Interaction of output impedance and input impedance
Any circuit can be represented by the following equivalent circuit. The
input of the circuit looks like a single resistor (the input impedance)
and the output looks like the Thevinin-equivalent circuit as seen from
the output.
A generic equivalent circuit. Input shown to the left and output to the
right. As a whole, any circuit will act like its equivalent circuit.
What is the load of a circuit
The load is the electronic component that ultimately converts
electrical energy into power. It is essentially the final input
impedance of the system. Like input impedance, a circuit load is
generically represented by a resistor.
Load
A generic equivalent circuit with a load
The load of a public address system is the loudspeaker. The load of a
radio transmitter is the antenna. The load of a computer power supply
is the computer. The ultimate function of any circuit is to deliver
power to the load. We will see this representation of a circuit load
often in this series of books. As will be shown below, it is important
to understand the interaction of the output impedance of a circuit and
the circuit's load.
The term “load” is also used as a verb meaning to cause a significant
current drain from the circuit, either by putting a useful load on the
circuit or by unintentional means. The result of loading a circuit is
always a drop in output voltage. How much drop and whether this is
good, bad or indifferent depends on the interaction of the output
impedance and the load.
Coupling subcircuits
Let's use a public address system as an example of a complex circuit.
At each stage there is an output impedance coupled to an input
impedance. For example, the output impedance of the microphone is
coupled to the input impedance of the preamplifier. The output
impedance of the preamplifier is coupled to the input impedance of the
power amplifier.
Microphone
Preamplifier
Power Amplifier
Speaker
A complete system represented by output impedances (Thevenin equivalent
circuits) connected to input impedances. This illustrates the
interaction of output impedances and input impedances at each stage.
Notice that the final input impedance in the public address system is
the speaker (the speaker is the load of the circuit). The ultimate
function of a public address system is to produce power in the speaker.
The speaker moves air (mechanical motion), thus producing sound waves
and therefore produces power (see Power above).
Output impedance determines the ability of a circuit to deliver current
to the next circuit in a system. When current flows through the output
impedance there is a drop in voltage across it. If a relatively large
current is passed through a relatively high output impedance, the
voltage drop could be enough that the next circuit in the system will
not operate correctly. The basic rule is as follows:
A circuit with a high output impedance can only deliver a small amount
of current. A circuit with a low output impedance can deliver a
relatively high amount of current.
"Relatively high amount of current" does not necessarily mean several
amps. Typically, the output of one circuit will deliver a few milliamps
to the input of the next circuit. This is a relatively high amount of
current compared to a circuit that can only deliver a few microamps to
the next circuit.
Input impedance determines the relative amount of current required to drive a circuit. The basic rule is:
A circuit with a low input impedance requires a relatively high amount
of current to operate. A circuit with a high input impedance requires a
relatively low amount of current to operate.