Equalising HEXFETs Circuit Diagram

When experimenting with audio output stages featuring multiple HEXFETs it quickly becomes apparent that the total power is not divided equally among the individual transistors. The reason for this lies in the wide part-to-part variations in gate-source voltage, which in the case of the IRFP240 (or IRFP9240) can be from 2 V to 4 V. Source resistors in the region of 0.22 Ω as commonly seen in amplifier circuits (see example circuit extract) help to counteract this, but usually not to a sufficient extent. One possible solution to this problem is to ‘select’ the transistors used so that their gate-source voltages match as closely as possible.

 Equalising HEXFETs Circuit Diagram

For building prototypes or very short production runs this is feasible, but requires additional manual effort in testing the components, and, of course, more transistors must be ordered than will finally be used. The circuit idea shown here allows differences in gate-source voltage between pairs of transistors to be compensated for by the addition of trimmer potentiometers: the idea has been tested in simulation using Simetrix. The second circuit extract shows the required changes.