Northampton Town loan defender John Johnson has praised manager Ian Sampson for improving the team's defence.The Cobblers have lost just twice in their last 16 matches in League Two and kept eight clean sheets. [LNB]Johnson told BBC Radio Northampton: "The manager and [coach] Malcolm Crosby turned it around, so credit to them. [LNB]"When we were struggling, they did a lot of work with the back four, taking us away from the rest of the team. It's obviously worked well." [LNB]The 21-year-old says the players are encouraged to see manager Ian Sampson sign a three-year deal at Sixfields. [LNB]"He's done a great job, so he deserves an extension," added Johnson. [LNB]"Those players [whose contracts are expiring] know the manager is going to be here, so it's an extra bonus for them." [LNB]Johnson is on-loan from Middlesbrough until the end of the season.