I'm a newcomer to analog electronics.
Currently, what I'm studying is Op-amp circuits.
The term I'm confused about is Input & Output impedance concept.
First lets get impedance, in general, out of the way. It is
a concept closely related to resistance. It has the same
units, volts per ampere, as resistance, but includes the
frequency dependence. But, in simplest terms, high
impedance is a lot like high resistance, and low impedance
is a lot like low resistance.
When applied to an amplifier circuit made with an opamp,
input impedance refers to how many volts it takes to drive
an ampere of current through the input of the amplifier (or
microvolts to drive a micro ampere, etc.). Output impedance
is how many volts the output signal is loaded down by, if
you pull an ampere out of it. A high input impedance reacts
to a signal without requiring much current from the signal
source, while a low output impedance drives a wide range of
load resistances without the output voltage drooping much.
For example, input impedance is differenf for inverting amplifier and
non-inverting amplifier.
The impedance of the amplifier, configured with programing
resistors is a combination of the opamp impedances and the
resistor impedances. The reason the inverting configuration
has a lower input impedance is that the output (which swings
in the opposite direction from the inverting input) is
connected back to that input through a feedback resistor.
This causes the inverting input to be held at an almost
fixed voltage (a voltage that matches whatever you have
connected to the non inverting input. So, whatever resistor
you connect between the signal source being amplified and
the inverting input is what determines how many volts per
ampere (ohms) the signal source must drive, since the entire
input signal voltage will be dropped across that resistor.
If the opamp is configured as a non inverting amplifier
(lets say, a follower with a gain of 1 and the output
connected directly to the inverting input) the inverting
input is forced to match whatever voltage you apply to the
non inverting input, by the output, so there is no other
path for input current than just what the opamp inputs leak.
The opamp is designed to keep that input leakage current
as low as possible, so the amplifier input impedance (volts
required to force an ampere of current through the input) is
very high.
How do you define the input impedance or output impedance?
I have defined input resistance and output resistance, but
I'll leave it there, for the moment as an approximation of
input and output impedance. The full definition involves
including how the volts per ampere changes with frequency.
and what does the term "Impedance matching mean?"
It is a signal concept that generally does not apply to
opamps, where lots of gain is available and gain precision
is important. It is an energy transfer concept. If a
signal source is loaded with a load that has a matching
impedance, the largest possible energy transfer is achieved.
This can be a useful thing to strive for when the signal
energy is th limiting factor. For instance, if you are
trying to force the maximum energy out an antenna you might
try to impedance match the antenna impedance to the
amplifier output impedance. In that case, you are not
trying to get some specific voltage across the antenna
terminals, but are just trying to radiate as much power as
possible from a given amplifier.
There are other reasons to be concerned with source and load
impedances that do not involve matching. Some components,
like transformers, have a frequency response that depends on
the impedances of the sources and loads connected to them.
So you have to design the rest of the circuit to produce the
impedances that will give the transformer the best frequency
response.