7 segment display BJT components what the use?

May i know what is the use of the npn and pnp bjt for my schematic? can someone please advise. Thank You

tnecniv said:

May i know what the use of the npn and pnp bjt for my schematic? can someone please advise. Thank You

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hi,
Are you asking WHY they are used or what TYPE to use.???

hi, i am asking why it is used.

May i know what is the use of the npn and pnp bjt for my schematic? can someone please advise. Thank You

Click to expand...

Hi,

The use of transistor in the ckt for two main reason:-
1.) To have large driving current
2.) To change the level of voltage as +5 to -5V or gnd.

in this ckt the load across 74xxx TTL chip is less it can drive it, so the 2nd reason is Ur answer.

It is a horrible circuit:
1) The output high voltage of the old TTL IC is typically only 3.1V which is not high enough to turn off the PNP transistors.
2) The 100 ohm base resistors for the PNP transistors provide a current of about 39mA which exceeds the maximum recommended output low current of 24mA for the old TTL IC.
3) Nothing limits the base current of the NPN trransistors which will exceed the recommended maximum recommended output high current of only 2.6mA for the old TTL IC.
4) The transistors are not given enough base current to saturate.