controlled impedance driver for distortion reduction


Paid Member
2002-07-12 9:55 am
San Diego
This is probably an impractical idea, but it gives me an excuse to point to one of my favorite research papers.

The basic idea is to control driving impedance to a single ended BJT common emitter stage, to an optimal value from the point of view of distortion.

Anyway, here's a nice paper about controlling impedance: [It's not about audio]

It is possible to show that by driving a bipolar common emitter stage from a resistance equal to 2Rpi, ie 2*beta*(Vth/Ic) where Vth is the thermal voltage kT/q, the third order distortion [ideally] disappears.

The silly/impractical idea is to use a circuit like the one described above in the paper, to control the output impedance of a driver stage, to reduce third order IM in a BJT amplifier. You cold presumably use some contraption of current mirrors and whatnot to (automatically and continuously) figure out the beta of the transistor, and adjust the drive impedance accordingly.

Another method, perhaps more applicable and more practical, of third order IM cancellation, is described inthe paper below.

The idea here is that if you use appropriate emitter degeneration, ie Re = 2Vth/Ic, you achieve the same result.

Anyway, I haven't experimented with either of these ideas. But the Bram Nauta paper is a nice read, and offers an elegant solution to a problem (that admittedly isn't directly related to audio) that shows good combination of sound engineering and creativity.