Hi: Many decades ago I worked on tunnel diode amplifiers. http://www.prc68.com/I/Aertech.shtml#TDA We used a 5.1 Volt Zener biased at a specific current. AFAICR this was a magic combination that resulted in a very temperature stable voltage. -- Have Fun, Brooke Clarke http://www.PRC68.com http://www.End2PartyGovernment.com/