Datasheet
G
2 R Cp
2
f =
P
G =
10
-2
(V + 1)
C
R
2
330W
VCA810
R
1
470W
V
R
-10mV
V
C
500W 500W
OPA698
V
IN
V
L
V
I
-3.4V
+0.5V
V = V- x 10
OL R
-2
R V
R + R
1 IN
1 2
+1
(
(
OPA820
VCA810
V
C
V
O
V
OA
R
2
330W
R
1
330W
C
0.047 Fm
V
I
G
2 R Cp
2
f =
P
V
V
O
I
R
R
2
1
= -
R C
G
2 2
1 + s
1
·
G = 10
-2
(V + 1)
C
Input Voltage (V)
+3.0 +2.5 +2.0 +1.5 +1.0 +0.5 0
1
0.1
0.01
0.001
Output Voltage (V)
f =
P
G
2 R Cp
2
VCA810
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SBOS275F –JUNE 2003–REVISED DECEMBER 2010
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
VOLTAGE-CONTROLLED LOW-PASS FILTER
achieved by using a voltage limiting amplifier.
In the circuit of Figure 39, the VCA810 serves as the
variable-gain element of a voltage-controlled
LOW-DRIFT, WIDEBAND EXPONENTIAL AMP
low-pass filter. This section discusses how this
A common use of the log amp above involves signal implementation expands the circuit voltage swing
compounding. The inverse function, signal capability over that normally achieved with the
expanding, requires an exponential transfer function. equivalent multiplier implementation. The circuit
The VCA810 produces this latter response directly, response pole responds to control voltage V
C
as shown in Figure 37. DC reference V
R
again sets according to the relationship in Equation 3:
the amplifier input voltage, and the input signal V
IN
now drives the gain control point. Resistors R
1
and R
2
(3)
attenuate this drive to prevent overloading the gain
control input. Setting these resistors at the same
where
values as in the preceding log amp produces an
exponential amplifier with the inverse function of the
With the components shown, the circuit provides a
log amp.
linear variation of the low-pass cutoff from 300Hz to
1MHz.
Figure 37. Exponential Amplifier
Testing the circuit given in Figure 37 gives the
exponential response shown in Figure 38.
Figure 39. Tunable Low-Pass Filter
The response control results from amplification of the
feedback voltage applied to R
2
. 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 .
Figure 38. Exponential Amplifier Response
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