Do not apply a voltage exceeding the rated emitter-base voltage (VEBO) shown in the Absolute Maximum Ratings table (Table 1) to the base terminal (negative bias in the case of npn transistors and positive bias in the case of pnp transistors). Doing so will degrade hFE and other parameters.
When a monostable multivibrator is composed of bipolar transistors, a bias voltage exceeding the rated VEBO might be applied to the base.
The following describes the operation of the multivibrator shown in Figure 1.
1.Suppose that Q1 is initially on. Therefore, the collector voltage of Q1 is close to the GND level.
C1 is charged via R3 while C2 is charged via R2.
2.As C1 is charged, the base-collector voltage of Q2 reaches its turn-on voltage (roughly 0.7 V), turning it on.
The collector voltage of Q2 drops to a level close to GND. Immediately before this point, the voltage across C2 is charged at VCC – 0.7 V. Even when the collector voltage of Q2 drops, C2 remains charged. Therefore, the voltage at the C2 terminal on the base side of Q1 drops to 0.7 V – VCC.
These steps are repeated to act as a monostable multivibrator.
Suppose that VCC = 10 V. Then, the emitter-base voltage of Q1 and Q2 reaches -9.3 V, exceeding the rated VEBO of -5 V.