Page 69 - Analog Circuit Design Art, Science, and Personalities

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

Figure 7-10.
Performance for
the "lampless"
oscillator.
Modern tech-
nology is almost
100 times worse!

itself and is (thank the gods) refreshingly simple. The JFET is a fairly symmetrical
structure, although this circuit drives it asymmetrically from gate to source. If you
arrange things so the gate is driven with a signal halfway between the drain and
source, symmetry is reestablished. This symmetrical drive eliminates all even-order
harmonics. Q1 's new companions make things look like Figure 7- 1 1.
With the trimmer set to provide the optimum amount of feedback, distortion
dropped to just 0.0018%-the analyzer's specified limit (see Figure 7-12).

Figure 7-11.
The local feed-
back network
around Q1,
intended to cure
channel resis-
tance modulation
effect.

Figure 7-12.
Results of Ql's
local feedback output ~V/DIV
fix. Distortion
improves to
0.0018% .... about
as good as the Distortion .0018%
light bulb.
A2 Output (AC
coupled) . 1 VDIV

Horiz. =
200psec/DIV

50