FET vs BJT: Fundamental Difference
| Parameter | FET / MOSFET | BJT |
|---|---|---|
| Control Mechanism | Voltage-controlled (like vacuum tubes) | Current-controlled |
| Transfer Characteristic | Square-law (quadratic) | Exponential |
| Harmonic Profile | Even-order dominant (2nd, 4th…) | Odd-order dominant (3rd, 5th…) |
| Perceived Sound | Warm, smooth, “tubey” | Direct, clean, analytical |
| Clipping Behavior | Soft (gradual, musical) | Sharp (abrupt, harsh) |
| Manufacturing | Static-sensitive, tight matching | Mature process, high yield |
| TRASAM Example | A12 PURE (16× Infineon MOSFETs) | AA3PRO, A12PRO (Sanken BJTs) |
Why FETs Sound “Tube-Like”
The square-law transfer function of FETs produces harmonic distortion that is dominantly even-order (2nd harmonic at -60dB, 4th at -80dB, etc.). The human ear perceives even-order harmonics as “warmth” and “musicality.” This is the same harmonic profile that gives vacuum tube amplifiers their characteristic sound — FETs replicate it in solid-state form, without the heat, fragility, and tube replacement costs.
Why FET Amplifiers Cost More
- Circuit design complexity: FET circuits require dedicated design — can’t reuse BJT circuit templates
- Matching requirements: A12 PURE’s 16 MOSFETs must be matched to ±2% for balanced operation
- Static sensitivity: MOSFETs are ESD-sensitive. Manufacturing, shipping, and QC require anti-static protocols throughout
- Lower yield: ~20-30% lower production yield compared to BJT manufacturing
Which to Choose?
- Prefer FET if: you love tube sound but don’t want tube maintenance, listen to vocals/jazz/acoustic, enjoy a more relaxed presentation
- Prefer BJT if: you want maximum accuracy and neutrality, listen to classical/orchestral/electronic, prefer fast transient response
