ON Semi’s output audio transistors eliminate thermal lag and bias in high-fidelity amps

Phoenix, Ariz. — Today On Semiconductor is using the International Consumer Electronics Showcase (CES) in Las Vegas to show off a new family of output audio transistors with internal bias control that eliminate amplifier warm-up period, enable instant temperature adjustment, upgrade sound quality, and improve bias stability in high fidelity amplifiers.

ON Semiconductor's ThermalTrak family of output transistors provide a simplified single-device solution for audio amplifier design. These devices feature patent-pending technology that enable quick and precise temperature adjustment (about 50 milliseconds). The initial devices will be suited for use in high-end consumer audio amplifiers and receivers, as well as professional audio applications such as theater, stadium and PA sound systems. ThermalTrak devices also simplify board design by eliminating the need for output drive compensation circuitry and costly manual bias adjustments.

“These devices provide a simplified single-device design solution that enhances the total harmonic distortion (THD) performance at lower power levels where it matters the most. The soon-to-be patented technology used to develop ThermalTrak provides a platform upon which the company plans to build a significant portfolio of audio output power solutions,” said Keith Nootbaar, ON Semiconductor's director of power discrete products.

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 heat sink 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 heat sink to be significant enough to be recognized, creating the potential for thermal runaway of an individual transistor before any compensation is made.

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 heat sink in close proximity to the output devices, ON Semiconductor'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 offer a 200-W power rating. Following the 200-W introduction, the company plans to introduce devices for 150-W and 250-W requirements. Further expansion of the ThermalTrak family for lower-watt applications is planned for later 2005.

The NJL1302D and NJL3281D ThermalTrak devices will be offered in a standard size 5-lead, TO-264 package. The devices are priced at $2.25 per unit in 10,000-unit quantities. Limited advance engineering samples are currently available. The devices are scheduled for full production early in the second quarter.

Click here for the NJL1302D data sheet.

Click here for the NJL3281D data sheet.

For more information, please call (602) 244-6600 or visit:

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