Understanding Logic gates at transistor level : Not Gate

Today we will talk about some basics of digital logic gates. It's about using the transistor for the construction of logic gates. Transistors are used in the construction of logic gates as they act as fast switches. 

When the base-emitter diode is turned on enough to be driven into saturation, the collector voltage with respect to the emitter may be near zero and can be used to construct gates for the TTL logic family. Let's start with simple NOT gate.

1. Not Gate using transistor

NOT gates are single-input devices which have an output level that is normally at logic level “1” and goes “LOW” to a logic level “0” when its single input is at logic level “1”, in other words, it “inverts” (complements) its input signal. The output from a NOT gate only returns “HIGH” again when its input is at logic level “0” giving us the Boolean expression A = Q.

The input of the NOT Gate is connected at the base of the transistor and the output is taken from the collector. The transistor here acts as the switch so when the voltage is applied at the base of the transistor the transistor starts conducting and shorts the output to the ground similarly when no voltage is applied at the input the output is connected to the Vcc as shown thus in this way the circuit implements the NOT function.