In the figure above, we can make the analogy of the operation of a transistor with that of a valve.
NPN: When a positive force (up and down) is applied to the valve lever, flow occurs from the manifold which is up (+) to the emitter which is down (-). The greater the force on the valve, the more the flow between the manifold and the emitter will be amplified.
PNP: When a negative force (from bottom to top) is applied to the valve lever, a flow appears from the emitter which is in height (+) towards the collector which is in bottom (-). The greater the force on the valve, the more the flow between the emitter and the collector will be amplified.
PNP: The base is driven by a negative (-), the "collector" signal output is positive (+).
By closing the switch (NO), we see that the bulb (L1) lights up. The light intensity changes according to the position of the potentiometer (Pot). The resistor (R) is here to protect the transistor from overcurrent.
The emitter/base pass current creates a current multiplied by the gain of the transistor between the emitter and the collector.
Gain: Ability of a device to increase the strength of a signal.
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NPN: The base is driven by a positive (+), the "transmitter" signal output is negative (-).
By closing the switch (NO), we see that the bulb (L1) lights up. The light intensity changes according to the position of the potentiometer (Pot). The resistor (R) is here to protect the transistor from overcurrent.
The base/emitter pass current creates a current multiplied by the gain of the transistor between the collector and the emitter.
Gain: Ability of a device to increase the strength of a signal.