Frequency (Hz)
10k 100k 1M 10M
V = 1.4V-
C
V = 2V-
C
V = 1.6V-
C
V = 1.8V-
C
3
0
3
6
9
12
15-
-
-
-
-
Gain (dB)
OPA846
VCA810
V
C
50W
50W
OPA820
V
OA
C
2 Fm
R
3
3W
V
O
R
2
750W
R
1
750W
V
I
f
Z
≈
1
2 (GR + R C)p
1 3
with G = 10
-2
(V + 1)
C
VCA810
SBOS275F –JUNE 2003–REVISED DECEMBER 2010
www.ti.com
For G > 1, the circuit applies a greater voltage to R
2
, TUNABLE EQUALIZER
increasing the feedback current this resistor supplies
A circuit analogous to the above low-pass filter
to the summing junction of the OPA820. The
produces a voltage-controlled equalizer response.
increased feedback current produces the same result
The gain control provided by the VCA810 of
as if R
2
had been decreased in value in the basic
Figure 41 varies this circuit response zero from 1Hz
circuit described above. Decreasing the effective R
2
to 10kHz, according to the relationship of Equation 4:
resistance moves the circuit pole to a higher
(4)
frequency, producing the response
control.
To visualize the circuit’s operation, consider a circuit
condition and an approximation that permit replacing
Finite loop gain and a signal-swing limitation set
the VCA810 and R
3
with short circuits. First, consider
performance boundaries for the circuit. Both
the case where the VCA810 produces G = 1.
limitations occur when the VCA810 attenuates, rather
Replacing this amplifier with a short circuit leaves the
than amplifies, the feedback signal. These two
operation unchanged. In this shorted state, the circuit
limitations reduce the circuit’s utility at the lower
is simply a voltage amplifier with an R-C bypass
extreme of the VCA810 gain range. For −1 ≤ V
C
≤ 0,
around R
1
. The resistance of this bypass, R
3
, serves
this amplifier produces attenuating gains in the range
only to phase-compensate the circuit, and practical
from 0dB to −40dB. This range directly reduces the
factors make R
3
<< R
1
. Neglecting R
3
for the
net gain in the circuit’s feedback loop, increasing gain
moment, the circuit becomes just a voltage amplifier
error effects. Additionally, this attenuation transfers
with a capacitive bypass of R
1
. This circuit produces
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,
a response zero at .
V
O
= G ● V
OA
. Thus, a G < 1 limits the maximum V
O
Adding the VCA810 as shown in Figure 41 permits
swing to a value less than the maximum V
OA
swing.
amplification of the signal applied to capacitor C, and
Figure 40 shows the low-pass frequency for different
produces voltage control of the frequency f
Z
.
control voltages.
Amplified signal voltage on C increases the signal
current conducted by the capacitor to the op amp
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 expression .
Figure 40. Voltage-Controlled Low-Pass Filter
Frequency Response
Figure 41. Tunable Equalizer
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Product Folder Link(s): VCA810
