In order to connect an alternating signal into the transistor amplifier
in a way that the circuitry that generates the signal doesn't interfere
with the operation of the amplifier, and also that the biasing and
operation of the transistor amplifier doesn't change the way the
circuitry of the source signal operates, we need a way to isolate them
from each other.
Since the only component of interest that needs to be shared by both
circuits is the alternating signal (AC signal), we need to use a
component that will let the ac component pass while blocking the any DC
of the bias circuitry or the signal generator.
As you learned in a previous lesson, a capacitor is a component that can
store energy in the form of an electric field created by lumping
charges close to each other but still isolated. Current cannot directly
cross the insulating layer inside the capacitor, effectively blocking
any direct current flow.
But something interesting happens when a capacitor is affected by an
alternating current. On the positive half of an AC wave, one side of the
capacitor is filled with an inrush of electrons, while on the other
side, electrons are pushed out to be replaced with holes, until the
capacitor is fully charged and no more charges move.
For the moment when the capacitor is charging, the amount of electrons
entering one plate of the capacitor is the same as the electrons being
pushed out from the other side, almost as if the electrons had just
crossed the insulating layer.
When the polarity is reversed the effect happens once again, the
electrons are now drawn towards the voltage source, leaving holes in the
plate of the capacitor. These holes draw the electrons that were
previously pushed away, into the plate of the capacitor. The net effect
is again as if the electrons crossed the insulating layer to get to the
voltage source.
In practice, it is not the actual crossing of the electrons that is of
use, but the movement of them on both sides of the capacitor that can be
used as a current in the circuit.
Summarizing, the capacitor blocks any current that tries to directly
cross the insulating layer, but it can't stop the electrons from being
drawn to or away from the plates, effectively letting alternating
voltages get through.
This effect is used to isolate the DC component from both sides while
allowing the ac to flow, and is called capacitive coupling.
