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Newbie in electronics here.

I think I know the basic theory behind transistors.I would like to know based on the schematic how you can know whether the transistor is used as a switch or an amplifier (as far as I know these or the two only things it can be used for). Appearantly you also have ta take into account something called "linear region". Could someone explain how you can mke the differenxe (prefferably using an example)

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    \$\begingroup\$ Measure Vce (or calculate it). If it's either 0.2V (or somewhere below 1V) or Vcc it's a switch. If it's about Vcc/2 (or anywhere above 1V but significantly below Vcc) it's an amplifier. \$\endgroup\$ – Brian Drummond May 15 '16 at 10:29
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    \$\begingroup\$ Is it maybe worth expanding this to cover other common patterns - darlington pairs, current mirror, etc.? \$\endgroup\$ – Roger Rowland May 15 '16 at 11:39
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If you see a circuit like this:

schematic

simulate this circuit – Schematic created using CircuitLab

It is being used as a switch.

A linear amplifier would look more like this

schematic

simulate this circuit

The specific resistor values will vary from circuit to circuit. I used the common emitter topology as it is very commonly used.

There are numerous resources for linear amplifiers, and you can read about the biasing requirements for this one at Wikipedia

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  • \$\begingroup\$ This is good. I would add something like 'if the base is biased by something which is not the driving signal, then it is an amplifier' \$\endgroup\$ – Vladimir Cravero May 15 '16 at 13:44

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