VCA810_12 Datasheet, PDF (15/30 Pages) Texas Instruments – High Gain Adjust Range, Wideband, VARIABLE GAIN AMPLIFIER

VCA810
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Zone III: V_C < −2V. The VCA810 control pin is out of
range, and some measure should be taken so that it
does not exceed –2.5V. A limiting action could be
achieved by using a voltage limiting amplifier.
LOW-DRIFT, WIDEBAND EXPONENTIAL AMP
A common use of the log amp above involves signal
compounding. The inverse function, signal
expanding, requires an exponential transfer function.
The VCA810 produces this latter response directly,
as shown in Figure 37. DC reference V_R again sets
the amplifier input voltage, and the input signal V_IN
now drives the gain control point. Resistors R₁ and R₂
attenuate this drive to prevent overloading the gain
control input. Setting these resistors at the same
values as in the preceding log amp produces an
exponential amplifier with the inverse function of the
log amp.
500W
V_IN
V_R
-10mV
+0.5V
V_I
OPA698
V_L
-3.4V
500W
VCA810
V_C
R₂
330W
( _{VOL = -VR x 10}^-2
R₁V_{IN +1}
R₁ + R₂
R₁
470W
Figure 37. Exponential Amplifier
Testing the circuit given in Figure 37 gives the
exponential response shown in Figure 38.
1
0.1
0.01
0.001
+3.0 +2.5 +2.0 +1.5 +1.0 +0.5
0
Input Voltage (V)
Figure 38. Exponential Amplifier Response
SBOS275F – JUNE 2003 – REVISED DECEMBER 2010
VOLTAGE-CONTROLLED LOW-PASS FILTER
In the circuit of Figure 39, the VCA810 serves as the
variable-gain element of a voltage-controlled
low-pass filter. This section discusses how this
implementation expands the circuit voltage swing
capability over that normally achieved with the
equivalent multiplier implementation. The circuit
response pole responds to control voltage V_C
according to the relationship in Equation 3:
f_P =
G
2pR₂C
(3)
where G = ^{10-2 (VC + 1)}
With the components shown, the circuit provides a
linear variation of the low-pass cutoff from 300Hz to
1MHz.
R₁
330W
V_I
C
0.047mF
R₂
330W
V_OA
OPA820
VCA810
V_O
V_C
V_O
V_I
=
-
R₂
R₁
·
1
1
+
s
R₂C₂
G
f_P =
G
2pR₂C
G = 10^{-2 (VC + 1)}
Figure 39. 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 circuit produces a response
pole
at
f_P
=
G
2pR₂C_.
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