Combinational logic design
- A transistor is the basic electrical component of digital systems.
- Combinations of transistors forms logic gates.
- Logic gates are the basic building block of digital systems.
- A transistor acts as a simple on/off switch.
- One type of transistor is CMOS transistor.
- n-MOS transistor is shown in Fig 4.1(a).
- The gate controls whether or not current flows from the source to the drain.
- When a high voltage (typically +5 Volts, which we’ll refer to as logic 1) is applied to the gate, the transistor conducts, so current flows.
- When low voltage (or) logic 0 is applied to the gate, the transistor does not conduct.
- Fig 4.1(c) shows an Inverter.
- When the input x is logic 0, the top transistor conducts (and the bottom does not), so logic 1 appears at the output F.
- When the input x is logic 0, then output becomes 1.
- Fig 4.1(d) shows NAND gate.
- x, y are inputs to NAND gate and F is the output.
- When at least one of the inputs x and y is logic 0, then at least one of the top transistors conducts (and the bottom transistors do not), so logic 1 appears at F.
- If both inputs are logic 1, then neither of the top transistors conducts, both of the bottom transistors conducts so logic 0 appears at F.
- Fig 4.1(e) shows NOR gate.
- When two inputs are at logic 0, then output F is 1.
- When two inputs are at logic 1, then output F is 0.
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