Page 63 - Analog Circuit Design Art, Science, and Personalities

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

Figure 7-1.
One of the
original Hewlett-
Packard Model
200A oscillators
-the good guys
won and nobody
lost. (Photo
courtesy of
Hewlett-Pa cka rd
Company.)

(DeForest hadn’t even dreamed of adding a third element to Edison’s Effect in
1891), so he couldn’t readily get anything to oscillate. Anyway, Wien was preoccu-
pied with other problems and developed the network for AC bridge measurements.
Hewlett saw that Wien’s network, combined with suitably controlled electronic
gain, offered significant potential improvements over approaches then used to make
oscillators. These included dynamic tuning range, amplitude and frequency sta-
bility, low distortion, and simplicity.
Hewlett had something else besides electronic gain available; he also had the
new tools of feedback theory. Harold S. Black’s pioneering work, “Stabilized Feed-
back Amplifier,” appears as the fourth reference in the thesis bibliography. Simi-
larly, Nyquist’s “Regeneration Theory,” a classic describing necessary conditions
for oscillation, is reference number three.
Hewlett synthesized all this nicely to show that Wien’s network could be made
to oscillate. Then he added a single (quite literally) crucial element. The oscillator’s
gain must be carefully controlled to support stable sinusoidal oscillation. If gain is
too low, oscillation will not occur. Conversely, excessive gain forces limit cycles,
creating a square wave oscillator. The problem is to introduce an amplitude regu-
lation mechanism that does not generate output waveform distortion. Hewlett
describes the elegant solution:

The last requirement, an amplitude-limiting
device that will not introduce distortion, is more
difficult to achieve. It is well known that the gain
of an amplifier with negative feedback is 1/R, providing
AD is large compared to 1. Thus if a resistance whose
value increases with the current through it is used as
past of the negative feedback network, the gain of the
amplifier may be made to decrease with an increase in
the input voltage. If an amplifier of this type is
used as part of the oscillator, it can be adjusted so
that oscillations will just start. As oscillations

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