Audio amplification circuit design method for eliminating thermal hysteresis and offset adjustment

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This article describes a new design approach to eliminating thermal hysteresis and offset adjustment in high-fidelity, high-power audio amplifiers using the ON Semiconductor ThermalTrak device.

For designing high-fidelity audio amplifiers, the challenge has been to maintain output bias while maintaining thermal stability within the AB output section over the entire operating temperature range of the amplifier. To adequately monitor and regulate the heat generated by each output device, the design engineer places a bias transistor (or multiple transistors) on the heat sink near the output device. The disadvantage of this design approach is that there is inevitably a delay before reaching thermal stability, and in some cases this warm-up time can be as long as 30 minutes. In addition, the amplifier design requires a slight under-biased output device for further adjustment to avoid heat loss. While these viable design techniques can accomplish design tasks, they sacrifice true high fidelity performance.

Figure 1: Standard single-channel audio amplifier solution.

Eliminating thermal hysteresis and offset adjustment in high-power audio amplifiers requires more precise bias control, and the challenge is here. The high performance amplifier circuit shown in Figure 1 is the standard design currently used in the industry. This design has been modified to improve performance and stability in low impedance loads.

The actual bias circuit consists of a small signal transistor on the heat sink between the drivers (TO-220). The bias stability of this design requires bias settings to avoid heat dissipation and no less than crossover distortion. (crossover distortion) at the point. The actual voltage drop across the bias transistor is set at 3.2 volts between the emitter and collector. Not only that, when the amplifier is driven to a low impedance load, a small amount of heat dissipation occurs, which is caused by a small amount of thermal hysteresis in the heat sink. In order to improve the small signal distortion of the amplifier, it is necessary to slightly increase the offset.

Since the output device has a minimum emitter resistance (0.1 Ω), it is difficult to handle in a production environment because a slightly larger bias produces a hot "bomb." A number of circuits are currently implemented in the industry to reduce this thermal effect, but they all increase system cost.

The new design approach (Figure 2) can be used to eliminate thermal hysteresis and improve amplifier performance and reliability without increasing component count. The new method allows for more accurate monitoring of the actual die operating temperature by integrating the biasing diode into the output transistor. The offset can now be controlled as soon as any changes are compensated in real time. Because the bias current can be quickly adjusted, there is no need to worry about heat dissipation or thermal hysteresis caused by the heat sink. Another advantage of this modification for the circuit is that it can be implemented without bias adjustment potentiometers, which also reduces the production steps and ensures that the static bias point is reached.

In this modified circuit design, passive components and active bias transistors are removed and replaced by diodes integrated into the output transistors. This results in a stable bias current and very accurate quiescent current that can be instantaneously adjusted to the load. And signal level. The distortion at the lower output voltage is also greatly reduced, and the noise reference of the amplifier is also improved by the absence of oscillation at the zero-crossing voltage.

Figure 2: SingleT audio amplifier ThermalTrak solution.

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