Page 67 - Analog Circuit Design Art, Science, and Personalities

Max Wien, Mr. Hewlett, and a Rainy Sunday Afternoon

Figure 7-7.
A wide range
version of the
basic circuit.
Multiple lamps
provided longer
gain loop time
constant,
improving low
frequency
distortion.

frequency descends towards the lamp’s thermal time constant. This effect can be
attenuated by increasing the lamp’s thermal time constant. The easiest way to do
this is to add more and bigger lamps. This causes longer amplitude settling times,
but low frequency distortion is reduced. Plotting distortion versus frequency clearly
shows this (see Figure 7-8).
Looking at the plot, I wondered just how far distortion performance could be
pushed using Hewlett’s suppositions and conclusions as a guide. The multi-lamp
experiment indicates that distortion rise at low frequencies is almost certainly due
to the lamp’s thermal time constant. But what causes the slight upward tilt around
15 to 20 kc? And just what limits distortion performance? Chasing all this down

Figure 7-8.
Distortion versus
frequency for the
wide range
oscillator. The
effect of the
multiple lamp
approach is
clearly evident,
but what causes
increasing high
frequency
distortion?

48