Question

The output impedance of a transistor connected in the...............arrangement is the highest,
A. common emitter
B. common collector
C. common base
D. none of the above

Answer 1

Hint: If we combine two extrinsic semiconductors we get diodes. Extrinsic semiconductor in which electrons are excess are called n-type semiconductor whereas extrinsic semiconductor in which holes are excess are called p-type semiconductor. Now if we combine two diodes we get a transistor. It can be a p-n-p transistor or n-p-n transistor. That transistor has emitter base and collector and can be used as switches, amplifiers and as many other devices
Formula used:
${\text{output impedance = }}\dfrac{{{\text{voltage}}}}{{{\text{current}}}}$

Complete answer:
The characteristics of emitter, base and collector will be different. Emitter is highly doped while the collector and base are lightly doped. The width of the base is very less. Moreover emitter-base junction is always forward biased whereas base-collector junction is always reverse biased. Forward biased in the sense diode provides very low resistance and reverse biased in the sense diode provides huge resistance.
Now coming to impedance in case of DC circuit if there is a resistor we can measure using an ohmmeter but if there are capacitors and resistors in the circuit we can’t measure the obstruction for the flow of current hence there comes a term impedance.
${\text{output impedance = }}\dfrac{{{\text{voltage}}}}{{{\text{current}}}}$
We measure the voltage drop across the device where we need impedance and we divide that voltage with current flowing through that device in order to get the impedance. Its units will be the same as resistance units that are ohms.
Output impedance or source impedance will be higher for the common base arrangement.

So, the correct answer is “Option C”.

Note:
Transistor can be used as an amplifier. An ideal amplifier should have high input impedance and very low output impedance i.e it should not draw any current from the input. Impedance is the combination of the resistance and the reactance.