Datasheet
R
2
453W
+V
IN
R
G+
R
G-
-V
IN
R
9
432W
C
9
10 Fm
R
8
50W
R
5
50W
R
1
20W
R
10
75W
VCA824
R
21
3kW
C
5
4pF
R
17
6kW
C
6
320 Fm
R
18
13.6kW
C
7
300 Fm
V
G DC
=+1V
75 LoadW
V
G
GND
V
REF
FB
V
OUT
V
IN
V
OUT
VOLTAGE-CONTROLLED LOW-PASS FILTER
G
2 R Cp
2
f =
8
(5)
V
OUT
V
IN
R
2
R
1
1
1+s
R C
2
G
= - ´
(4)
- £0.8V V 0.8V
G
£
(6)
R
F
1kW
+V
IN
R
G+
R
G-
-V
IN
FB
R
G
200W
20W
V
OUT
VCA824
Out
V
G
OPA690
24pF
C
V
IN
R
332W
2
R
332W
1
24pF
50W
V +1
G
2
R
F
R
G
G=2 ´ ´
(7)
VCA824
SBOS394C – NOVEMBER 2007 – REVISED DECEMBER 2008 .......................................................................................................................................
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Figure 83. Differential Cable Equalizer
by a short circuit. Visually replacing the amplifier by a
short leaves a simple voltage-feedback amplifier with
a feedback resistor bypassed by a capacitor.
In the circuit of Figure 84 , the VCA824 serves as the
Replacing this gain with a variable gain, G, the pole
variable-gain element of a voltage-controlled
can be written as shown in Equation 5 :
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
Because the VCA824 is most linear in the midrange,
control voltage, V
G
, is calculated as according to the
the median of the adjustable pole should be set at V
Gsimplified relationship described in Equation 4 :
= 0V (see Figure 24 , Figure 44 , Figure 65 , and
Equation 6 ). Selecting R
1
= R
2
= 332 Ω , and targeting
a median frequency of 10MHz, the capacitance (C) is
24pF. Because the OPA690 was selected for the
circuit of Figure 84 , and in order to limit peaking in
the OPA690 frequency response, a capacitor equal to
C was added on the inverting mode to ground. This
architecture has the effect of setting the
high-frequency noise gain of the OPA690 to +2V/V,
ensuring stability and providing flat frequency
response.
Once the median frequency is set, the maximum and
minimum frequencies can be determined by using V
G
= – 0.8V and V
G
= +0.8V in the gain equation of
Equation 7 . Note that this is a first-order analysis and
does not take into consideration the open-loop gain
limitation of the OPA690.
Figure 84. Voltage-Control Low-Pass Filter
The response control results from amplification of the
feedback voltage applied to R
2
. First, consider the
With the components shown, the circuit provides a
case where the VCA824 produces G = 1V/V. Then
linear variation of the low-pass cutoff from 2MHz to
this circuit performs as if the amplifier were replaced
20MHz, using – 1V ≤ V
G
≤ +1V.
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