thanks for all your replies. one question tip 120 has a high gain of 800-1000 and i want just collector current of 0.15 amp. so we need to reduce the base current to 0.15/800=1.875 x 10^-4. so a resistance of atleast 30 k will be sufficient. why do you advice i should use 330......1k? please explain me and pardon me if i am wrong somewhere.