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SBOS275F – JUNE 2003 – REVISED DECEMBER 2010
STABILIZED WEIN-BRIDGE OSCILLATOR
Adding Wein-bridge feedback to the above AGC
amplifier produces an amplitude-stabilized oscillator.
As
shows, this alternative requires the
addition of just two resistors (R
W1
, R
W2
) and two
capacitors (C
W1
, C
W2
).
Connecting the feedback network to the amplifier
noninverting input introduces positive feedback to
induce oscillation. The feedback factor displays a
frequency dependence due to the changing
impedances of the C
W
capacitors. As frequency
increases, the decreasing impedance of the C
W2
capacitor
increases
the
feedback
factor.
Simultaneously, the decreasing impedance of the
C
W1
capacitor decreases this factor. Analysis shows
1
f
W
=
2
p
R
W
C
W
Hz,
that the maximum factor occurs at
making this the frequency most conducive to
oscillation. At this frequency, the impedance
R
W2
300W
C
W2
4700pF
magnitude of C
W
equals R
W
, and inspection of the
circuit shows that this condition produces a feedback
factor of 1/3. Thus, self-sustaining oscillation requires
a gain of three through the amplifier. The AGC
circuitry establishes this gain level. Following initial
circuit turn-on, R
1
begins charging C
H
negative,
increasing the amplifier gain from its minimum. When
this gain reaches three, oscillation begins at f
W
; the
continued charging effect of R
1
makes the oscillation
amplitude grow. This growth continues until that
amplitude reaches a peak value equal to V
R
. Then,
the AGC circuit counteracts the R
1
effect, controlling
the peak amplitude at V
R
by holding the amplifier gain
at a level of three. Making V
R
an ac signal, rather
than a dc reference, produces amplitude modulation
of the oscillator output.
C
W1
4700pF
R
W1
300W
VCA810
V
C
f = 1/2
p
R
W1
C
W1
R
W1
= R
W2
C
W1
= C
W2
V
O
V
OPEAK
= V
R
R
3
1kW
HP5082
OPA820
R
4
100W
R
1
50kW
R
2
50kW
C
H
1
m
F
V
R
0.1 VDC
C
C
10pF
V-
Figure 34. Amplitude-Stabilized Oscillator
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