impedance of amplifier

[bhuvaneshnick]

i am studying bjt amplifiers,i am often getting through impedance of input and outputs . Its messing up with me
example:common collector configuration have high input impedance and low output impedance
does that input impedance is impedance between the terminal( base and collector)?
it is also said as impedance matching circuit.why common base and common emitter are not being said impedance matching circuits.Thank you in advance

 

[Tony Stewart]

Always refer impedance with respect to ground the common reference point, regardless of the BJT configuration of "BJT Common pin"

The "common BJT pin" is normally ( but not always) bypassed or direct connect to power or ground to shunt the signal and not used for input or output. such as CE, CC, CB.

So for CE the emitter impedance is boosted by hFE towards the input and the input bias resistance is reduced by hFE as seen by the emitter so as a unity gain device it acts an impedance transformer or buffer.
The CB is not precisely 50Ohms input impedance but is typically used for this when the bias current is set for this. Transconductance, gm and Ie affects it.

See https://en.wikipedia.org/wiki/Common_base for details.

So from these 4 configurations one can choose gain for voltage, current, power or , impedance ratio, each limited by the gain bandwidth product of the device.

 

[audioguru]

Most things are not impedance matched;
1) A low impedance microphone feeds a higher impedance preamp input so that the very small voltage of the signal is not reduced.
2) The fairly low output impedance of a CD player feeds the much higher input impedance of a power amplifier.
3) The extremely low output impedance of an audio power amplifier (0.04 ohms or less) feeds an 8 ohm speaker.
4) The extremely low output impedance of an opamp feeds the very high input impedance of a following opamp.

But radio signals have the output impedance of a transmitter matched to the impedance of the antenna so that there is maximum power output.

 

[crutschow]

As AG noted most electrical signals are not impedance matched.
Besides RF circuits where maximum power transfer is desired, another place where impedances are matched is for high frequency signal transfer, such as cables for Ethernet and TV. In that case the line impedance is typically matched by the source and receiver to avoid signal reflections and degradation.