Datasheet

LM13700
SNOSBW2E –NOVEMBER 1999–REVISED MARCH 2013 www.ti.com
Figure 26. Amplitude Modulator
Figure 27. Four-Quadrant Multiplier
Noting that the gain of the LM13700 amplifier of Figure 24 may be controlled by varying the linearizing diode
current I
D
as well as by varying I
ABC
, Figure 28 shows an AGC Amplifier using this approach. As V
O
reaches a
high enough amplitude (3V
BE
) to turn on the Darlington transistors and the linearizing diodes, the increase in I
D
reduces the amplifier gain so as to hold V
O
at that level.
Voltage Controlled Resistors
An Operational Transconductance Amplifier (OTA) may be used to implement a Voltage Controlled Resistor as
shown in Figure 29. A signal voltage applied at R
X
generates a V
IN
to the LM13700 which is then multiplied by
the g
m
of the amplifier to produce an output current, thus:
(10)
where g
m
≈ 19.2I
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 LM13700 input.
Figure 30 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 31, where each “end” of the “resistor” may be at
any voltage within the output voltage range of the LM13700.
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