Why we need high differential input resistance in an OPAMP?

[graciousparul]

input resistance

Hi...
Why we need high differential input resistance in an OPAMP?
(as µAF771 FET has input ressitance of 1000GΩ)

thanks

 

[FvM]

Re: input resistance

Hello,

I could say: If you don't know, you probably don't need, no problem. Consider a high impedance sensor, e. g. a pH-electrode (an instrument to measure some chemical property), it has > GΩ impedance. To measure the voltage with small error, you need a buffer amplifier with several orders of magnitude higher impedance.

Regards,
Frank

 

[ehsanelahimirza]

input resistance

a very high resistance means that the orignal signal will not have to provide power at the input.

this almost infinite resistance causes negligible power requirements at the signal input hence we can amplify the signal without loading the source.

this thing is also done in analog voltage meters

 

[subharpe]

Re: input resistance

High input impedence assures that the device does not load to the circuit. That is why if you want to design a cascade of two operations with OPAMPs you can simply design the two stages separately and simply connect them. OPAMP has very high input impedence and very low output impedence. But suppose the same thing you want to do with two transistors. In that case designing the two stages separately and conecting them does degrade the performance. This is because the moderate input impedence of the second stage couples with the finite output resistance of the first stage. In quantitative terms in normal transistor operation the no load condtions and the loaded conditions are not same. But in OPAMP they are same upto certain limit of input level and frequency.

Consider a cascade amplifier made of transistor. The two stages are coupled through a capacitor. What the capacitor does is tha it blocks the dc part of theoutput of the first stage and couples the ac part only. The role of the capacitor is quite like an increase in the dc input resistance of the second stage s that loading of the second stage does not affect the dc bias condition of the second stage designed searately.

 

[pngzhi]

Re: input resistance

For a large input impedence.

 

[ronyutp]

Re: input resistance

Also a high impedance input is helpfull in linear OP AMP circuits. If the impedance input is not high enough some simple formulation could not be applied. For example, inverted amplifier A=-R2/R1 or not inverted amplier A=(1+R2/R1).