Diode - BJT Three-Input Majority Voting Logic

This is a three-input majority voting logic circuit using diodes and BJT. Diodes D1 and D2 perform the AND logic function for the first and second input variables. Diodes D3 and D4 ANDs the second and third input variables, and diodes D5 and D6 ANDs the third and first variables. Resistors R1, R2, and R3 provide pull-up to Vcc for the three AND gates. The results of the three AND functions are ORed together using diodes D7, D8, and D9. To provide adequate fanout, the output of the OR gate is amplified using a BJT. When the output of the diode OR gate is connected to Q1, the logic state at the emitter follows the OR output. When output is a logic HIGH it suffers a large voltage loss due to the diode OR gate and Vbe drop. When the output of the diode OR gate is connected to Q2, the logic state at the collector is the complement of the OR output.