Applications:
Voltage Controlled Amplifiers
(Continued)
maximizes their linearizing action when balanced against
R
IN
. A value of 1 mA is recommended for I
D
unless the spe-
cific application demands otherwise.
DS007981-9
FIGURE 2. Voltage Controlled Amplifier
DS007981-10
FIGURE 3. Equivalent VCA Input Circuit
Stereo Volume Control
The circuit of
Figure 4
uses the excellent matching of the two
LM13700 amplifiers to provide a Stereo Volume Control with
a typical channel-to-channel gain tracking of 0.3 dB. R
P
is
provided to minimize the output offset voltage and may be
replaced with two 510Ω resistors in AC-coupled applications.
For the component values given, amplifier gain is derived for
Figure 2
as being:
If V
C
is derived from a second signal source then the circuit
becomes an amplitude modulator or two-quadrant multiplier
as shown in
Figure 5,
where:
The constant term in the above equation may be cancelled
by feeding I
S
x I
D
R
C
/2(V− + 1.4V) into I
O
. The circuit of
Fig-
ure 6
adds R
M
to provide this current, resulting in a
four-quadrant multiplier where R
C
is trimmed such that V
O
=
0V for V
IN2
= 0V. R
M
also serves as the load resistor for I
O
.
7
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