Electrical – Protecting BJT base-emitter junction from breakdown voltage

The diode is in the best position, and is of an appropriate type.

It conducts when the input is negative, the same as the transistor base conducting when the input is positive. The 47K resistor is about 1/10 of a normal RS-232 load. One could also block the voltage, but then a -100V spike (ESD say) could break down the 1N4148 and break down the E-B junction, causing irreversible damage.

Also, a 1N4148 is an appropriate diode for this application. It's a "switching diode", low capacitance and fast reverse recovery. A 1N4001 would also likely work okay, at least at slow baud rates. The 200mA rating means that even if a very high voltage were to appear at the input the transistor is fully protected, at least up until the resistor arcs over,.

This is a guaranteed maximum number, with a relatively heavy base and collector current (although only at 25°C). There is no typical given.

Compare these typical curves for the common MMBT4401 transistor:

At the Ic=500mA, Ib=50mA level, the \$V_{BE}\$ is typically more than 1 volt.

The manufacturer chooses to guarantee it will be less than 2V (compare to the '4401 where the guarantee is 1.2V). That's about all you can say about it.. it's probably closer to 1V than 2V typically, and certainly much less than 2V (and actually well under 1V) at more reasonable currents such as 10mA base current/100mA collector current.

It's not unusual for manufacturers to have loose guarantees.. leakage currents are often guaranteed at 1uA levels when actual typical leakage may be more like 1nA or 100fA. That speeds up the testing. In the case of the Vbe, it may reduce false fallouts due to bad connections in their test jigs or whatever.