Temperature compensating an EF3 audio power amplifier output stage is not a trifling task. There are 6 Vbe junctions, running at 3 different current densities, different temperatures and neither is the thermal performance of the heatsink assembly characterized in most DIY cases. This makes deploying a conventional 2 transistor Vbe spreader (or Vbe multiplier as it is sometimes called) a little less than straight forward if one is to secure decent temperature compensation – i.e. to within 10% over the full operating range. The article below explores a slightly different approach to the problem in which, during the development phase, the compensation is designed to intersect the ideal bias voltage at two different temperatures, ideally the first at ambient and the second up above 50 deg C – a technique I later called ‘two point temperature compensation’. Some time after writing this in 2010, I started the design and then construction work of the e-Amp, and used what I learned writing this up to solve the problem using an NTD thermistor, which you can read about in the e-Amp article on pages 37 to 40.
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