BTL (Bridge-Tied-load) means bridge-tied load. The load connected on both ends of the two amplifier output. One of the amplifier output is another image output of an amplifier, that increases the signal across the load and differ only in phase 180 °. The original load will be 2 times the single-ended output voltage. Theoretically, the circuit will increase the output power of 4 times. BTL circuit can take full advantage of the system voltage, so often used in low-voltage BTL structure, systems or battery-powered systems. When the car stereo power more than 10w per channel, the most used form of BTL. BTL is different from the push-pull form form, BTL amplifier for each signal the signal is complete, but the two amplifier output signal inverting it. Integrated amplifier with a BTL amplifier blocks requires a two-channel amplifier or two mono blocks. But not all of the power amplifier block form for BTL, BTL is also connected in the form of several advantages and disadvantages, the following means of making experiments on a variety of connection elaborate. Manifold amplifier used in experiments with LM1875, LM3886, TDA1514, LM4766. Personally prefer the LM4766 sounds, and other IC applications are also many magazines have introduced. Therefore, application of the LM4766's BTL example.
Figure 1 shows the LM4766 BTL application of the first connection. LM4766 amplifier input signal from the inverting input B, input, R6, R4, C2 is the negative feedback network. Amplifier A inverting input signal passes through R9 output from the amplifier B, the introduction. By R9, R1, C1 partial pressure. While R1, C1, R3 Peng is A negative feedback amplifier network. C6-ended input signal is assumed V, B, amplifier gain is PB = (R4 + R6) / R4, B the output voltage of PB * V. The input amplifier A is the result of partial pressure, voltage coefficient of R1 / (R9 + R1), in the circuit generally take R9 = R6, R4 = R1. Therefore, the partial pressure coefficient 1/PB, inverting amplifier A input signal is also V, whose gain is - (R4 + R6) / R4, R3 generally equal to R6, so the A's gain is-PB * V. The load of the output signal of the superposition of this two, that 2PB * V. However, the application of this form of audio link exists. First, R9 must be strictly equal to R6, otherwise the error will be magnified by the amplification factor A, so A and B of the output signal amplitude lot of difference.
Figure 2 shows the LM4766 BTL application of the second connection. As an inverting amplifier A, B as in phase amplifier. A gain of -R17/R1, B's gain (R4 + R6) / R4, desirable to gain the appropriate values so that the two are equal. Because A, B amplifier input signal in phase is absolutely equal, even after amplification phase delay, but as long as the phase delay equal superposition phase of confusion after the problem does not exist. The adaptability of this circuit is very ideal. Because of its demanding input source. Signal source must be added before the power circuit, and resistance should be small enough. Without access to the signal source, A follower will be output as a DC voltage damage the speakers. When the signal source resistance is too large, A magnification factor becomes smaller. For such a connection in the circuit has low output resistance increases first-class circuit system.
Figure 3 shows the LM4766 BTL application of the third connection. Add a piece of circuit NE5532. It improves the connection in Figure 2, the instability. NE5532 amplifier for the voltage follower, A11, A27 for the inverter. Therefore, LM4766 two-phase inverting amplifier input, their output is inverted. All C7 used in capacitors, the output should be this is the A11 without DC component, so A27 negative feedback can be DC, which makes the output of A27 and A11 just the difference between the output of 180 °, which does not exist in Figure 1 that the phase delay. While the output of the NE5532 resistance is quite small, it solved the problem of instability in Figure 2. Just the circuit adds an IC, the complexity of the circuit increased.
Of the above three methods were used National Semiconductor's LM1875, LM3886, LM4766 and Philips TDA1514 to do the test.
First is the LM1875. LM1875 high-voltage up to ± 30V, output current up to 3A maximum output power of 30W. No over-current and temperature protection circuit. Datasheet given in NS, it is not recommended BTL application methods. Access method used in Figure 1, the sound a bit the feeling of floating. In Figure 3, unplug the audio input terminal when the speaker in dash of sound, proved it in the self-excited. 80W output power up to three circuits around.
Figure 3 and then way on the LM3886 and TDA1514 do some comparison. TDA1514 maximum supply voltage is. ± 30V, when the difference between positive and negative voltage exceeds 60V, the manifold will be very easy to damage. When the load is 8O, the maximum voltage of ± 28V, recommended ± 24V. LM3886 maximum supply voltage is ± 42V, 8O time when the load is recommended for ± 35V, 4O when the load is recommended ± 28V. Power supply current best not less than 5A. If the current is not enough, will cause voltage fluctuations, can not reflect the BTL output power. Test, using sine wave signal generator and adjust its amplitude until clipping distorted. Heat sink temperature to be large enough to avoid over-current protection action.
TDA1514 BTL method used should be cautious, or easy self-excited. Output of each amplifier must be a 1/4W, 10O serial resistance and 0.047pF capacitor to ground. Feedback resistor (typically 27kO to 33kO) is reduced to 20k see to reduce the gain. Speaker with 8O's. TDA1514 limited flow protection and maximum internal power limiting circuit, the maximum output current of about 3A. Maximum output power of 50W. Into BTL in the next, we found the maximum power up to 100W or so, limiting circuit start to operate. Peak of the sine wave output when turned off. After a peak and then open, although not very easy to listen to it, but has seriously affected the sound quality. When more than 110w power limit circuit, the output was completely turned off.
LM3886 has a unique "SPIKE" protection circuit, which makes it in many ways better than the Philips TDA1514. LM3886 output over time is not to simply turn off the circuit. SPIKE protection of a conversion interval. LM3886 also has a fixed temperature sensing over-current protection. Maximum output current is slightly larger than the TDA1514. Actual maximum output power of 60W. Into BTL in the next, the maximum power up to 120W or so. Both sound equally matched, but when the LM3886 high-voltage applied, the sound quality in the strong side to win a chip.
Finally, the LM4766 way access into the experiment in Figure 3, the circuit output power of 100W or so, the sound altogether powerful, dynamic range is excellent. In addition, conditional to the NS website readers can ask for free samples (free sample). Website is: www.national.com . In summary, the use of BTL, the circuit to improve power output, be sure to select the appropriate circuit according to the system circuit form, and to understand the parameters used in Manifold, to design a suitable application circuit.