Posted by Al Sekela on February 16, 2003 at 14:40:27:
In Reply to: Anyone heard of Chapter Audio Amps? posted by Rick on February 15, 2003 at 10:46:49:
I've not heard the Chapter amplifier, but I reviewed the web site. It sounds like competent people designed the amplifier and wrote the web site technical description.
This is another solid-state amplifier that uses feedback. It uses complementary MOS transistors, according to the website. This is a fundamental weakness in the design because complementary transistors means the output stage requires both NMOS and PMOS power FETs. NMOS means N-channel Metal Oxide Semiconductor, while PMOS means P-channel MOS. N-channel transistors use p-type silicon and the charge carriers are electrons, while P-channel transistors use n-type silicon and the charge carriers are holes. The NMOS devices conduct during the positive half-cycle of the output wave, while the PMOS devices conduct during the negative half-cycle.
This is convenient for circuit designers, because the voltage offset between the two types of output device is small and the layout is simple.
However, holes and electrons have different mobilities in silicon. This means it takes larger PMOS transistors to get the same overall conductivity compared to NMOS. Larger transistors have more capacitance, so the driver stage is faced with an asymmetrical load when the positive and negative-going swings of the output signal are compared. Odd-order harmonic distortion would be the primary result of this asymmetry, and driver stage impedance and feedback characteristics would have to be chosen carefully to minimize it. Like all things in audio, "minimize" is a matter of taste, and what one person thinks is negligible distortion might be excruciating to another. Careful auditioning is advised.