Application of BTL power amplifier circuit composed of LM3886

At present, the output power of mainstream audio amplifier integrated circuits is generally not very large. Except for some STK thick film amplifiers, it generally does not exceed 1OOW. For home audio, if the sensitivity of the speakers used is not very high and the listening room area is relatively large, a simple integrated circuit amplifier may be difficult to create a satisfactory sound effect. In view of this situation, this article introduces an effective method to increase the output power of ordinary integrated circuit amplifiers, which can achieve more ideal dynamic performance and more sufficient power reserve under limited conditions. In fact, the easiest way to increase the output power is BTL, i.e. bridge connection. The output load of the traditional OCL amplifier corresponds to the common ground, so the maximum voltage obtained by the two sections of the load can only be close to and less than the voltage of the power supply to the common ground. For example, the power supply voltage of the OCL amplifier uses ±28V. Under ideal conditions, the maximum output voltage of 28V can be obtained with an 8Ω load, and the maximum effective output power calculated by the sine wave is about 49W. If the BTL load is connected between the output terminals of two OTL amplifiers and the same power supply voltage is used, ideally, the 8Ω load can get a maximum output voltage of 56V, and the maximum output power is about 196W, which is 4 times the former. It can be seen that the BTL method is powerful. However, there is a considerable gap between the actual situation and the theoretical calculation. For example, when the power supply voltage is ±28V, the load impedance of 8Ω can only output 38W power. Under the same conditions, the maximum output power of two LM3886 connected in BTL mode is about 150W. In fact, if four LM3886s are connected in pairs by BTL to form a set of dual 150W stereo amplifiers, it is a very cost-effective solution in terms of performance and cost.

Next, we will introduce the BTL power amplifier circuit composed of LM3886, as shown in the figure above. Only one channel circuit is drawn here, and the circuit of the other channel is exactly the same. The power rectifier filter circuit is common to both channels. The filter capacitors C1 and C2 are 22000μF, which is enough to cope with the demand for instantaneous current of explosive signals. Discharge resistors R1 and R2 are connected in parallel at both ends of the capacitor. The rectifier diodes VDI-VD4 use MOTOROLA's MR752 or other high-current rectifier diodes. ±28V voltage is directly supplied to LM3886 for use, and no voltage stabilization is required. The purpose is to ensure that the LM3886 obtains charge from the filter capacitor as little as possible without affecting the dynamic effect. A pair of 7815 and 7915 three-terminal voltage regulator ICs are used to generate ±l5V stable voltage for the input op amp and inverting amplifier. The entire amplifier consists of an input circuit, an inverting circuit and a BTL power amplifier circuit, in which the input circuit is completed by a single op amp OPA604 to amplify the input small signal by 1 times. At the same time, the operational amplifier adopts differential input, and the ground wire is regarded as a signal input terminal. This treatment can effectively suppress the ground wire interference noise and improve the signal-to-noise ratio index of the input stage. The following DRVl34 is a dedicated single-ended conversion balanced drive circuit, which is generally used at the transmitting end of balanced transmission equipment. It can convert the single-ended input signal into two inverted signal outputs. DRVl34 plays the role of an inverting stage circuit here. The single-ended signal amplified and output by OPA604 enters DRVl34 and is divided into two outputs. The phase difference is exactly 180 degrees. After these two signals drive a LM3886 for power amplification, they are synthesized into a complete power output signal at the output end of the speaker. This is the principle of BTL working mode. LM3886 adopts a standard 11-fold in-phase voltage amplifier circuit here. Due to the effect of BTL, the voltage amplification factor of the power amplifier stage is twice that of a single amplifier, that is, the total voltage amplification factor of the BTL amplifier stage composed of two 11-fold amplifiers will be 22. Due to the LM3886's own reasons, its voltage amplification factor should not be designed too small. Generally, it needs to be more than 10 times to work stably. Therefore, 11 times is taken here. If the value is too large, the distortion and noise of LM3886 will increase, which is not advisable for designing audio amplifiers with high performance indicators. The voltage amplification factor of the whole circuit is 22 times, that is, the voltage gain is 27dB. For a 150W power amplifier, the gain is too small, so there is no volume adjustment potentiometer at the input end. The purpose is to design this power amplifier into a pure post-stage amplifier. When used, it needs to be combined with an independent pre-stage voltage amplifier to form a front- and rear-stage split power amplifier. This treatment is a common practice for high-performance amplifiers and is worth promoting. In addition, LM3886 has a complete internal protection circuit, so the power amplifier output does not have an independent external protection system. It should be noted that the 8th pin of LM3886 is a mute control pin, which can be used to suppress the current impact sound at the moment of power-on. For the specific power-on mute circuit, please refer to the circuit in the dotted box in Figure 1. The circuit is relatively simple and its working principle can be analyzed by the reader by himself. It is not described in detail here.

The BTL circuit introduced in this article is widely used. Here we only use the commonly used LM3886 as an example. In fact, any form of power amplifier circuit can be connected in BTL mode, including integrated circuit power amplifiers such as LMl875, LM4766 and LM4780. If you use an LM4780 dual-channel power amplifier integrated circuit to form a BTL mode, it can replace two LM3886s at the same time. The circuit is simpler and smaller in size. Interested readers may wish to give it a try.