VCA810_16 Datasheet, PDF (26/40 Pages) Texas Instruments – VCA810 High Gain Adjust Range, Wideband and Variable Gain Amplifier

VCA810
SBOS275G – JUNE 2003 – REVISED DECEMBER 2015
Application Information (continued)
R₁
330W
V_I
C
0.047mF
R₂
330W
V_OA
OPA820
VCA810
V_O
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V_C
V_O
V_I
=
-
R₂
R₁
·
1
1
+
s
R₂C₂
G
f_P =
G
2pR₂C
G = 10^{-2 (VC + 1)}
Figure 41. Tunable Low-Pass Filter
The response control results from amplification of the feedback voltage applied to R₂. First, consider the case
where the VCA810 produces G = 1. Then, the circuit performs as if this amplifier were replaced by a short circuit.
Visually doing so leaves a simple voltage amplifier with a feedback resistor bypassed by a capacitor. This basic
G
circuit produces a response pole at ^{fP = 2pR}2^C.
For G > 1, the circuit applies a greater voltage to R₂, increasing the feedback current this resistor supplies to the
summing junction of the OPA820. The increased feedback current produces the same result as if R₂ had been
decreased in value in the basic circuit described above. Decreasing the effective R₂ resistance moves the circuit
G
pole to a higher frequency, producing the ^{fP = 2pR}2^C response control.
Finite loop gain and a signal-swing limitation set performance boundaries for the circuit. Both limitations occur
when the VCA810 attenuates, rather than amplifies, the feedback signal. These two limitations reduce the
circuit’s utility at the lower extreme of the VCA810 gain range. For −1 ≤ V_C ≤ 0, this amplifier produces
attenuating gains in the range from 0 dB to −40 dB. This range directly reduces the net gain in the circuit’s
feedback loop, increasing gain error effects. Additionally, this attenuation transfers an output swing limitation from
the OPA820 output to the overall circuit’s output. Note that OPA820 output voltage, V_OA, relates to V_O through
the expression, V_O = G × V_OA. Thus, a G < 1 limits the maximum V_O swing to a value less than the maximum
V_OA swing.
Figure 42 shows the low-pass frequency for different control voltages.
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