Active devices Vs passive devices

A transistor can amplify, it can control a large current by supplying it with a small current.

Passive devices don't amplify, some devices are harder to categorise...

Resistors, capacitors, inductors are considered as passive.

Hard to give an all encompasing concise definition.

Active devices do something. Passive ones don't.

A transistor is an active device. It can amplify. It can switch. Etc.

This is a reply to Del too...

A transformer can amplify the voltage (or current) too... But it's considered as passive... An inductor connected in series with a switch and a supply can, also, increase the voltage: During the opening of the switch, a high voltage is applied at its terminals instantly and a corresponding spark (i.e. current) may occur (between the terminals) (and if you connect a small lamp at the terminals of the switch you'll see an instant, intense flash of the lamp)... But the inductor is considered as passive... ...

A silicon diode doesn't amplify the voltage (or current)... It is considered active though... ...

I believe your explanations and rules are inadequate... ...

Yes..I tried to word my answer carefully and use words like 'considered' and I carefully avoided diodes...I was waiting for someone to jump in with these...

It's a tricky question to answer concisely.... It's as much about convention as anything else.

Ah, I forgot to mention the relay: is activated by a small current on his coil and gives a large voltage (and current) to a load through its contacts (you could consider this as a kind of "power amplification")... Although, a relay is a passive component... ...

In the wikipedia is mentioned that: "active components are those that have gain or directionality (in contrast to passive components which have neither)..." ...

I conclude that with "directionality" refers to the diodes and with gain refers to the transistors (BJT, FET e.t.c.)... Also, any compination of these could have "gain" (i.e. an op.amp.) or "directionality" (i.e. a logic gate)...

(Active components could be solid state (silicon) or electronic valves)...

I'm not sure that the above definition is the proper one though... The examples that I gave (inductor, transformer, relay) gives a (kind of) gain... ...

Maybe is more correct to think about the relatioship between the voltage and current in active components: this relationship is not linear... and maybe this the property that distinguish them from the other (passive) components... ... (???) ...

[As a second thought maybe we could consider the relay as an exception: it's an electromechanical component (and not a pure electric one)... (e.g. it supplies power to a load but it cannot be considered as an active component...)...]

Dear guest=

Since you posted your querry on the "instrumentation" section one has to assume your question applies to "instrumentation" --that being the case----------

Passive devices will show you in real time , those functions you wish to monitor (pressure, voltage,etc. ) and do not play any part in the overall function of the device.

Active devices can show you in real time/ those functions you wish to monitor, but may play an intregal part in the function of the device (low voltage shut down, low oil pressure alarms, etc.)

However you did mention "transistor" in your querry, so i think maybe your hunting for a definitation as it relates to electronic audio equipment--If this is true, please so state and I can be more specific with my answer. Hope this helps!

Donzi

A transistor is active;

Although, if you have downstream sensitivity to an upstream event, anything before or in between that final stage, could be considered active.

There is one informal rule as well how to separate active and passive devs (no matter for electronics or instrumental circuits):

The device which needs specially involved power supplying for its operating can be classified as active. As rule this device has at least two PWS inputs.

Why as rule? Because of considering for instance thermocouple or photodiode --- its get energy "wireless" owing to known physical effects.

Everything needs meal to be active .

transistor is an active devices because it can enhance its energy i.e.there is an amplification