Why sometimes there is a diode on the base of BJT?

 

To prevent the base emitter junction from being reverse biased sufficiently to breakdown the junction. That would kill the transistor beta.

 

The diode is using to protects the b,e diode of bjt when there is a negative voltage appearing on the input, normally if the input voltage as +V, 0V, -V and the circuit only needs the 0, +V, so you need to cut off the negative voltage, if the input voltage only with +V, 0V then you don't need the diode.

 

To prevent the base emitter junction from being reverse biased sufficiently to breakdown the junction. That would kill the transistor beta.

The diode is using to protects the b,e diode of bjt when there is a negative voltage appearing on the input, normally if the input voltage as +V, 0V, -V and the circuit only needs the 0, +V, so you need to cut off the negative voltage, if the input voltage only with +V, 0V then you don't need the diode.

Thank you! Understood!