MORE ON IMPEDANCE

If a device has a HIGH INPUT IMPEDANCE, that means low current will flow which in turn means that the device requires low current to work!  Take for example the following statements

The FET input means that the op-amp has a very high input resistance and only needs a tiny input current to work. This means it is easy to ‘drive’ - i.e. the signal source doesn't have to supply much current. The Bipolar output means that the op-amp can provide just as much output current as an all-bipolar design could.

source:http://www.st-andrews.ac.uk/~jcgl/Scots_Guide/datasheets/Opamps/071.html

IMPEDANCE MATCHING

If maximum current is desired at the input to a circuit, should the input impedance of that circuit be lower than, equal to, or higher than the output impedance of the previous stage?


Answer:  The circuit should have a lower impedance compared to the circuit the circuit of the previous stage.  This will allow more current to flow given the lower resistance.

What transistor circuit configuration should be used to match a high output impedance to a low input impedance?

Answer:  The question is thus: Circuit A has high output impedance - MyCircuitB - Circuit C has a low input impedance
Hence MyCircuitB should have a Common Collecter configuration since a Common Collector configuration provides high input impedance and low output impedance.

Low Feeds High
In order to preserve signal level and frequency response, it's important
to drive equipment with a source signal that is lower in impedance than
the destination equipment's input impedance. If the input impedance of a
device is not significantly higher than the source impedance, the signal
will be reduced or "loaded down" and its signal to noise ratio and
frequency response will suffer.

Think of this as having a nozzle at the end of a garden hose. The garden
hose is a low impedance source (there is little resistance to the flow
of water) and the nozzle is the higher impedance of the input being fed
by the hose.

When the nozzle valve is closed (open circuit):

* Input impedance is VERY high
* Pressure (voltage) is at maximum
* Flow (current) is zero

Now open the nozzle just a little:

* Input impedance reduces but remains high
* Pressure reduces but remains high
* Flow is small

As you continue to open up the nozzle:

* Input impedance reduces further
* Pressure reduces
* Flow increases


With the nozzle open all the way:

* Input impedance is very low
* Pressure falls dramatically
* Flow is greatest


In the case of a high impedance guitar output (7,000 to 15,000 Ohms or
more) driving a relatively low impedance input of a mixer (2,000 to
10,000 Ohms), it's like connecting a garden hose to a fire nozzle. The
hose just can't produce enough flow (current) for the size of the
opening (impedance) to maintain the pressure (voltage).

If the load impedance (nozzle/FET) is 10 times or more the source
impedance (hose/AC Signal), it is called a "bridging" impedance.
Bridging results in maximum VOLTAGE transfer from the source to the
load. Nowadays, nearly all devices are connected bridging -- low-Z out
to high-Z in -- because we want the most voltage transferred between
components.