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From what I know, potentiometers vary an analog current with an input usually in the form of a knob. And transistors vary an analog current with an input in the form of a smaller current.

Is there any practical difference between the two other than one varies between the position of a knob and the other varies between a smaller current?

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They are entirely different.

Any resistor is a linear device, a transistor is nonlinear. A linear device has a linear relationship between voltage and current, simply put. Transistors show very much more complex behavior.

One is an interface, the other not. You cannot twiddle a transistor to change its characteristics.

A potentiometer is used to change a setting of a device, permanently at production, or during use. As long as it is not touched, the setting stays the same, being a passive device. Transistors can't be used to do that.

A transistor, being an active device, puts some of its characteristics, say the output resistance in relation to one of its inputs, say the collector current.

A transistor does exhibit resistance in the sense there is a voltage and a current, and when I measure those, it looks like a resistance. But when you change the CE-voltage, the current will not change in the same ratio. Say, you double CE-voltage, you will not get double the current, but rather nearly the same current mostly, making it a differential resistance. On any resistor, you would get a linear response, double voltage ==> double current.

This leads to the possibility of self-feedback on a transistor, making it possible to really regulate something, say in a constant current source. Twice the voltage, same current? Completely impossible with only passive devices, no resistor can do that.

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  • \$\begingroup\$ can a primitive/limited potentiometer be built from a transistor? i.e. making the input current to the transistor static, therefore imitating a "knob". \$\endgroup\$ – Erik Kaplun Nov 22 '15 at 13:22
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A potentiometer varies a voltage, rather than a current. You presumably mean a rheostat, a BJT is a current-operated device. The word transistor was coined from "transfer resistor".

Transistor operation is a bit more complex than simply varying the setting of a rheostat. The Art of Electronics uses this cartoon with a "Transistor man" to explain how a transistor works:

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  • \$\begingroup\$ Why the downvote? \$\endgroup\$ – Leon Heller May 18 '11 at 4:49
  • \$\begingroup\$ Perhaps because of the 263(!) character URI. Ever heard of TinyURL? \$\endgroup\$ – stevenvh May 18 '11 at 6:01
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    \$\begingroup\$ It's not like the URL is visible to readers. And it's nicely hidden away in it's own line, in the source code! \$\endgroup\$ – Andres Riofrio Mar 21 '12 at 3:04
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    \$\begingroup\$ @AndresRiofrio - the URL was visible before Leon edited. It took half of the answer! (See edit history) \$\endgroup\$ – stevenvh Apr 3 '12 at 7:18
  • \$\begingroup\$ According to The Oxford Dictionary, the word "transistor" has the following etymology: "ORIGIN 1940s: from transconductance, on the pattern of words such as varistor." \$\endgroup\$ – Erik Kaplun Nov 22 '15 at 13:24
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Yes, a transistor is like a variable resistor, in that the resistance between collector-emitter or drain-source is variable. It's like a valve, and either allows current to flow or blocks it from flowing. (I don't like it when people say "transistors are amplifiers". They're a part used to build amplifiers, but they can't do anything themselves, without a power supply, biasing circuitry, etc.)

The difference from a potentiometer is that the resistance of the output is not directly proportional to the current. It's more like a current-regulating valve. It adjusts its resistance as well as possible so that the current at the output is proportional to the input.

This is from the Art of Electronics Student Manual:

transistor is a valve not a pump

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