Output transistors with internal bias control eliminate amplifier warm-up period

9 February 2005 Analogue, Mixed Signal, LSI

An innovative family of output transistors with internal bias control that provide a simplified single-device solution for audio amplifier design has been released by On Semiconductor. The devices eliminate tedious warm-up time, enable quick and precise temperature adjustment and improve both sound quality and bias stability, says the company.

The ThermalTrak devices simplify board design by eliminating the need for output drive compensation circuitry and costly manual bias adjustments. They are intended for use in high-end consumer audio amplifiers and receivers, as well as professional audio applications such as theatre, stadium and PA sound systems.

In audio output applications, transistor overheating leads to increased harmonic distortion and can potentially cause catastrophic failure of the end product. Designers have traditionally mounted discrete bias protection circuitry to the heatsink to monitor temperature changes. While relatively effective, this standard industry solution still results in a lag time before thermal stability is achieved. Ultimately, it can take up to 5 minutes for the temperature change of the heatsink to be significant enough to be recognised, creating the potential for thermal runaway of an individual transistor before any compensation is made.

Solution

More accurate bias control is needed to overcome this design challenge and eliminate both the thermal lag and bias trimming. Rather than mounting bias transistor (or transistors) to the heatsink in close proximity to the output devices, ON's ThermalTrak technology thermally integrates bias compensation within an output transistor. This provides precise detection and immediate correction of thermal changes - improving both sound quality and bias stability. This technology establishes a superior level of bias compensation performance not previously achievable using traditional discrete biasing circuitry.

The first ThermalTrak transistors to be introduced are the NJL1302D (PNP) and the NJL3281D (NPN). These 230 V devices in standard 5-lead, TO-264 package, offer a 200 W power rating. The company plans to introduce devices for 150 W and 250 W requirements.

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