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VCA810 データシートの表示(PDF) - Texas Instruments

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VCA810 High Gain Adjust Range, Wideband and Variable Gain Amplifier Texas-Instruments
Texas Instruments Texas-Instruments
VCA810 Datasheet PDF : 40 Pages
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Application Information (continued)
VCA810
SBOS275G – JUNE 2003 – REVISED DECEMBER 2015
3
0
-3
VC = -1.4V
-6
VC = -1.6V
-9
VC = -2V
VC = -1.8V
-12
-15
10k
100k
1M
10M
Frequency (Hz)
Figure 42. Voltage-Controlled Low-Pass Filter Frequency Response
9.1.7 Tunable Equalizer
A circuit analogous to the above low-pass filter produces a voltage-controlled equalizer response. The gain
control provided by the VCA810 of Figure 43 varies this circuit response zero from 1 Hz to 10 kHz, according to
the relationship of Equation 9:
G
fZ 2pGR1C
(9)
To visualize the circuit’s operation, consider a circuit condition and an approximation that permit replacing the
VCA810 and R3 with short circuits. First, consider the case where the VCA810 produces G = 1. Replacing this
amplifier with a short circuit leaves the operation unchanged. In this shorted state, the circuit is simply a voltage
amplifier with an R-C bypass around R1. The resistance of this bypass, R3, serves only to phase-compensate the
circuit, and practical factors make R3 << R1. Neglecting R3 for the moment, the circuit becomes just a voltage
1
amplifier with a capacitive bypass of R1. This circuit produces a response zero at fZ 2pR1C.
Adding the VCA810 as shown in Figure 43 permits amplification of the signal applied to capacitor C, and
produces voltage control of the frequency fZ. Amplified signal voltage on C increases the signal current
conducted by the capacitor to the operational amplifier feedback network. The result is the same as if C had
been increased in value to GC. Replacing C with this effective capacitance value produces the circuit control
1
expression fZ 2pR1GC .
R1
750W
R2
750W
OPA820
VI
50W
C
R3
2mF 3W
VOA
VCA810
OPA846
VO
50W
VC
fZ
1
2p(GR1 +
R3C)
with G = 10-2 (VC + 1)
Figure 43. Tunable Equalizer
Copyright © 2003–2015, Texas Instruments Incorporated
Product Folder Links: VCA810
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