A transistor separates a control side from a controlled path.
That separation is the reason a small electrical condition can influence a larger circuit behavior.
The Two Sides
For a first model, think of a transistor as having:
- a control input
- a controlled current path
flowchart LR control["Control signal"] --> transistor["Transistor"] supply["Power source"] --> path["Controlled path"] transistor --> path path --> load["Load or next circuit"]
The control signal does not need to provide all output energy. It changes how the controlled path behaves.
This is why a transistor can be used for both switching and amplification.
Switching
In switching, the circuit uses the transistor mostly near two extremes:
- blocked enough to count as off
- conductive enough to count as on
Digital circuits care about this because they need stable states. A transistor switch can help create a node voltage that is reliably low or high.
The switch model is not perfect, but it is useful:
The control input selects whether another path is available.
Amplification
In amplification, the circuit uses a region where the controlled path varies with the input.
The input does not create the larger output energy. The power supply provides the energy. The transistor shapes how that energy reaches the output.
This is less central to modern digital logic than switching, but it explains why the same device family appears in analog circuits.
Boundary
The control side and controlled path are not infinitely isolated. Real devices have leakage, capacitance, limits, and timing effects.
The durable distinction is:
The control signal changes the device condition. The controlled path carries the circuit energy affected by that condition.