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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
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