Question

Since the input impedance of an ideal operational amplifier is infinite.
A. Its input current is zero
B. Its output resistance is high
C. Its output voltage becomes independent of load resistance
D. it becomes a current controlled device

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 an emitter base and collector and can be used as switches, amplifiers and as many other devices.

Formula used:
${\text{input impedance = }}\dfrac{{{\text{voltage}}}}{{{\text{current}}}}$

Complete step by step 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.
The voltage which we applied will be lesser than the output voltage. That means there will be some voltage gain. That voltage gain depends upon the load resistance and the input resistance.
Now that input voltage we apply determines the input current.
${\text{input impedance = }}\dfrac{{{\text{voltage}}}}{{{\text{current}}}}$
For DC circuits we use the term resistance and for AC circuits we use the term called impedance
Here impedance is given as infinity that means obstruction to flow of current is high. So input current will be zero.
Hence option A will be the answer.

Note:
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. 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.