Re: Beginner's question: NPN common-emitter amplifier setup?

A bipolar resistor can be thought of as a current controlled switch. A
small current flowing into (out of) the base of an NPN (PNP) transistor
causes a larger current to flow in the collector - emitter. While the
collector current is not quite equal to the emitter current (due to the
base current) for easy analysis they are assumed to be since the
difference is usually very small. The ratio between base current and
the collector current is called the transistor Beta (current gain) and
is usually specified as a minimum value according to the data***.

If on the resistor to which you are referring, one end connects to
ground and the other end connects to the emitter, any current flowing
through the emitter will flow through the resistor, bringing the
emitter voltage above ground. In turn, the base voltage will be about
..6V - .7V above the emitter voltage, assumig the transistor is on. If
enough current is flowing, such that the collector voltage drops to
about or below the base voltage, the transistor will enter saturation
which means that an increase in base current will produce little, if
any increase in collector - emitter current. In this mode of
operation, the CE voltage is typically about .2V to .3V. This assumes,
that there is a collector resistor, which is generating a voltage drop
from the supply to the collector terminal. A circuit of this
configuration will exhibit an electrical "gain" equivalent to Rc / Re.
The idea is to design the circuit to be dependant on the resistors
rather than the inherent properties of the transistor which can vary
significantly

Sometimes a capacitor is placed in parallel with the emitter resistor.
This is done so that at DC the emitter resistor is present to "bias"
the transitor into a certain region of operation, while providing a
high gain to AC signals. Note also, that the emitter has an inherent
resistance associated with it that is proportional to the emitter
current.

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