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Oct 16 2020

NE5532 and LM311 Class D Power Amp

I Description

Class D power amplifiers are often used for music playback of various electronic products in daily life. The Class D power amplifier in this blog uses the NE5532 operational amplifier and LM311 to form a self-oscillation to generate a 150kHz carrier wave and modulate the input low-frequency signal into a constant-amplitude pulse. Then the power amplifier pushes the moving tube. Finally, the low-frequency signal is separated by filtering.

Figure 1. Class-D Power Amplifier


I Description

II How to Choose an Amplifier

III Working Principle of Class D Amplifier

IV Principle Block Diagram

V Working Principle

5.1 Channel Selection and Display Principle

5.2 Carrier Generation Circuit

5.3 Comparator Circuit

5.4 Drive Circuit and Power Amplifier Circuit

5.5 Low-pass Filter

VI Conclusion


II How to Choose an Amplifier

Traditional power amplifiers mainly fall into the following three categories:

  • Class A power amplifiers,;
  • Class AB power amplifiers;
  • Class B power amplifiers.

Class A power amplifiers mainly amplify small signals and need to set a bias voltage to stabilize the circuit, so the efficiency is low.

Class B power amplifiers do not rely on bias voltage for amplification but use the principle that when the input signal is greater than the conduction voltage of the transistor, the transistor is turned on for amplification. Since the input signal is divided into positive and negative, it needs to use 2 triodes for amplification. Because when the input signal is less than the turn-on voltage, that is, in the range of -0.6V to 0.6V, the transistor cannot be turned on. Therefore, the input signal cannot be amplified and there is distortion. Therefore, a Class AB power amplifier is proposed.

Class AB power amplifier, because it directly amplifies the analog signal when it works, it requires the transistor to be in a linear amplification state. Therefore, too much power consumption needs to be dissipated, which also has drawbacks.

Class D power amplifier overcomes the drawbacks of the above class A power amplifier, class AB power amplifier, and class B power amplifier. It uses pulse high and low-level control switching devices to turn on and off. Then the voltage and current of the output signal have been amplified, that is, the power is amplified, so the power consumption is very small.

III Working Principle of Class D Amplifier

First, use the op amp self-oscillation to generate a high-frequency carrier, and then modulate the input low-frequency analog signal to a high-frequency signal through a comparator. This modulating wave is a series of pulse signals of constant amplitude whose width is modulated and the frequency changes with the amplitude of the low-frequency signal. This process is also called pulse width modulation, or pulse width modulation for short.

This system is generally composed of a comparator, the input signal is a triangle wave (carrier) and a low-frequency signal, and the two signals are compared. If the low-frequency signal is greater than the triangle wave signal, the comparator outputs a constant. If it is less than, 0 is output. Therefore, the comparator output is a series of modulated pulse width modulated waves. The output modulation wave is amplified by the switching power and filtered to output a low-frequency signal.

IV Principle Block Diagram

principle block diagram

Figure 2. Principle Block Diagram

V Working Principle

5.1 Channel Selection and Display Principle

Channels can be divided into 1 channel and 2 channels. The selection circuit uses the 74ls74 chip, which is a dual-rising-edge D flip-flop chip with a total of 14 pins. Since the input signal is a low-frequency analog audio signal, the multiplexer of this blog uses a 74HC4052 chip.

When the output terminal of the D flip-flop is high, the digital tube displays 2. At the same time, the signal enters the 10th pin of 74HC4052. The chip's digital selection terminal selects Y1 channel signal input and output Y1 signal, that is, channel 2 is selected.

When the output of the D flip-flop is low, the digital tube displays 1. At the same time, the signal enters the 10th pin of 74HC4052. The digital selection terminal of the chip selects Y0 channel signal input and output Y0 signal, that is, channel 1 is selected.

5.2 Carrier Generation Circuit

The carrier wave used in this blog is a triangular wave with a frequency of 170kHz.

The function of the carrier is to modulate the input analog low-frequency signal to high-frequency to form a modulated wave.

The circuit uses NE5532 operational amplifier and voltage comparator LM311 to form a self-oscillation.

When setting the circuit parameters, it should be noted that the chips are all powered by a 5V power supply, and the operational amplifier can be divided by a 10kΩ resistor to make the voltage divided into 2.5V. At the same time, it is required to set the parameters so that the amplitude of the carrier output is 3V.

5.3 Comparator Circuit

The circuit in this article uses LM311 chip. The power supply voltage of the chip is 5V. In order to obtain a static potential of V+=V-=2.5V, the resistance value of the 4 resistors is 10kΩ.

Since the carrier Vp-p=3V, Vp-p of the audio signal is required to be 4V or less. Otherwise, it will cause distortion. This part is used as a modulator circuit, and the original input audio signal is input to the 2-pin positive input terminal of the operational amplifier LM311 after channel selection, amplification, and DC bias. Generate a triangular carrier wave through the self-oscillation circuit and input it to the 3-pin negative input terminal of the comparator LM311.

When the voltage on the positive terminal of pin 2 is greater than the voltage on the negative terminal of pin 3, the output will be high, otherwise it will be low. When there is no input signal, the voltage value of the DC bias is half of the peak value of the triangular wave. At this time, the output of the comparator is a square wave.

When there is an audio signal input and it is a positive half cycle, the comparator output high level time is longer than the low level time, and the duty cycle of the square wave is greater than one-half;

In the negative half cycle, the voltage at the positive input of the comparator is greater than zero. However, the time for the audio signal amplitude to be higher than the triangular wave amplitude is greatly reduced, and the square wave duty cycle is less than one-half. Therefore, the waveform output by the comparator is a waveform whose pulse width is amplitude modulated by the audio signal.

In this way, low-frequency audio information is modulated into the pulse waveform. The waveform is shown in Figure 3.

modulation waveform graph

Figure 3. Modulation Waveform Graph

5.4 Drive Circuit and Power Amplifier Circuit

The drive circuit adopts IR2110 full bridge drive. The power amplifier circuit uses a class D power amplifier, which is a pulse-controlled high-current switching amplifier. It turns the PWM signal output by the comparator into a high-voltage, high-current, high-power PWM signal.

5.5 Low-pass Filter

The 150kHz carrier uses a fourth-order LC filter. The function is to separate the low frequency signal from the PWM waveform.

Working principle: When a pulse with a duty cycle greater than one-half arrives, the charging time of the capacitor is longer than the discharging time, and the output level rises. When the narrow pulse arrives, the discharge time of the capacitor is longer than the charging time, and the output level decreases, which coincides with the amplitude change of the original audio signal. So that the original audio signal is separated.

VI Conclusion

Use NE5532 and LM311 operational amplifiers to design class D power amplifiers. By making PCB boards, soldering, debugging and realizing corresponding functions, the sound quality is better. So it is a more practical class D power amplifier. The circuit design and production is suitable for students who have a good grasp of analog and digital electronics and have a certain design foundation. This experiment will enable them to master electronic technology.


  • What is NE5532?

The NE5532 is a Dual Low Noise Op-Amp in 8-pin package commonly used as amplifiers in audio circuits for its noise immunity and high output drive capability. The Op-Amp is internally compensated for high unity gain with maximum output swing bandwidth, low distortion and high slew rate.

  • How to check NE5532 IC with digital multimeter?

  • Whats the size of NE5532 produced by Texas Instruments? Anything to compare?

You will find all physical sizes in inches as well as millimeters in page 19 of the official TI datasheet at:

  • How to use LM311?

LM311 is a single-channel comparator. When using it, connect the reference voltage and the compared signal voltage to its non-inverting and inverting input terminals (pin 2 and pin 3), and its output is the result of the comparison. If you want the foward output result, pin 7 is connected to the positive power supply and pin 1 is the output. If the result is to be output in reverse, pin 1 is grounded and pin 7 is the open collector output.

  • lm311 and lm393 are both voltage comparators, so what is the difference between them?

LM311 is single voltage comparaotor, LM393 is dual voltage copatpr. LM311 has a load current of up to 50MA and a voltage of 40V. It can drive relays with a minimum power supply voltage of 5V.

The LM393 load current is 16MA, and the minimum voltage is 2V for a single power supply.

  • Whats the difference between LM311 and LM111?

Their functions are the same, and the 1XX series can be used in harsher environments.

The 3XX series can only be used in a commercial environment, typically the applicable temperature range of the device.

The price of 1xx is much more expensive than 3xx.

  • What does the 5 and 6-pin balance strobes of LM311 mean?

The function of balancing the mirror current of the reverse circuit is realized by connecting a potentiometer in the middle. In addition to the balance function, the 6 pin also has a strobe function, and the 6 pin can be grounded through the transistor drive circuit for strobe output.

  • What is the difference between lm311 voltage comparator dual power supply and single power supply?

The comparators are all open-collector outputs, without load resistance, they cannot output voltage signals.

Dual power supplies can detect signals lower than 0, and single power supplies can only detect signals higher than 0.

  • Can the lm311 comparator be powered by a positive and negative five-volt dual power supply?

Of course, LM311 can be powered by ±5V dual power supply. Its requirement for working power supply is that the voltage difference between the positive and negative power supply (or single power supply voltage) is at least 3.5V and the maximum is 30V, as long as it is within this range.


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