Datasheet

LM13700
SNOSBW2E –NOVEMBER 1999–REVISED MARCH 2013 www.ti.com
Figure 48. True RMS Converter
The circuit of Figure 49 is a voltage reference of variable Temperature Coefficient. The 100 kΩ potentiometer
adjusts the output voltage which has a positive TC above 1.2V, zero TC at about 1.2V, and negative TC below
1.2V. This is accomplished by balancing the TC of the A2 transfer function against the complementary TC of D1.
The wide dynamic range of the LM13700 allows easy control of the output pulse width in the Pulse Width
Modulator of Figure 50.
For generating I
ABC
over a range of 4 to 6 decades of current, the system of Figure 51 provides a logarithmic
current out for a linear voltage in.
Since the closed-loop configuration ensures that the input to A2 is held equal to 0V, the output current of A1 is
equal to I
3
= −V
C
/R
C
.
The differential voltage between Q1 and Q2 is attenuated by the R1,R2 network so that A1 may be assumed to
be operating within its linear range. From Equation 5, the input voltage to A1 is:
(11)
The voltage on the base of Q1 is then
(12)
The ratio of the Q1 and Q2 collector currents is defined by:
(13)
Combining and solving for I
ABC
yields:
(14)
This logarithmic current can be used to bias the circuit of Figure 25 to provide temperature independent stereo
attenuation characteristic.
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