AND gate

Shihab Uddin Ahmed | January 15, 2017

AND gate is a digital circuit which performs AND operation on inputs. As I discussed in previous section that AND gate gives high output when all inputs are high. In other cases its output must be low (i.e. 0). So we need to design a circuit in that way.

logic_gate_basic-2

Truth table for AND gate

Figure below shows an AND gate circuit designed only with diodes and resistors:

and-gate-with-diode

Let’s discuss about all four possible input states of the circuit:

  • If A and B both are low, both diodes will be forward biased and conduct current. So grounded diodes will bring output to low state.
  • If A is grounded and B is taken to high state, A will be forward biased and B will be reverse biased. Reverse biased diode will not conduct electricity. But forward biased diode A alone is enough to bring output at low state.
  • If A is taken to high state and B is grounded it will the same thing like previous.
  • If A and B both are at high state, both diodes will be reverse biased and will not conduct current. So output will be high, because in that condition there will be no current through the resistor.

Figure below shows an AND gate circuit using resistors and transistors:

AND gate with transistor

So, how this AND gate with transistors work? It’s nearly the same as previous one. You get high at output only when both transistors are biased. It happens when both inputs are at high state.

Usually AND gate circuit is made for demonstration purposes only, where resistor, diode and transistors may be used. But for practical purposes we can use integrated circuit (IC), where several AND gates may be packed in a single IC.

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