The BTL power amplifier is composed of two amplifiers, and the two ends of the load are respectively connected to the output ends of the two amplifiers. The output of one of the amplifiers is the mirror output of the other amplifier, that is, the signals at both ends of the load differ only in phase by 180 degrees. The load will get twice the voltage of the original single-ended output. Theoretically speaking, the output power of the circuit will increase by 4 times. Not all power amplifier blocks are suitable for the BTL format, and several connection methods of the BTL format have their own advantages and disadvantages. The following describes each connection one by one.
Figure 1 is the first BTL connection of LM4766. The input signal is input from the non-inverting input terminal of the LM4766 amplifier B, and the signal of the inverting input terminal of the amplifier A is introduced from the output terminal of the amplifier B through R9. The output obtained on the load is the superposition of these two signals. But R9 must be strictly equal to R6, otherwise this error will be amplified by the amplification factor of A, making the output signal amplitudes of A and B very different. Second, due to the presence of C1 in the negative feedback of A, the output of A does not completely differ from the output of B by 180 degrees in phase. In actual listening, I also found that the output of the circuit is really confusing in terms of positioning and layering. But whether this circuit is no-load or no input signal, or the internal resistance of the signal source of the input signal is very large, the circuit can work normally. Stability and simplicity are its advantages.
Figure 2 is the second BTL connection. A acts as an inverting amplifier and B acts as an in-phase amplifier. The gain of A is -R17 / R1, and the gain of B is (R4 + R6) / R4. You can take appropriate values ​​to make the gains of the two equal. Since the input signals of the A and B amplifiers are absolutely equal in phase, even if the phase is delayed after amplification, as long as the delayed phases are equal, there will be no problem of phase confusion after superposition. This circuit has high requirements on the input signal source. The signal source must be added before the circuit is powered on, and the internal resistance must be small enough. When the signal source is not connected, A is basically self-excited because A is in an open loop state. When the internal resistance of the signal source is too large, the amplification factor of A will become smaller. This connection method is suitable for the circuit system with a low output resistance front stage added to the circuit. Figure 3 is the third BTL method. A NE5532 is added to the circuit. The C7 capacitor is used in A1, so there is no DC component in the output of A1, so A2 can use DC negative feedback, which makes the output of A2 and the output of A1 exactly 180 degrees different, so there is no connection in Figure 1 Phase delay problem. At the same time, because the output internal resistance of NE5532 is quite small, it solves the problem of instability in the connection method of Figure 2.
The authors tested the above three methods with National Semiconductor's LM1875, LM3886, LM4766 and Philips TDA1514.
The maximum voltage of LM1875 can reach ± ​​30V, the output current can reach 3A, and the maximum output power is 30W. No over-current and temperature protection circuit. When applied to the connection in Figure 1, the sound is a bit floating. In the applications of Figures 2 and 3, the sound is strong and strong. In Figure 2, when the audio input terminal is unplugged, there is a bald voice in the speaker, confirming that it is self-excited. The output power of the three circuits can reach about 80W.
Then in the way of Figure 3, make a comparison between LM3886 and TDA1514.
The maximum supply voltage of TDA1514 is ± 30V. When the load is 8Ω, it is recommended to use ± 24V. It is recommended to use ± 35V when the load of LM3886 is 8Ω, and ± 28V when the load is 4Ω.
TDA1514 is easily self-excited when applied to the BTL method. At the output of each amplifier, a 1 / 4W 10Ω resistor must be connected in series with a 0? 047μF capacitor to ground. The feedback resistance (generally 27kΩ to 33kΩ) is reduced to 20kΩ to 22kΩ to reduce the gain. The internal current limiting protection and maximum power limiting circuit of TDA1514, when connected to a BTL circuit and found that the maximum power reaches about 100W, the current limiting circuit starts to operate. The output is turned off at the peak of the sine wave. After the peak, it is turned on again. Although it is not easy to hear, it has seriously affected the sound quality. When it exceeds 110W, the power limiting circuit works, and the output is completely turned off.
The LM3886 has a unique SPIKE protection circuit. It does not simply turn off the circuit when the output is exceeded. SPIKE protection has a conversion interval. The LM3886 also has fixed temperature detection overcurrent protection. The maximum output current is slightly larger than TDA1514, and the actual maximum output power is 60W. When connected to BTL, the maximum power can reach about 120W. The sound quality of the two is indistinguishable, but when the LM3886 is applied to high voltage, its sound quality is superior in terms of robustness.
Finally, the LM4766 is connected to the experiment in the mode of Figure 3. The output power of the circuit is about 100W, and the sound quality is simply and powerful, and the dynamic range is excellent.
In summary, when using BTL circuits to increase the output power, it is necessary to select the appropriate circuit form according to the system circuit, and to understand the parameters of the integrated block used to design a suitable application circuit.
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