Little Boy -- An LDMOS HF Amplifier

Little Boy — AN LDMOS HF Amplifier
by W6KANI recently watched the movie Fat Man and Little Boy, a 1989 great movie about the Manhattan project and the development of the atomic bomb. Paul Neman is the lead character as an army general assigned the responsibility of delivering the bomb. John Cusack plays the role of a very bright mathematician and scientist from the University of Chicago. Toward the end of their efforts many of the scientist, especially Cusack’s character, begin to think about the moral issues of what they are about to achieve. The movie centers around the collaborative effort to affect the end result with a lot of very independent minds and ideologies.

The film reminded me of how we amateurs work together exchanging ideas and helping each other with all sorts of issues – antennas, radios, projects – etc. So it has been with this LDMOS amplifier project for me. (Silicon-based LDMOS FETs are widely used in RF power amplifiers for base-stations where requirements for high output power with a corresponding drain to source high breakdown voltage ratings are required – typically cabable of very high frequencies.) Exchanging ideas with many of my ham friends for this project has been the key for its success. After two failures with LDMOS chips it was pointed out to me that the source to drain voltage ratings of the devices being tested was not high enough to take the 55VDC analog power supply being used. As a rule of thumb the rating should be divided by a factor of “2”, and that would mark the upper limit. The original unit we used had a rating of 100 volts so anything over 50 volts was likely to result in a “failure”. The NXP BLF 188XR has a rating of 135 volts…high enough to use a 50VDC source…..