Two Transistors. One Inverter.
A digital inverter performs the simplest possible logic operation: it changes 1 into 0 and 0 into 1. So why does it need two transistors?
The challenge
Why two transistors are needed for a digital switch
The simplest logic gate
A digital inverter reverses its input signal:
- Input 1 → Output 0
- Input 0 → Output 1
A modern CMOS inverter contains two complementary transistors: one PMOS and one NMOS. Their gates receive the same input, and their drains are connected to the same output.
Your task
Why does a CMOS inverter use two transistors instead of only one?
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Two transistors guarantee a clear answer for both possible inputs.
A single transistor behaves like one controlled switch:
- When it is ON, it connects the output to one voltage.
- When it is OFF, it disconnects the output.
But disconnection does not automatically create the opposite signal. Without another component, the output may be left floating—neither a reliable 0 nor a reliable 1.
A CMOS inverter solves this problem with two complementary transistors:
Input is 0
- PMOS switches ON and connects the output to the positive supply.
- NMOS switches OFF.
- The inverter produces a clear output 1.
Input is 1
- NMOS switches ON and connects the output to ground.
- PMOS switches OFF.
- The inverter produces a clear output 0.
The two transistors work like two doors: one opens the path to 1, while the other opens the path to 0. One door is always closed while the other is open.
Input 0 → definite output 1
Input 1 → definite output 0
This is why the CMOS inverter always gives a clear opposite answer while consuming very little power in a stable state.
Why this works
Thinking principle: If one element cannot provide both required states, add a complementary element that provides the missing state.
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