No base resistor for NPN driven by opto output?

[cupoftea]

Hi,
Please could i just do a reality check and confirm that no base resistor is needed in the attached NPN base drive?
LTspice and jpeg schem attached.

The NPN is just for a digital high/low output (from its collector) when the opto_diode current source is turned on (5mA) and off (0mA)

 

[danadakk]

The design is depending on the CTR and input current. If limited on coupler input
current and the CTR limitations will limit current into external transistor
base. That being said I would think about using a R to limit input current
to external transistor should the coupler input driver fail with high current.
Keep in mind graphs are typical values, not worst case, in general in datasheet.

Regards, Dana.

 

[KlausST]

Hi,

What´s the benefit of the circuit?
To drive a BJT with some miliamperes base current while the bjt just drives below 1mA...

The bjt "overdrive" will lead to quite unsymmetrical timing behaviour.

Klaus

 

[FvM]

Asymmetrical timing is primarily caused by the optocoupler itself, but no timing requirements have been mentioned at all. You can however omit Q1, it has no purpose with 10k load.

 

[Easy peasy]

a base or series resistor will linearise the circuit substantially - work out using the worst case gains of the bjt and opto ....

 

[danadakk]

OP states application is digital. so probably not a consideration.


Regards, Dana.