Design question about PNP transistor

[Dornado]

Wondering why a PNP transistor (MSP2307A) is used instead of using another 2N3904 to drive the bases of the LED transistors?

In the design page it says:

Code: Select all

(Note we do not use base resistors because the power supply droop, the NPN's Vce_sat and the IR LED voltage drop will max out and limit the base current and then the Vce of the PNP will sit at the 'right' voltage.)

If that's the reason why the PNP is used could someone elaborate further? My transistor fu is weak.

[cstratton]

When switching with transistors, NPN transistors are usually used as low side switches - they switch the ground side of the circuit. And they take a more positive voltage on the base to do it.

Conversely PNP transistors are can be more easily used as high side switches, using a more negative voltage on the base.

Since each functions in this inverter-ish fashion, if you want to use a transistor to turn on a transistor, it's easiest if you drive NPN with PNP or PNP with NPN.

Of course there are other tricks that can be employed, but for small signal you can buy decent pnp devices so why bother.