Accueil > Electronic > Discrete element logic circuits
ElectronicThe basics of automation

Discrete element logic circuits

These circuits are made with diodes or bipolar transistors. Let us take as an example the realization of the 3 basic logical operators.

Realization of the AND:

Realization of the AND

Operation:
Let us consider that the 1 corresponds to the voltage Vcc (5V for example), and the 0 to the mass: GND (ground) = 0V.

   

  • if A = B = 0 then the 2 diodes are forward biased => They are conducting => in Y, we have 0.6V or approximately 0V => Y = 0.
  • if A = 1, B = 0 then D1 is reverse biased => D1 is blocked. On the other hand, D2 is forward biased => D2 is conductive and imposes in Y: 0.6V (approximately 0V) => Y = 0.
  • if A = 0, B = 1 then symmetrically to the previous case, we have D2 blocked and D1 open => Y = 0.
  • if A = B = 1 then the 2 diodes are reverse biased => in Y we have about Vcc => Y = 1.

This operation is indeed that of an ET!

Realization of the OR:

Realization of the OR

Operation: 1 <=> Vcc (5V for example)
0 <=> GND (0V)

   

  • if A = B = 0 then the 2 diodes are blocked => point Y is linked to GND => Y = 0.
  • if A = 1, B = 0 then D2 is blocked but D1 is forward biased => D1 is conductive => in Y we have Vcc (Vcc – 0.6 in true) => Y = 1.
  • if A = 0, B = 1 then in the same way we have D1 blocked and D2 open => Vcc on Y => Y = 1.
  • if A = B = 1 then the 2 diodes are forward biased => they are both on => Vcc on Y => Y = 1.

This operation is indeed that of an OR!

Realization of NO:

Realization of NOT

Operation:
1 <=> Vcc (5V for example)
0 <=> GND (0V)

  • if A = 1 then the transistor T is saturated => Y = Vce_sat = 0.2V = 0 approximately (GND) => Y = 0.
  • if A = 0 then the transistor T is blocked => Y = Vcc => Y = 1.

This operation is indeed that of a NO!

Leave a Reply

Your email address will not be published. Required fields are marked *

Solve : *
17 × 20 =