Common base - decouple or not?

[throbscottle]

I decided to use a npn tranny in common base mode (because it has to be non-inverting) to drive a P-type mosfet from a 555. Works well in LTSpice. I tried simulating it both with and without a decoupling cap connected to the base - seemed to make very little difference, so I can live without it. However, is there a good reason I should decouple the base, considering the transistor is just being used as a switch-with-gain?

Thanks folks

 

[throbscottle]

Ah, OK, makes a lot of difference if I drop the input voltage right down though!

 

[Nigel Goodwin]

Fairly obviously the capacitor only has effect on using the transistor as an AC amplifier, where it's pretty important.

 

[throbscottle]

Oh yeah, of course. IMA Dummass.

 

[tomizett]

I can't quite visualise the cct you're describing, but do remember that a lot of the problems that decoupling is used to cure (things like track inductance, finite supply impedance, EMI, noise) are real-world problems that won't occur in a simulation.

 

[throbscottle]

Ah, good point. It did make the switching slightly sharper in the sim as well. Also less noise going back to the power rails (not that it really matters for this app)