3-13
Figure 3-14.—Waveforms of a monostable multivibrator (triggered).
The only part of the operation not described so far is the short C1 charge time that occurs right after
Q1 and Q2 return to their stable states. This is simply the time required for C1 to gain electrons on its left
side. This charge time is determined by the R1C1 time constant.
Another version of the monostable multivibrator is shown in figure 3-15. View (A) is the circuit and
view (B) shows the associated waveforms. In its stable condition (T0), Q1 is cut off and Q2 is conducting.
The input trigger (positive pulse at T1) is applied to the collector of Q1 and coupled by C1 to the base of
Q2 causing Q2 to be cut off. The collector voltage of Q2 then goes
-
VCC. The more negative voltage at
the collector of Q2 forward biases Q1 through R4. With the forward bias, Q1 conducts, and the collector
voltage of Q1 goes to about 0 volts. C1 now discharges and keeps Q2 cut off. Q2 remains cut off until C1
discharges enough to allow Q2 to conduct again (T2). When Q2 conducts again, its collector voltage goes
toward 0 volts and Q1 is cut off. The circuit returns to its quiescent state and has completed a cycle. The
circuit remains in this stable state until the next trigger arrives (T3).
Figure 3-15A.—Monostable miltivibrator and waveshapes. Schematic.
