noise coupling through a BJT transistor base?

[SArepairman]

So if I am using a 2n2222 as a low side switch from a filtered supply, how much junk will go through the capacitor into my load?

Am I correct in assuming that the parasitic base-collector capacitance is the main problem (how noise gets in).

This is for driving relays. I don't know if I should have a individual filter for each relay or if I can use a low side switch and use only one filter.

[c4757p]

Base!!

If I needed to switch relays from a low-noise circuit, I'd either 1) find a way to switch them while signal stuff isn't being done, or 2) power them from an isolated supply and switch them through optocouplers.

[SArepairman]

Base!!

If I needed to switch relays from a low-noise circuit, I'd either 1) find a way to switch them while signal stuff isn't being done, or 2) power them from an isolated supply and switch them through optocouplers.

well their being switched through a isolator and it is just range relays. I was going to use a simple capacitance multiplier filter to power the relays, I doubt much will couple through them.

its probably completely unnecessary to do so BUT i still want to know the basic theory of how noise gets in.