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Design >> Analog Design >> OPAMP input resistance
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Message started by extremis on Dec 29^th, 2010, 5:15am

Title: OPAMP input resistance
Post by extremis on Dec 29^th, 2010, 5:15am

Could an OPAMP have different input resistances for +/- inputs? Would that be a problem?

Title: Re: OPAMP input resistance
Post by rfmagic on Dec 30^th, 2010, 1:17am

In open loop condition the +/- input of the opamp are typically identical. the probelm is in closed loop condition. this is the main reason for using an instrumentation amplifier

take a look at the attached document

Title: Re: OPAMP input resistance
Post by nus_lin on Jan 15^th, 2011, 7:06pm

for bipolar opamp, you will see input resistance, but for cmos opamp, ideally, the resistance approaches infinity. in reality, due to the esd, cmos opamp also has a resistor in the range of 100M ohm.

Title: Re: OPAMP input resistance
Post by raja.cedt on Jan 16^th, 2011, 5:28am

due to leakge there may be chance of having differential resisatnce in cmos.

Thanks.

Title: Re: OPAMP input resistance
Post by thechopper on Jan 17^th, 2011, 4:43pm

It is true that in closed loop the input resistance might be different in the + and - inputs, but only for single ended configurations.
For fully balanced opamps both inputs have the same circuit connected to them, and due to the balanced nature of the circuit, the input resistance for both of them will be ideally identical, the same way they are under open loop conditions.

Concerning MOS inputs, the input resistance will be almost infinite but only at DC. At higher frequencies the unbalance will be noticeable again only for SE topologies, while for FB configurations they should be identical.

Best
Tosei