Astable Multivibrator

Astable multivibrators are widely used in electronic equipment and are widely used in pulse signal generation circuits. Its working principle is
shown in Figure 17-18. As can be seen from the figure, the two transistors VT1 and VT2 are connected by coupling circuit A in the form of C2 and Rz. The output of VT2
is sent to the input end of VT1 through the coupling circuit B (C,, R,) to form positive feedback, thus forming an oscillator circuit. Replace
    the transistor VT1 in Figure 1 with a switch Sl, omit the coupling circuit B (Ci, Rl) from VT2 to VT1, and it
becomes Figure
    (a) is the case where the switch sl is turned off, and the base of VT2 The power supply adds positive voltage through R2 , causing V-12 to conduct. At this time,
the dotted line in the circuit is the current charging capacitor C2.
    Then make the switch sl close t as shown in figure (b). Since the capacitor C2 has been charged at this time, the voltage is reduced to o. Therefore, at the moment when the switch is closed,
it is equivalent to adding negative voltage to the base of VT2, causing VT2 to cut off. The direction of the dotted line in Figure (b) is the direction in which the charge of capacitor c7 is discharged.
Then the base voltage of VT2 changes from negative to positive, and then VT2 is turned on again.