Class D audio amplifiers are highly efficient, energy efficient, and compact, and are widely used in portable applications, home AV equipment, and car audio. The Class D audio power amplifier designed in this paper is mainly based on the following three aspects: ensuring high fidelity, improving efficiency and reducing volume. This paper designs a Class D audio power amplifier that operates at 5V supply voltage and uses PWM. The whole system includes input amplifier stage, error amplifier, comparator, internal oscillator circuit, drive circuit, full bridge switch circuit and reference circuit. . By introducing feedback technology to reduce the THD index of the system, the dual-channel inverse wide modulation scheme not only suppresses the static power consumption of the class D audio power amplifier, but also achieves the purpose of removing the low-pass filter at the output of the class D audio power amplifier. Reduce the size of the system.
1 System design of Class D audio amplifierThe system structure of the Class D audio power amplifier designed in this paper is shown in Figure 1. The amplifier structure is implemented based on the bilateral natural sampling technology scheme. At any time, the output contains twice the amount of information of the single-sided sampling scheme. By bilateral natural sampling, a large amount of distortion components in the output audio signal can be removed to The low-pass filter at the output of the Class D audio power amplifier is removed beyond the range of audio bandwidth that can be sensed by the human ear.
Figure 1 Class D audio power amplifier structure
The system is powered by a single power supply. The high and low levels of the pulse signals “out1†and “out2†are VDD and GND respectively. The input amplifier stage is realized by the closed-loop structure of the operational amplifier OTA. The error amplifier is composed of the operational amplifier OTA and the capacitor Cs. When the system is working, the audio input signal Vin first outputs two differential signals after passing through the input amplification stage, and then sums the feedback signals to the error amplifier to generate error signals VE1 and VE2, modulates the triangular wave carrier signal VT, and outputs two pulses. The signals "out1" and "out2" drive the speaker to sound. The system consists of two feedback loops, the first consisting of R1, Rf1 and OTA to set the gain of the input amplifier stage and the entire Class D audio power amplifier, and the second consists of R2, Rf2 and the back-end audio signal processing circuitry. Used to reduce the THD index of the system.
In Figure 1, the current I1 and I2 for charging and discharging the capacitor Cs are determined by Vout1, Vout2, Vin, R1, Rf1, R2 and Rf2, where the resistance and capacitance must have good linearity and matching to obtain good Closed loop performance.
The model of an open-loop Class D audio power amplifier is shown in Figure 2.
Figure 2 Open-loop Class D audio power amplifier model
At this point the system output is:
The total harmonic distortion of the open loop system is: 
Vin in equation (2) is the input signal of the amplifier, Vn is the introduced harmonic distortion, and Hf is the transfer function.
The model of a Class D audio power amplifier with a feedback loop is shown in Figure 3.
Figure 3 Closed-loop Class D audio power amplifier model
At this point the output of the system is:
Where Hfb is the transfer function of the closed-loop model and G is the feedback gain. To get equal magnification, the design transfer function is:
Then equation (3) becomes:
The total harmonic distortion of the closed system is:
Comparing equations (2) and (6), it can be seen that the feedback loop closed-loop system THD is 1/(1+HfbG) of the open-loop system THD, that is, the THD of the system is reduced by the feedback structure.
2 unit circuit design implementationThe system unit circuit mainly includes: an input amplification stage, an error amplifier, a comparator, a drive circuit, a full bridge switch circuit, an internal oscillation circuit, and a reference circuit.
2.1 Input amplification stageThe input amplification stage of the Class D audio power amplifier is implemented based on the closed-loop structure of an operational amplifier (OTA). The structure is shown in Figure 4. It is used to level and signal the input audio signal as needed. The input signal can meet the requirements of the latter stage in terms of amplitude. The gain of the input amplifier stage can be determined by setting the resistance of Rf1 and R1.
Figure 4 input amplifier stage circuit structure
The comparator circuit used in this paper is shown in Figure 5. The comparator circuit consists of three stages, namely the input preamplifier stage, the decision stage (or positive feedback stage) and the output digital shaping buffer stage. The preamplifier stage is implemented with a differential amplifier of active load. The amplification factor is not large enough to amplify the input signal to improve the sensitivity of the comparator, and the input signal of the comparator and the switch from the positive feedback stage. The noise is isolated; the judgment stage is used to further amplify the signal of the preamplifier stage as the core part of the comparator, and the circuit realizes positive feedback by cross-interconnecting the gates of m8 and m9 to have a signal capable of distinguishing very small signals. Capabilities and increase the gain of this stage of the circuit; the output buffer stage is a self-biased differential amplifier whose input is a pair of differential signals that are used to convert the output signal of the decision stage to a logic level (0V or 5V). That is, the output high level VOH=VDD, output low level VOL=GND.
Figure 5 Comparator circuit diagram
2.3 internal oscillator circuitThe triangular wave generating circuit structure used in this paper is shown in Figure 6, where m5, m6 and m7, m8 constitute two sets of constant current sources, and m9~m13 and Q1 constitute the output stage. In the circuit, the output signal VT is fed back to the comparators comp1 and comp2, respectively, with reference levels VREF1 and VREF2 (VREF2).
Figure 6 Triangle wave generation circuit
As can be seen from Figure 6, the initial voltage value of VT is zero, when the circuit is powered up, due to 0
2.4 full bridge switching circuitThe output stage adopts N- and P-type power switches to realize the full-bridge switching circuit composed of tubes. The structure and load current flow direction are shown in Figure 7.
Figure 7 Schematic diagram of full-bridge circuit structure and load current
The full-bridge switching circuit operates in the switching mode. As the input signal changes, the state of m1~m4 changes, and only one pair of power switches is turned on and the other pair is turned off.
2.5 drive circuitThe structure of the drive circuit is shown in Figure 8. This circuit can effectively adjust the dead time (N-type, P-type power switch tube is turned off at the same time), to prevent the single-arm "shoot-through" phenomenon, and to protect the shutdown function. The input signal is the PWM pulse signal output by the comparator, PWM1 is used to drive the N-type power switch, and PWM2 is used to drive the P-type power switch. In order to avoid the single-arm "shoot-through" phenomenon in the full-bridge switching circuit, when the PWM signal changes from low level to high level, PWM2 should first go high, turn off the PMOS power switch, and then PWM1 changes again. To be high, turn on the NMOS power switch, as shown in Figure 9. Otherwise, when the PWM signal goes from high to low, PWM1 goes low first, turns off the NMOS switch power transistor, and then PWM2 goes low again. Level, turn on the PMOS switch power tube. In the actual circuit, the length of the dead time can be adjusted by controlling the delay bit control bit Tc as needed. In order to reduce the distortion, the dead time must be reduced. The driving circuit drives the power tube by increasing the driving capability step by step, thereby reducing the necessary dead time and ensuring low distortion.
Figure 8 drive circuit structure
Figure 9 Dead time
EN is the enable signal of the control module, and the normal operation is high level; when there is over current, over temperature, etc., it becomes low level, and the full bridge power switch circuit is turned off.
2.6 reference circuitThe bandgap voltage reference source structure designed in this paper is shown in Figure 10. It consists mainly of the core circuit and the startup circuit.
Figure 10 reference circuit
In the core circuit, M1~M12 together form a cascode current mirror to provide DC offset, and op amp op1 uses two-stage cascode amplification. In addition, negative feedback is introduced in the circuit of Figure 10 to ensure the accuracy of the current mirror of the bias circuit, and has nothing to do with the power supply, and has a high power supply rejection ratio.
When the circuit is powered up, the bias circuit may have a zero current condition, and the startup circuit is required to ensure that the circuit can work normally. When the circuit is not working, EN and Vs1 are 0, Vs2 and Vs3 are 1, M15 and M17 are not connected, the output of the op amp is high, and M3~M6 are not connected. The whole circuit does not consume current. When EN changes from 0 to 1, due to the action of C1, Vs1 remains at 0, Vs2 is 1, and Vs3 becomes 0. At this time, M15 and M17 are turned on, and inp and inn are pulled to 0 and 1, respectively. It becomes 0, M3~M6 turns on, M13, M14 branch starts to have current, and charges C1 until Vs1 is higher than I2 threshold voltage, Vs2 becomes 0, Vs3 becomes 1, and M15 and M17 turn off. The final circuit deviates from the zero current state and begins normal operation, and Vs1 is charged to the supply voltage, and the entire startup circuit no longer consumes current.
3 ConclusionIn this paper, the working principle of Class D audio power amplifier based on PWM modulation technology is studied. The THD of Class D audio power amplifier is reduced by introducing feedback technology. The necessary dead time is reduced by increasing the driving capability step by step. Lower distortion; using a two-way inverse wide modulation scheme, on the one hand, suppresses the static power consumption of the system, and on the other hand removes the LC low-pass filter of the output stage, thereby achieving the purpose of reducing system cost and volume.
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