About Class D Amplifier Design

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About Class D Amplifier Design [Copy link]

System overall design
This system consists of a high-efficiency power amplifier (Class D audio power amplifier), a signal conversion circuit, and an external test instrument.

Class D power amplifier design
Class D power amplifier consists of PWM circuit, switching power amplifier circuit and output filter.

A fixed frequency PWM circuit composed of a comparator and a triangle wave generator is used to modulate the triangle wave with the input audio signal amplitude to obtain a square wave whose duty cycle changes with the audio input signal amplitude, and drives the power tubes of the upper and lower bridge arms with opposite phases, so that one power tube is turned on and the other is turned off. The square wave is then converted into an audio signal through the output filter to drive the speaker to sound. The full-bridge Class D amplifier can achieve balanced output, which is easy to improve the output filtering characteristics of the amplifier and reduce interference. The peak-to-peak voltage on the load of the full-bridge circuit is close to twice the power supply voltage, and a single power supply can be used. When implementing it, the method of two output pulses with opposite phases is usually adopted.

Hardware circuit design

Principle analysis:
When an analog audio signal is input, the analog audio signal is converted into a high-frequency PWM pulse signal with a pulse width corresponding to its amplitude through a PWM modulator, which controls the on/off of the switch unit, drives the pulse power amplifier to work through a pulse driver, and then drives the speaker to work through a power low-pass filter.

Comparator
The comparator circuit is composed of a low-power, single-power dual-channel comparator chip LM311. In order to improve system efficiency and reduce unnecessary opening and closing of the CMOS tube in the subsequent H-bridge, two triangular waves with different biases are used to compare the upper and lower halves of the audio signal. When the potential on the positive end is higher than the potential on the negative end, the comparator outputs a high level, otherwise it outputs a low level. In this way, two corresponding PWM wave signals are generated and sent to the subsequent drive circuit for processing.

Note:
The upper half is compared and 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 is compared in the opposite way, 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, thereby reducing system power loss. The circuit uses the audio signal as the modulation wave and the triangle wave with a frequency of 70kHz as the carrier wave. A 2.5V DC bias voltage is added to both signals and compared through a comparator to obtain a pulse signal with the same amplitude and a duty cycle that changes with the audio amplitude.

The power supply voltage of the LM311 chip is a single 5V power supply. To provide a static potential of 2.5V for V+=V-, R10=R11, R8=R9, and all four resistors are 10kΩ. Since the triangle wave Vp-p=2V, the Vp-p of the audio signal must not be greater than 2V, otherwise the amplifier will be distorted. Since the output stage of the comparator chip LM311 is an open collector structure, a pull-up resistor must be added to the output end, and the pull-up resistor uses a 1kΩ resistor.

The drive circuit, complementary symmetric output and low-pass filter circuit
transform the PWM signal into a complementary symmetric output drive signal, and use the CD40106 Schmitt trigger in parallel to obtain a larger current output, which is sent to the output tube driven by the complementary symmetric emitter follower composed of transistors, ensuring fast driving. The transistors in the drive circuit use 9012 and 9014 pairs of tubes.
The requirements of the H-bridge complementary symmetric output circuit for VMOSFET are small on-resistance, fast switching speed, and small turn-on voltage. Because the output power is slightly greater than 1W, it is a low-power output, so a pair of tubes with relatively small power, small input capacitance, and easy to drive quickly can be selected. The requirements for the filter are that the upper limit frequency is
≥20kHz
, and the characteristics are basically flat within the passband. The complementary PWM switch drive signal turns on Q6 and Q8 or Q12 and Q10 alternately, and drives the speaker after being filtered by two 4th-order Butterworth filters.

Data Testing

1) Passband measurement: When the amplifier voltage gain is 10, measure the upper and lower boundary frequencies of the 3dB passband. The RC filter used for the test should be removed during the passband test.
2) Maximum undistorted output power: When the gain is 10, input a 1kHz sinusoidal signal, use a millivoltmeter to measure the effective value of the amplifier output voltage, and calculate the maximum output power Po-max.
3) Input impedance: When a 10kΩ resistor is connected in series in the input circuit, the voltage drop at the amplifier input should be less than 50%.

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How to achieve reversing operation for a single-power Class D amplifier?