifiers have a power device conduction time of 50% during the entire input signal cycle. Its advantage is that the efficiency can reach 78.5% under ideal conditions, but its disadvantage is that crossover distortion will be generated and noise will increase. Class AB (Class A and Class B) power amplifiers are a combination of the above two amplifiers. The conduction time of each power device is between 50% and 100%. It has the characteristics of low distortion of Class A and high efficiency of Class B, and its working efficiency is between the two. The contradiction between the low efficiency and large size of traditional audio power amplifiers and the development trend of high efficiency, energy saving and miniaturization of audio power amplifiers has given rise to the emergence and development of Class D (Class D) audio power amplifiers. This system is implemented using Class D power amplification and is powered by a single power supply, which meets the actual needs of modern society for small and portable power supplies.

1 System solution demonstration and selection

1.1 Overall solution

Solution ①: Digital solution. After the input signal is conditioned by the pre-amplifier, it is collected by the A/D into the single-chip microcomputer for processing. The generation of the triangular wave and the comparison with the audio signal are completed by the software part. Then the single-chip microcomputer outputs two completely opposite PWM waves to the post-stage power amplifier part for amplification. This solution has a simple hardware circuit, but it will introduce large digital noise.

Solution ②: Hardware circuit solution. The generation and comparison of triangular waves and PWM generation are still realized by hardware circuits. This solution has low noise and can achieve a larger amplitude, so the effect is better. Therefore, this solution is adopted.

1.2 Triangular wave generation circuit design

Solution ①: Use NE555 to generate triangular waves. The characteristics of this circuit are that a constant current source is used to linearly charge and discharge the capacitor to generate a triangular wave. The waveform linearity is good, the frequency control is simple, and the signal amplitude can be controlled by adding an attenuation potentiometer.

Solution ②: Integrate the square wave to generate a triangular wave. The integrator and the comparator are cascaded, and the triangular wave is obtained by integrating the square wave generated by the comparator. The frequency and amplitude control only need to adjust some resistor values, and the control is simple. However, considering the integral drift of the integral circuit.

Here, the selection scheme ① is adopted.

1.3 Design of PWM wave generation scheme

Scheme ①: Direct comparison. Take a triangular wave signal with the same bias as the input audio signal and a slightly larger amplitude and directly compare it with the audio signal to generate a PWM wave, and then generate a PWM wave signal completely opposite to it through the inverter to the post-amplifier circuit.

Scheme ②: Dual-channel comparison. Use two triangular wave signals with different biases to compare with the upper and lower halves of the audio signal respectively. This scheme can reduce the number of opening and closing of the CMOS tube in the suffix H-bridge circuit, reduce power loss, and improve efficiency.

Scheme ③: Directly reverse the audio signal. After amplifying and conditioning the audio input signal, directly reverse it, and then compare the processed signals with triangular waves respectively, so as to generate two reverse PWM waves.

Because scheme ② has a higher efficiency and has a certain effect on suppressing common-mode noise, scheme ② is selected.

1. 4 Short-circuit protection scheme design

Scheme ①: Current transformer method. Use a current transformer to sense the current passing through the load resistor, and then process this current to determine whether the circuit is over-current.

Scheme ②: Sampling resistor method. Connect a small value resistor in series to the circuit to sample the current flowing through the load of the system to determine whether the circuit is over-current. This scheme is simple to implement, and the connection of a small value resistor has little effect on the system, so this scheme is adopted.

2 System overall design scheme and implementation block diagram

As shown in Figure 1, the overall implementation block diagram of the system is composed of four main modules: high-efficiency power amplifier, signal conversion circuit, over-current protection and power measurement. The core high-efficiency power amplifier is composed of five parts: preamplifier, triangle wave generation circuit, comparator circuit, drive circuit, and H-bridge complementary symmetrical amplifier. After the input audio signal is amplified and conditioned by the preamplifier circuit, it is divided into upper and lower parts and compared with two triangular wave signals to obtain two corresponding PWM waves; that is, the audio signal is pulse-width modulated, and then the driving circuit increases its signal driving ability, and then it is fed into the H-bridge module, and the duty cycle is changed to control the conduction and cutoff of the power switch tube to achieve power amplification, and then the output on the load is low-pass filtered to filter out the original audio signal. The signal is fed into the signal change circuit on the load, and the double-ended signal is converted into a single-ended signal. After passing through an RC filter with a cutoff frequency of 20 kHz, it is connected to the test instrument for testing. At the same time, the true effective value of the single-ended signal is detected here, and after AD sampling, it is sent to the single-chip microcomputer for power calculation and display. The system also has an overcurrent protection function. The 0.1Ω sampling resistor is connected in series with the load to sample the current value flowing through the load. After amplification and comparison, the power supply of the power amplifier part is controlled by a relay, thereby achieving protection. The maximum undistorted output power of the system is greater than or equal to 1 W, and the voltage amplification factor can be continuously adjusted from 1 to 20. Due to the use of Class D amplification, it can achieve higher efficiency, very low output noise, and very small power display error.


3 Main Function Circuit Design

3.1 Preamplifier Module The

preamplifier circuit uses the high-efficiency, rail-to-rail, low-noise op amp chip OPA350 to form a common-phase broadband amplifier circuit. The signal input end is connected in series with a capacitor to achieve DC isolation coupling. At the same time, due to the single power supply, a 2.5V bias is given to the same-direction end of the op amp. The feedback resistor is set as a potentiometer, which can dynamically change the gain of the amplifier from 1 to 20 times and the gain is continuously adjustable.

3.2 Triangle wave generation circuit

The triangle wave generation circuit is shown in Figure 2. The NE555 chip is used to form a triangle wave circuit, and the capacitor C1 is linearly charged and discharged through a constant current source to obtain a triangle wave. When it starts working, the pin 3 of the 555 chip is at a high level, the diode D4 is turned on, and D3 is turned off, so D1 is turned on, and D2 is turned off. The constant current source composed of T1, T2, and R1 charges C1 linearly through D1. When the voltage across C1 reaches 2/3Vcc, the output level of pin 3 is reversed and becomes low. At this time, the conduction states of D1, D2, D3, and D4 are also completely opposite. The constant current source composed of T3, T4, and R2 below discharges C1 linearly through D2. When the discharge makes the voltage across C1 reach 1/3Vcc, pin 3 is reversed to a high level again. This cycle repeats to achieve the generation of a periodic triangular wave signal. The output is drawn from both ends of C1 to obtain a triangular wave signal with good linearity. A first-level in-phase follower is connected to achieve the purpose of isolating the front and rear stages. C1 uses a polystyrene capacitor with low leakage current and fast response speed to ensure good performance.

The frequency and amplitude of the triangle wave are calculated as follows: Let the charge and discharge current through the resistors R1 and R2 be Io, where Io=Vbe/R (where Vbe is the conduction voltage of the transistor), then

the triangle wave period T=t1+t2, the frequency is f=1/T, and the measured triangle wave frequency of this circuit is 120 kHz (which may deviate from the calculated value because the conduction voltage drop of the transistor is not strictly 0.7 V).

3.3 Dual comparator circuit (PWM wave generation circuit)

The dual comparator circuit is composed of a low-power dual comparator chip LM393 that can work with a single power supply. In order to improve the system efficiency and reduce the unnecessary opening and closing of the CMOS tube in the subsequent H bridge, two triangular waves with different biases are used to compare with the upper and lower halves of the audio signal respectively, and two corresponding PWM wave signals are generated for the subsequent drive circuit to process. The dual comparison waveform is shown in Figure 3. It is worth noting here that the upper half comparison is processed as the audio signal connected to the negative end of the comparator and the triangular wave signal connected to the positive end; the lower half comparison is the opposite, so that a mutual correspondence is formed. When the corresponding PWM wave is formed in one half of the audio signal, the other half is low level, which can ensure that the CMOS tube in the subsequent H bridge does not open and close unnecessarily, so as to reduce the power loss of the system. The upper half comparison triangular wave bias is adjusted to 3 V by using a potentiometer, and the lower half comparison triangular wave bias is adjusted to 2 V. It should also be noted that the triangle wave signal should be slightly larger than the audio signal amplitude within the comparison range, and the bias adjustment should be more accurate to prevent certain points of the audio signal from being compared, resulting in distortion when the original signal is restored by subsequent filtering.


3. 4 H-bridge complementary symmetrical output circuit (fourth-order Butterworth filter added later) The

H-bridge complementary symmetrical circuit is shown in Figure 4. The field effect tube IRF9540 and IRF540 with low on-resistance, fast switching rate and little temperature influence are used to form a complementary push-pull amplifier circuit. Using the symmetrical output mode and making full use of the power supply voltage, the peak-to-peak value of the floating output carrier can reach 10 V, which effectively improves the output power.

The two signals amplified by the H-bridge complementary symmetrical circuit are respectively low-pass filtered by a fourth-order Butterworth filter to filter out the high-frequency carrier, and the amplified audio signal is added to both ends of the 8 Ω load. The upper cutoff frequency of the filter is about 20 kHz, and the characteristics are flat within the passband, which has a good effect. Note that a high-power inductor should be selected here, otherwise it will reduce the signal amplitude and cannot achieve a higher power.

3. 5 Short-circuit protection module

The short-circuit protection circuit is shown in Figure 5. A 0.1Ω small resistor is connected to the system and connected in series with an 8Ω load resistor. The sampled voltage at both ends of the sampling resistor is amplified and then compared with the set reference voltage to control the power supply and power failure of the functional part, which plays a protective role. The amplification part uses the chip NE5532 to form a subtraction amplifier. While amplifying, the two-end signal at both ends of the resistor can be converted into a single-end signal. The amplifier gain is:

The amplified signal passes through the peak detection part composed of D1, C1, and R5, and the detected signal amplitude value is sent to the comparator for comparison with the set reference voltage. The comparator uses the low-power, fast-response dual-channel comparator chip LM393. The negative end of the comparator is set to 5.1V using the voltage regulator D6 and C3 and R7. The comparator is connected in a hysteresis comparison mode. Once overcurrent occurs, it can be self-locked. At this time, the high level output by the comparator turns on the transistor T1, the ground control end of the relay is connected to the ground, the relay is energized, and the power supply of the power amplifier part is cut off to achieve the protection purpose. Because the comparator is self-locking, after solving the overcurrent problem, the power supply of the protection module is turned off to re-enter the protection state. D2, D3, R6, and C2 form a power-on delay circuit. After power failure, C2 quickly discharges through D2 to prevent the residual voltage on C2 at the beginning from affecting pin 3, and prevent the comparator from entering the self-locking state under abnormal conditions, so that the protection module cannot function normally.

3.6 Power measurement and display circuit (RMS detection and AD conversion circuit)

The power measurement circuit uses the true RMS detection chip AD637 to detect the true RMS value of the signal, and then samples it through the 12-bit serial interface and 20kHz sampling rate AD chip ADS1286 and invites it to the FPGA for processing by the program, calculates the power and displays it, as shown in Figure 6.

The input is equipped with OPA604 to form an emitter follower to isolate the front and back stages. The average time constant can be set by changing the value of the average capacitor, and the low-frequency accuracy, the size of the output ripple and the stabilization time can be determined. The AC ripple component can be reduced by increasing the value of this capacitor, but this will increase the settling time, so a second-order active low-pass filter is used to reduce the output ripple. After the true effective value is obtained, it is directly fed into the ADS1286 for analog-to-digital conversion, and then processed by the FPGA to calculate the output power of the system and display it.

4 System software design

According to the requirements of the topic, in order to realize the measurement and display function of the system power, the hardware uses an 8-bit CPU AT89S52 and is implemented through C51 programming. The single-chip microcomputer must complete the functions of controlling the ADS1286, collecting data, calculating power and sending it for display. The FPGA (using the EP1C6QC240 of the Cyclone series of Atera) is used as a bus controller to manage the data exchange between the LCD and the A/D and the single-chip microcomputer. Verilog HDL language is used to program the implementation in Quartus9.1 environment.

5 Test methods and results

5.1 Test instruments

15 MHz function signal generator model: Agilent33120A

Digital oscilloscope model: Tektronix TDS 1002, dual channel, 60 MHz

DC power supply model: SG173SB3, voltage and current stabilization type

Four and a half digit digital multimeter model: Fluke 45 dual display multimeter

5.2 Test plan and results analysis

1) Power display error measurement Agilent signal source is used to provide input audio signal, and the oscilloscope measures the peak voltage Vo on the load at the single-ended output test point. The actual power is calculated according to the formula, and then the display error is calculated. The results are shown in Table 1.

From the data in the table, it can be seen that the system power display module has a 4-digit display, with an accuracy better than 5% and a small error.

2) Noise Use the Agilent signal source to give the input audio signal (to ensure that the signal frequency is below 20 kHz), use a 0.1μF capacitor to short-circuit the input end to the ground, and use an oscilloscope to measure the noise level at the output end. At this time, the measured noise is 2.96mV.

3) Efficiency measurement Use a DC power supply to power the power amplifier circuit separately to test the efficiency. The power supply voltage is +5 V. Use the same method as the passband measurement to give the input signal, use an oscilloscope to observe the output signal amplitude, adjust the output to 200 mW and 500 mW, connect the four-and-a-half-digit digital multimeter in series to the amplifier circuit, and measure the circuit current I. Calculate the power amplifier efficiency according to the formula, and the results are shown in Table 2.

As can be seen from the table, when the output power is 500mW, the efficiency of the power amplifier circuit is as high as 64.10%, which greatly meets the requirements of the topic; when the output is 200mW, the efficiency also reaches 43.96%. The system can achieve high-efficiency audio amplification.

4) Overcurrent protection measurement Use the same method as the passband measurement to give the input signal, use an oscilloscope to observe the output signal amplitude, short-circuit the load ends, and you can see that the short-circuit module warning light is on, the power supply of the power amplifier part is cut off, and the output becomes zero, achieving the purpose of protection.

6 Conclusion

The system realizes the amplification of audio signals and completes the functions of high-efficiency power amplification, signal conversion, power measurement and display, and overcurrent protection. The system has good performance and high indicators in power and efficiency. The amplifier circuit, signal conversion, power measurement and short-circuit protection have achieved good results. Especially in terms of power, it can reach 1.16W, the efficiency can reach 64%, the noise is very low, and the power measurement display error is small. The operation is simple and the human-computer interaction is flexible.