Pulse and Digital Circuits

7.5 EMITTER-COUPLED ASTABLE MULTIVIBRATORS

The main drawback of a collector-coupled astable multivibrator is that the circuit may not necessarily oscillate when V_CC is switched ON. It is possible that both Q₁ and Q₂ may be ON because of the inherent symmetry in the circuit. For oscillations to build up, one device may need to be switched into the OFF state by connecting its base to the emitter for a short duration. This disadvantage can be overcome in an emitter-coupled astable multivibrator as shown in Fig. 7.16(a). The circuit operates in such a fashion that Q₁ switches between cut-off and saturation, and Q₂ switches between the cut-off and active regions.

When Q₁ is OFF, V_CN1 = V_CC1 = V_BN2. If Q₂ (when ON) is to be in the active region,