Everytime I start writing about three letter acronym devices like FETs and BJTs I am reminded of the fondness our chief engineer, Bob Stadtherr, has for the best recursive acronym of all time, TLA.
Acronyms aside, every schooled electrical engineer is familiar with the two main types of transistors used in audio circuits: FETs and BJTs, but how many understand their fundamental differences as it impacts sound quality?
Bipolar transistors, or BJTs, are what most circuits are built around. They are current-controlled devices meaning they operate by allowing a small current at their input to control a larger current flowing between the collector and emitter (their outputs). They are very linear, meaning they handle the audio signal with minimal distortion, especially at lower volumes. Bipolars can bring punch and dynamics to the music although used incorrectly they can also harden the sound and cause one's teeth to gnash.
FETs, on the other hand, are voltage-controlled. Unlike bipolars, which require a small current to function, FETs just need a change in voltage at their input to control the flow of current between their source and drain (outputs). This quality makes FETs quite different in character. Because of their high input impedance and voltage-driven nature, FETs tend to sound warmer and smoother. They’re often compared to tubes (also voltage driven devices), with a more relaxed presentation that can make music sound richer and less clinical. On the downside, used incorrectly they can soften to the point of blurry mush.
This knowledge of using one type of device over another in an effort to shape the sound quality of an amplification device is not taught in engineering school.
Which is why I think it's important to keep those who care about music and how it performs in the home informed about all the little stuff that isn't taught in school.
