If a transistor is a solid state digital switch, how can it also function as an analog amplifier?


If a transistor is a solid state digital switch, how can it also function as an analog amplifier?

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Transistors only act like switches if they are configured to, as on Complimentary Metal Oxide Semiconductor transistors used in most digital systems.

Its not exactly a switch, it can just act as one, but it can act in different ways also.

A bjt transistor (one of the most common ones) is essentially two diodes strapped together, and they retain those properties when as a transistor.

The transistor operates in two “modes” – a linear mode and a saturated mode.

Certain transistors are designed to have a wide linear operating area and biasing the transistor in this area allows it to behave like an amplifier.

Other transistors are designed to have a narrow linear operating area and they are easily used as switches.

Ultimately, for the practitioner, just calling something a “transistor” is more or less not helpful. You have to specify the exact kind of transistor and the circuit it operates in and what that transistor is supposed to do.

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

A digital switch does not mean that it can’t also let a controlled current through, the behavior depends on the control sitgnal.

Compare it to a water walve. If you only have it fully open or fully closed it is like a digital switch. But you can only just open it a bit and control the water flow,

When transistors are used for digital logic the control signal you use are the min or max voltage so it is fully open or fully closed. But as an analog amplifier, you have it in the middle range then it is partially open.

So what you consider the transistor as will depend on how you use it.

You can look at a digital switch as an amplifier with very high amplification and a large input signal. Above half the max voltage is increase the voltage and below half the max voltage it decreases the voltage. It can only amplify it to max voltage and decrease it to zero volt

Add to that a signal with voltages up to max voltage and down to zero voltage and you will the majority of the time overdrive it and the output is just maxed or min.

Bot the amplification and input signal lever is inappropriate for an analog amplifier where you like a higher voltage variant of the input signal. There has to be a balance between input signal voltage, amplification and max output voltage to avoid you overdriving the amplifiers. A digital switch are a deliberate design so the vast majority of the time you overdrive it

If you look at how a transistor in digital circuits works in reality the voltage does not imitate change from min to max. It rises from min to max and during that time the transistor is partially open or partially closed. In digital logic when you try to do stuff as fast as possible the clock frequency is so high that this is a very relevant effect.

Just consider them a switch and you have a signal that just is at max and min is an idealization. The analog properties of the switch can often be very important.

Digital signals do not really exist. They are always analog in reality and change at a limited speed, there is also noise in them.

Here is a for example oscilloscope image of the signal in an ethernet cable