Designing a High-Performance Audio BJT

An ideal high fidelity audio amplifier would perfectly reproduce the input signal on the output plus add voltage, current, or power gain. It would add no spectral content. It would be perfectly transparent. BJTs have many flaws due to the fundamental nature of the device that keeps them from being ideal in audio applications. These flaws include: noise, non-linear transfer function, low input impedance, limited gain and voltage breakdown, thermal sensitivity, limited current and power handling capability, gain and device capacitances that vary with the applied voltage and temperature, asymmetrical amplification, non-linear input current, and overload issues. Noise can mask the signal of interest. The non-linear transfer function contributes to an inability to exactly reproduce the input signal resulting in poor THD performance. Low input impedance can load the driving circuit producing signal distortion and reduced input signal levels. Device gain and capacitance that varies with the signal level acts to modulate the signal and limits its ability to reproduce the input signal exactly. Many of the problems are a result of the application circuit. But most result from the BJT. This article shows how to correct for many of the inherent BJT defects using a circuit that controls many of the inherent parameters and can improve the BJT THD performance by as much as 20X. Even better, the circuit looks from the outside as a much improved 3-pin BJT that can be plugged in for the original BJT with little or no circuit modification.