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| VCA810_16 Datasheet, PDF (23/40 Pages) Texas Instruments – VCA810 High Gain Adjust Range, Wideband and Variable Gain Amplifier | |||
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Application Information (continued)
VCA810
SBOS275G â JUNE 2003 â REVISED DECEMBER 2015
0.15
0.10
0.05
0
VIN = 1VPP
VIN = 100mVPP
-0.05
-0.10
VIN = 10mVPP
-0.15
-0.20
Time (5ms/div)
Figure 37. AGC Output Voltage for 100-kHz Sinewave at 10 mVPP, 100 mVPP, and 1 VPP
9.1.4 Stabilized Wein-Bridge Oscillator
Adding Wein-bridge feedback to the above AGC amplifier produces an amplitude-stabilized oscillator. As
Figure 38 shows, this alternative requires the addition of just two resistors (RW1, RW2) and two capacitors
(CW1, CW2).
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 CW
capacitors. As frequency increases, the decreasing impedance of the CW2 capacitor increases the feedback
factor. Simultaneously, the decreasing impedance of the CW1 capacitor decreases this factor. Analysis shows
1
that the maximum factor occurs at fW = 2pRWCW Hz, making this the frequency most conducive to oscillation. At
this frequency, the impedance magnitude of CW equals RW, 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, R1 begins charging CH negative,
increasing the amplifier gain from its minimum. When this gain reaches three, oscillation begins at fW; the
continued charging effect of R1 makes the oscillation amplitude grow. This growth continues until that amplitude
reaches a peak value equal to VR. Then, the AGC circuit counteracts the R1 effect, controlling the peak amplitude
at VR by holding the amplifier gain at a level of three. Making VR an AC signal, rather than a DC reference,
produces amplitude modulation of the oscillator output.
RW2
CW2
300W 4700pF
CW1
4700pF
RW1
300W
VCA810
f = 1/2pRW1CW1
RW1 = RW2
CW1 = CW2
R1
50kW
VC
R3
1kW
HP5082
R2
CH
50kW
1mF
OPA820
CC
10pF
VO
VOPEAK = VR
R4
100W
VR
0.1 VDC
V-
Figure 38. Amplitude-Stabilized Oscillator
Copyright © 2003â2015, Texas Instruments Incorporated
Product Folder Links: VCA810
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