CMOS Inverter

• The circuit below is the simplest CMOS logic gate.
• When a low voltage (0 V) is applied at the input, the top transitor (P-type) is conducting (switch closed) while the bottom transitor behaves like an open circuit.
• Therefore, the supply voltage (5 V) appears at the output.
• Conversely, when a high voltage (5 V) is applied at the input, the bottom transitor (N-type) is conducting (switch closed) while the top transitor behaves like an open circuit.
• Hence, the ouput voltage is low (0 V).
• The function of this gate can be summarized by the following table:
  

  Input	Output
  High	Low
  Low	High

• The output is the opposite of the input - this gate inverts the input.
• Notice that always one of the transistor will be an open circuit and no current flows from the supply voltage to ground.

  

Figure 3.2: Inverter Circuit and Standard Symbol