Stereo Volume Control (Continued) DS007980-12 FIGURE 5. Amplitude Modulator The constant term in the above equation may be cancelled Figure 8. A signal voltage applied at R generates a V to X IN by feeding I x I R /2 (V− + 1.4V) into I . The circuit of Fig- the LM13600 which is then multiplied by the g of the ampli- S D C O m ure 6 adds R to provide this current, resulting in a fier to produce an output current, thus: M four-quadrant multiplier where R is trimmed such that V = C O 0V for V = 0V. R also serves as the load resistor for I . IN2 M O Noting that the gain of the LM13600 amplifier of Figure 3 may be controlled by varying the linearizing diode current ID as well as by varying I , Figure 7 shows an AGC Amplifier ABC where g ≈ 19.2 I at 25˚C. Note that the attenuation of m ABC using this approach. As V reaches a high enough amplitude O V by R and R is necessary to maintain V within the linear O A IN (3 V ) to turn on the Darlington transistors and the lineariz- BE range of the LM13600 input. ing diodes, the increase in I reduces the amplifier gain so D Figure 9 shows a similar VCR where the linearizing diodes as to hold V at that level. O are added, essentially improving the noise performance of the resistor. A floating VCR is shown in Figure 10, where Voltage Controlled Resistors each “end” of the “resistor” may be at any voltage within the An Operational Transconductance Amplifier (OTA) may be output voltage range of the LM13600. used to implement a Voltage Controlled Resistor as shown in DS007980-13 FIGURE 6. Four-Quadrant Multiplier 9 www.national.com
