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