VCA810_16 Datasheet, PDF (27/40 Pages) Texas Instruments – VCA810 High Gain Adjust Range, Wideband and Variable Gain Amplifier

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Application Information (continued)
VCA810
SBOS275G – JUNE 2003 – REVISED DECEMBER 2015
3
0
-3
V_C = -1.4V
-6
V_C = -1.6V
-9
V_C = -2V
V_C = -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
f_Z ≈ _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 ≈ 2pR}1^C.
Adding the VCA810 as shown in Figure 43 permits amplification of the signal applied to capacitor C, and
produces voltage control of the frequency f_Z. 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 G_C. Replacing C with this effective capacitance value produces the circuit control
1
expression ^{fZ ≈ 2pR}1^GC .
R₁
750W
R₂
750W
OPA820
V_I
50W
C
R₃
2mF 3W
V_OA
VCA810
OPA846
V_O
50W
V_C
f_Z
≈
1
2p(GR₁ +
R₃C)
with G = 10^{-2 (VC + 1)}
Figure 43. Tunable Equalizer
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