If I use a quarterstaff as my Arcane Focus, can I add my Proficiency Bonus, the way a Bard does when using an instrument as their arcane focus?

Question

A Bard can add their Proficiency Bonus to checks they make with a Musical Instrument they're proficient with. It says on p.154 under Musical Instruments in the Player's Handbook:

If you have proficiency with a Musical Instrument you can add the proficiency for what ever you do with it. A bard can use a musical instrument as a spellcasting focus,...

I wonder, can I do the equivalent as a Wizard, with an Arcane Focus (staff) doubled as a Quarterstaff?

Related Question: Is it possible to use a weapon as a normal weapon and an arcane focus at the same time?

Answer 1

Your proficiency bonus is already applied to spell save DCs and spell attacks

The Basic Rules clarify, in the chapter on Spellcasting (emphasis mine):

The DC to resist one of your spells equals 8 + your spellcasting ability modifier + your proficiency bonus + any special modifiers.

and...

Your attack bonus with a spell attack equals your spellcasting ability modifier + your proficiency bonus.

---

Using a spellcasting focus only replaces certain material components...

A character can use a component pouch or a spellcasting focus in place of the components specified for a spell.

...so being proficient with an arcane focus has no affect on the details of spellcasting.

As an additional note, this is true not only for arcane foci that double as quarterstaffs, but also for such characters as bards of the College of Swords who gain:

If you’re proficient with a simple or martial melee weapon, you can use it as a spellcasting focus for your bard spells.