Equalising HEXFETs


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.

Circuit

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.

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