31-year-old full-back Kevin Amankwaah, freed by Exeter at the end of last season, has signed a one-year deal after impressing Boothroyd in pre-season.
"He's 6'3 tall and he makes us a bit bigger as a team - I think we've lost a lot of height from last year," Boothroyd told BBC Radio Northampton.
"He's been in promotion-winning teams and I think he's a really good addition."