Stereo Volume Control
(Continued)
DS007980-12
FIGURE 5. Amplitude Modulator
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
.
Noting that the gain of the LM13600 amplifier of
Figure 3
may be controlled by varying the linearizing diode current I
D
as well as by varying I
ABC
,
Figure 7
shows an AGC Amplifier
using this approach. As V
O
reaches a high enough amplitude
(3 V
BE
) to turn on the Darlington transistors and the lineariz-
ing diodes, the increase in I
D
reduces the amplifier gain so
as to hold V
O
at that level.
Figure 8.
A signal voltage applied at R
X
generates a V
IN
to
the LM13600 which is then multiplied by the g
m
of the ampli-
fier to produce an output current, thus:
where g
m
≈
19.2 I
ABC
at 25˚C. Note that the attenuation of
V
O
by R and R
A
is necessary to maintain V
IN
within the linear
range of the LM13600 input.
Voltage Controlled Resistors
An Operational Transconductance Amplifier (OTA) may be
used to implement a Voltage Controlled Resistor as shown in
Figure 9
shows a similar VCR where the linearizing diodes
are added, essentially improving the noise performance of
the resistor. A floating VCR is shown in
Figure 10,
where
each “end” of the “resistor” may be at any voltage within the
output voltage range of the LM13600.
DS007980-13
FIGURE 6. Four-Quadrant Multiplier
9
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