Power Transistors Warm Up To Thermal-Sensing Diodes

High-fidelity audio amplifiers consistently encounter a major hurdle: maintaining output bias while achieving thermal stability within the AB output section over the amp's full operating temperature range. The usual approach mounts one or more bias transistors on the heatsink near the output devices. Yet there's always some lag time before reaching thermal equilibrium, sometimes as much as 30 minutes.

To eliminate thermal lag and boost amp performance and reliability, ON Semicon-ductor unveiled output transistors with integrated bias diodes. This permits immediate control of the bias current and eliminates the need for a bias trimming potentiometer. Distortion levels at lower output voltages are measurably improved, and the lack of oscillation at the zero-volt crossing enhances the amp's noise floor.

ON Semiconductor's complementary NJL3281D (NPN) and NJL1302D (PNP) ThermalTrak audio output devices use Ultrafast diode technology as the temperature-sensing device, along with the audio output transistor. These 230-V devices are rated for 200 W. Devices rated for 150 and 250 W will follow. Sampling now in five-lead, TO-264 packages, they cost $2.25 each in 10,000 unit lots