Applications-Voltage Controlled Amplifiers (Continued) DS007980-10 FIGURE 3. Equivalent VCA Input CircuitStereo Volume Control If V is derived from a second signal source then the circuit C becomes an amplitude modulator or two-quadrant multiplier The circuit of Figure 4 uses the excellent matching of the two as shown in Figure 5, where: LM13600 amplifiers to provide a Stereo Volume Control with a typical channel-to-channel gain tracking of 0.3 dB. R is P provided to minimize the output offset voltage and may be replaced with two 510Ω resistors in AC-coupled applications. For the component values given, amplifier gain is derived for Figure 2 as being: DS007980-11 FIGURE 4. Stereo Volume Control www.national.com 8
