Sonny,
I really wouldn't spend much time trying to track down the article. The first 3/4s of it is simply a reprise of Self's article on amplifier distortion which is accessable at Self's website. So here is what you're missing -
"Supringly, the output devices do not contribute increased distortion with increased loading- it all comes from the drivers.
There are good SPICE-simulator evidence that this expanation is correct and LSN (Large Signal Nonlinearity) is entirely due to the beta-droop [what an unfortuneate choice of terminology] causing exta current to be drawn from the drivers, degrading their linerarity. Here is the evidence:
- Simulated output stages buit from output transistors modified to have no beta-droop at all (simply by increasing the SPICE model parameter IKF) show no LSN...
-Simulated output devices driven with zero-impedence voltage sources instead of transistor drivers show no LSN. This shows that LSN does not occur in the output transistors themselves, but in the drivers.
- If you look at the output stage distortion as an error voltage between input and output, the double emmitter follower (EF) stage errror is error=(driverVbe + OutputVbe +Re drop)
A simulated EF output stage with the usual drivers shows that of these three terms, it is primatily nonlinerarity in the driver Vbe that increases as the load resestance reduces, while th output Vbe nonlinerarity is hardly changed. The voltage drop across Re is essentially linerar; as it is in series with the load."
Thats about it, until the second part is published.
I do find Self's comment about the inviolate nature of a single transistor driver somewhat specious. Among those who've incorporated Self's findings, AKSA & LC Audio, both have adopted the EF topology (including the speedup cap accross the resistor linking the emmitters of the driver) through the unique approach of actually listening prior to choosing an output topology that incorporates extended beta outputs. In addition to that adoption, LC has chosen to address the current drive limitations of the driver current by using a cascade of 4 emmitter followers for each rail....
http://www.lcaudio.com/temil.htm
Golly, talk about brute force!
I'm certainly not criticizing Self's (or anyone's) simulation approach to design. It is through the use of these powerful tools that the state of the art is advanced. I just don't happen to agree that objective measures should necessarily superceed subjective metrics.
