Maker Pro
Maker Pro

impedance of amplifier

bhuvanesh

Aug 29, 2013
201
Joined
Aug 29, 2013
Messages
201
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
 
Last edited by a moderator:

witsender

Dec 12, 2013
22
Joined
Dec 12, 2013
Messages
22
Hi Bhuvanesh :)
Transistor circuits are either common base, common emitter or common collector.
In the case of a common emitter circuit whose input is at the base of the transistor, the input impedance is measured between base and emitter. Output impedance for such a circuit is measured between collector and emitter.
Hope this helps...
 

bhuvanesh

Aug 29, 2013
201
Joined
Aug 29, 2013
Messages
201
my question is why we are calling common collector configuration as impedance matching network
 

LvW

Apr 12, 2014
604
Joined
Apr 12, 2014
Messages
604
my question is why we are calling common collector configuration as impedance matching network

Well - a common collector stage can placed BETWEEN a signal source having a high source impedance and another stage having rather small input impedance.
Thus, the common-collector stage acts like a "matching network" between source and load.
Without this stage (large input resistance, small output resistance) we would have a huge and unwanted voltage drop across the source output impedance.
 

Colin Mitchell

Aug 31, 2014
1,416
Joined
Aug 31, 2014
Messages
1,416
"why we are calling common collector configuration as impedance matching network"

There are three uses for an amplifier. To increase the voltage (the amplitude of the voltage)
To increase the current. To increase both the voltage and current.
Since a common collector configuration does not increase the voltage of the signal, we have to talk about the feature or quality that it will provide.
The only thing it does is increase the current.
So we call it a CURRENT AMPLIFIER.
It just happens to have a large input impedance due to the way the transistor is connected and the gain of the transistor increases the input impedance by a factor equal to the amplification of the transistor.
When a load is placed between the emitter and 0v rail, the transistor increases the impedance or resistance of the load by a factor of about 100 to 200, depending on the gain of the transistor.
This means the input of the stage has a very high impedance and when you connect a signal to the input, the amplitude of the signal will not be reduced (attenuated) or it will be attenuated a very small amount.
However if you connect the signal to the load, the amplitude of the signal will be reduced ENORMOUSLY.
This means the stage has the effect of "changing the load" by a factor of 100 or 200 and the input signal thinks it is delivering its signal to the load.
The transistor is converting or changing the load into a higher impedance or resistance.
We call this feature MATCHING.
You can also call it CONVERTING or INCREASING.
 
Top