Answer / jayesh pakhale

Because Inputs of the opamp go to the base of the
transistors where transistors are in CE mode which provide
high base to emitter resistance while low collector to
emitter resistance so the input resistance of opamp is high
while ouput resistance of opamp is low which are required
to work opamp as a amplifier.

Answer / raj

for avoiding loading and providing impedance matching.

Answer / prabhu

Because this will help op amp to draw very negligible current from the input.

Answer / shethia.tejas

because first stage of op-amp is FET DIFF AMP and FET has
highest i/p imp.
The output resistance is LOW as with negative feedback it
approaches zero at low frequencies.

Answer / lodhi

If input resistance is high, any signal source can drive
the op-amp and to reduce the loading , o/p resistance
should be low...

Answer / dlhylton

An op amp has high input impedance to prevent loading. When
the impedance is very high, the current that the op amp
draws is very low.

Op amps also have high input impedance because it's a
voltage gain device. In order for the op amp to amplify the
voltage, the voltage signal must drop fully across. And for
this to happen, the op amp must have high input impedance,
v=ir. The greater the impedance, the greater the voltage
drop.

Check out:
http://www.learningaboutelectronics.com/Articles/Why-does-
an-op-amp-need-a-high-input-impedance-and-a-low-output-
impedance

for full explanation

Answer / manzar hussain

if input impedance is high it means voltage drops is high,ohms law V=IR it proves that the voltage drops is high because of high input impedance,,and if voltage drops is high it means current drops low and the max current will reaches to output

Answer / shad

bcoz if current is coming from high rating aqup high res
can make it accordance to opamp remaining ll be in loss.
(to avoid the damage)and output resistanca is low so that
mim loss occures at output end and max current can flow to
device at o/p.

Answer / sirisha

if the i/p impedance is high the i/p weak signal will
develop sufficient voltage so that there would be no
attenuation at the output and also there would be no loading
of the preceding stage and the o/p impedance is low so that
the o/p can drive other devices

Answer / swathidstar

Loading at the input point and to pass the maximum output
to the load is the requirement of Op-Amp which is a current
exchange device from input side to output side.

Why bjt is called current control device?

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