Shepherd, who owns around 29% of the club, with the Sir John Hall family controlling almost 41% of shares, insists he will never bow to pressure from fans to sell.
Shepherd Insists Fans Won't Get Him Out
Shepherd has faced two supporter's demonstrations after home games this season, and Newcastle were targeted by both the Belgravia and Polygon investment groups this season, but the takeovers failed to materialise.
Manchester City are on the verge of being taken over by a consortium headed by the former prime minister of Thailand, Thaksin Shinawatra, Shepherd is adamant Newcastle will stay in his hands.
Shepherd: "It's a plc matter, but none of these talks came to anything because I'm not selling.
"It's an impossible club to buy. If the Halls don't sell and I don't sell there's no way anybody can buy this club.
"The both of us need to agree to sell the club or nobody will ever buy it without getting the 75%."
It is the worst kept secret on Tyneside that the Halls are looking to sell their stake, but Shepherd holds a large enough stake to obstruct any takeover.
He has even talked of buying the Halls' stake himself, although Sir John continues to hang onto them for the immediate future.
Shepherd: "I've never heard of any Newcastle manager saying we didn't provide the funds when it was necessary. Being a plc we cannot talk money and it would be commercially stupid to do that anyway.
"I can never understand clubs saying they've got this, that and the other because you're just giving ammunition to the ones you're buying from. We've always supported the manager money-wise.
"We always go for the best manager available and he's (Sam's) been the best for us at Newcastle since 2004, but circumstances didn't come out that way (until now). I'm confident he'll do a great job here. I'm very confident that he'll be the man to end the long wait for trophies."