Bipolar transistors have the highest concentration in the emitter, followed by the base, then the collector. The concentration of the emitter is over 100 times higher than that of the base. The higher the concentration, the lower the withstand voltage tends to be.
VEBO has only a few volts. Care must be taken when inputting a large-amplitude signal to the base. In addition, reverse bias is applied to some circuits such as the circuit below. Please consider the usage conditions when designing.
When a monostable multivibrator is composed of bipolar junction transistors (BJTs), a bias voltage exceeding the rated VEBO might be applied to the base.
The following describes the operation of the multivibrator shown in Fig. 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.
- 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.