A multiple-valued current-mode adder has a current-to-voltage converter for converting a total current into an input voltage level, the total current comprising current values respectively representative of the values of input integers a transistor with a negative transconductance due to a resonant tunneling...http://www.google.com/patents/US4982356?utm_source=gb-gplus-sharePatent US4982356 - Multiple-valued current mode adder implemented by transistor having negative transconductance
