LM13600M datasheet(9/24 Pages) NSC

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Stereo Volume Control (Continued)

The constant term in the above equation may be cancelled

by feeding I

S xIDRC/2 (V

− + 1.4V) into I

O. The circuit of Fig-

ure 6 adds R

to provide this current, resulting in a

four-quadrant multiplier where R

C is trimmed such that VO =

0V for V

IN2 = 0V. RM also serves as the load resistor for IO.

Noting that the gain of the LM13600 amplifier of

Figure 3

may be controlled by varying the linearizing diode current I

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.

Voltage Controlled Resistors

An Operational Transconductance Amplifier (OTA) may be

used to implement a Voltage Controlled Resistor as shown in

Figure 8. A signal voltage applied at R

X generates a VIN 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 IABC at 25˚C. Note that the attenuation of

O by R and RA is necessary to maintain VIN within the linear

range of the LM13600 input.

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.

DS007980-12

FIGURE 5. Amplitude Modulator

DS007980-13

FIGURE 6. Four-Quadrant Multiplier

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