LM13600M datasheet(19/24 Pages) NSC

19 / 24 page

Additional Applications (Continued)

The circuit of

Figure 28 is a voltage reference of variable

temperature coefficient. The 100 k

Ω 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 ac-

complished by balancing the TC of the A2 transfer function

against the complementary TC of D1.

The log amplifier of

Figure 29 responds to the ratio of cur-

rents through buffer transistors Q3 and Q4. Zero tempera-

ture dependence for V

OUT is ensured because the TC of the

A2 transfer function is equal and opposite to the TC of the

logging transistors Q3 and Q4.

The wide dynamic range of the LM13600 allows easy control

of the output pulse width in the Pulse Width Modulator of

Fig-

ure 30.

For generating I

ABC over a range of 4 to 6 decades of cur-

rent, the system of

Figure 31 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 = −VC/RC.

The differential voltage between Q1 and Q2 is attenuated by

the R1, R2 network so that A1 may be assumed to be oper-

ating within its linear range. From

Equation (5), the input volt-

age to A1 is:

The voltage on the base of Q1 is then

The ratio of the Q1 and Q2 collector currents is defined by:

Combining and solving for I

ABC yields:

This logarithmic current can be used to bias the circuit of

Fig-

ure 4 provide a temperature independent stereo attenuation

characteristic.

DS007980-35

FIGURE 28. Delta VBE Reference

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