# Power amplifier principle, circuit diagram and commonly used chips

#### BySipxmach

Jan 22, 2022

Use the current control function of the triode or the voltage control function of the field effect tube to convert the power of the power supply into a current that changes according to the input signal. Because sound is waves of different amplitudes and frequencies, that is, AC signal current, the collector current of the triode is always β times the base current, and β is the AC amplification of the triode. Applying this point, if a small signal is injected into the base, Then the current flowing through the collector will be equal to β times the base current, and then this signal is isolated with a DC blocking capacitor, and a large signal with the current (or voltage) being β times the original is obtained. This phenomenon becomes the amplification of the triode. effect.

The original power amplifierreason

Use the current control function of the triode or the voltage control function of the field effect tube to convert the power of the power supply into a current that changes according to the input signal. Because sound is waves of different amplitudes and frequencies, that is, AC signal current, the collector current of the triode is always β times the base current, and β is the AC amplification of the triode. Applying this point, if a small signal is injected into the base, Then the current flowing through the collector will be equal to β times the base current, and then this signal is isolated with a DC blocking capacitor, and a large signal with the current (or voltage) being β times the original is obtained. This phenomenon becomes the amplification of the triode. effect. After continuous current amplification, power amplification is completed.

Basic composition of power amplifier

The power amplifier is usually composed of 3 parts: preamplifier, driver amplifier, and final power amplifier.

1. The preamplifier plays a matching role. Its input impedance is high (not less than 10kΩ), which can absorb most of the previous signals, and the output impedance is low (below tens of Ω), and most of the signal can be transmitted. At the same time, it is itself a current amplifier, which converts the input voltage signal into a current signal and gives it appropriate amplification.

2. The driver amplifier acts as a bridge. It further amplifies the current signal sent by the preamplifier and amplifies it into a medium-power signal to drive the final power amplifier to work normally. If there is no driver amplifier, the final power amplifier cannot send out high-power sound signals.

3. The final power amplifier plays a key role. It forms a high-power signal from the current signal sent by the drive amplifier to drive the speaker to sound. Its technical indicators determine the technical indicators of the entire power amplifier.

1. LM1875

LM1875 is one of the commonly used power amplifier chips. It is designed for mono. It not only has the advantages of mellow sound quality and high power, but also has a complete protection circuit. It belongs to the model of the same type of chips.

2. LM3886

It is also a mono design with 11 pins. Compared with LM1875, LM3885 has more power, wider dynamics, and other parameters. Therefore, LM3886 will be used as audio power amplifier only in end multimedia speakers. chip.

3. LM4766

The usual saying on the Internet is that LM4766 is equivalent to encapsulating two LM3886 together. Why do you say that? In terms of performance parameters, LM4766 is exactly the same as LM3886, and even the tone color is exactly the same. However, because the LM4766 has many pins, people in the industry often call it a “centipede chip”, which is difficult to solder.

4. LM1876

m1876 is a commonly used dual-channel audio power amplifier circuit for short as a power amplifier. Its fidelity is very high, and the power of a single channel can reach 20 watts.

5. TDA7240

Currently LM1875 is also one of the commonly used power amplifier chips.

A typical BTL power amplifier is shown in the figure below. The power amplifier tube of the BTL power amplifier is a four-leg circuit composed of two complementary symmetrical circuits. The load RL is connected to the output ends of the two complementary symmetrical circuits, and a direct coupling output mode is adopted.

What are the commonly used power amplifier chips

The power supply voltage VCC is applied to the collectors of the amplifier tubes VT1 and VT2 to supply power to them. In the static state, since there is no signal input, the amplifier tubes VT1~VT4 are cut off, there is no voltage output, and no current flows on the RL. When the positive half-cycle signal of Ui is input, VT1 and VT4 are turned on, and their collector currents form a loop from VCC to ground through VT1, RL, VT4, forming the first half-cycle of the output signal; when the negative half-cycle signal of Ui is input, VT2 , VT3 is turned on, and their collector current forms a loop from VCC, VT2, RL, and VT3 to ground, forming the negative half cycle of the output signal. In this way, a complete signal can be obtained.