Pulse and Digital Circuits

7.3 AN ASTABLE MULTIVIBRATOR AS A VOLTAGE-CONTROLLED OSCILLATOR

A voltage-controlled oscillator (VCO) is one in which the frequency of oscillations varies as a function of voltage. The same circuit is also called a voltage-to-frequency converter (VFC) because a given voltage gives rise to a specific frequency. An astable multivibrator is used as voltage-controlled oscillator [see Fig. 7.13(a)]. A comparison of this with the circuit shown in Fig. 7.1 shows that here the two resistors R and R are returned to a separate source V_BB rather than to V_CC.

When Q₁ is OFF and Q₂ is ON, C₁ charges. When Q₁ is ON, the charge on C₁ decays with a time constant τ₁ = RC₁ as shown in Fig. 7.13(b). As a result, the voltage at the base of Q₂, V_B2 varies with time, ...