If a transistor is a solid state digital switch, how can it also function as an analog amplifier?
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Transistors have two modes depending on how much voltage you apply to the gate(we’ll be working with FETs here, BJTs have bases and are current based)
If you just slam the gate on and off then the transistor switches from off to saturation and back. In saturation its doing its best to just let current flow through it
If you keep your gate voltage in the middle region you can hold the transistor in the linear region. The amount of current allowed to flow through the transistor is directly proportional to the gate voltage – threshold voltage. If you move the input voltage up then it lets more current flow, move it down and less flows, this also manifests as the voltage across the transistor so you can use that too.
Digital applications use transistors as switches because it keeps the circuitry simple, but analog applications make use of the full range of the transistor to make amplifiers
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