What is load capacitance ? How does it affect the circuit ?
First, a "load" is an impedance to which power from a circuit is
delivered (and either dissipated in the load or reflected backed to
the source circuit). Impedance is, very approximately, "The AC
equivalent of DC resistance with phase shifting added, both of which
change of over frequency". Examples could include antennas, loud
speakers, AC motors and appliances, lights, logic gate inputs, etc.
If you can assume only linear circuit components are used, you can
reduce *every* circuit, without approximation to just three
components: a power source (voltage or current), a source impedance
and a load impedance. The power source and source impedance
combinations are also known as Thevenin and Norton sources, thus every
circuit can be reduced to two components: a Thevenin/Norton source and
a load impedance. If you can't assume linear, you can still reduce to
three or two, but with certain limitations. Components like
resistors, capacitors and inductors are practically linear.
Semiconductors and vacuum tubes are definitely non-linear, thus there
are limitations.
If the load impedance has a primarily capacitive reactance, or
negative imaginary impedance, then you have capacitive load, which can
be modeled as a load capacitance. The load could also be inductive
(positive imaginary impedance), or resistive (zero imaginary
impedance). You can have combinations also forming
resistive-capacitive or resistive-inductive impedances. I'm assuming
you know
complex numbers, or at least,
trigonometry and right
triangles: impedance is always the hypoteneuse, resistance one side
and (inductive - capacitive) reactance the other side.
The specific effect on a circuit depends on what the circuit is. In
digital circuits a capacitive load acts to slow down pulse rise times.
In RF/microwave it creates a phase shifted reflection of incident
power back into the power source. In audio it can create a low-pass
filter effect. In power electronics (e.g. AC motors) it creates a
phase shift for starting a motor or for compensating power factor
shift caused by the motor. Sometimes these are intentional effects;
sometimes not. A common thread is that power is not fully used to its
theoretical maximum: only purely resistive loads dissipate any power;
"reactive" power is reflected back.
MM
