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Need help in understanding input offset curent of Op-Amp

by null void
Tags: curent, input, offset, opamp
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null void
#1
Nov30-13, 06:02 AM
P: 92

This picture is from http://www.ecircuitcenter.com/Circui...s/op_ibias.htm

I am not very sure if I am getting the right idea of how the IB+ and IB- works, but when i solve it open up one of the IB to solve the problem.

for IB+, (IB- is opened)
Vout = V-( 1 + R2/R1), ..... V- = IB+R3
Vout = IB+R3( 1 + R2/R1)

for IB- (IB+ is opened)
IB- = (V- - 0) / R1 + (V- - Vout) / R2
Vout = V-(1+ R2/R1) + IB-....can i consider that when IB+ is opened, V+ = 0 = V- ?
if I assume that I am right, i get what the page gives:

Vout = -R2(IB-)

one more thing, the IB+ is positive input biase current while the IB- is the negative input bias current? And the Iinput offset = difference of IB- and IB+ ->|IB+ - IB-| ?

These two input bias current and input offset current are the parameters that always appear in the datasheet right?
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Jony130
#2
Nov30-13, 06:23 AM
P: 410
Try watch this video
http://www.youtube.com/watch?v=TxBJb-Z0XFI

Also how can you add current into resistor ?
Vout = IB+R3( 1 + R2/R1)
null void
#3
Nov30-13, 07:01 AM
P: 92
Quote Quote by Jony130 View Post
Try watch this video
Also how can you add current into resistor ?
Vout = IB+R3( 1 + R2/R1)
it is (IB+)R3(1 + R2/R1), sorry for the confusing writing.

I consider that the IB+ current will pass throught the R3 and therefore the V+ is equivalent to (IB+)R3 . And i just apply the normal concept for finding the voltage amplification of operational amplifier circuit.

Jony130
#4
Nov30-13, 07:31 AM
P: 410
Need help in understanding input offset curent of Op-Amp

For sure we can use a superposition to find Vout.

First we assume IB+ > 0A and IB- = 0A
So have

Vout'1 = IB+*R3 * (1 + R2/R1) and this voltage is positive

Next we open IB+ = 0A and we left with IB-.

Vout'2 = - R2*IB- and for this IB current direction the output voltage is negative.

And finally

Vout = Vout'1 + Vout'2 = IB+•R3•(R2/R1 + 1) - IB-•R2
Jony130
#5
Nov30-13, 08:07 AM
P: 410
Quote Quote by null void View Post

These two input bias current and input offset current are the parameters that always appear in the datasheet right?
No, in datasheet you will find the value for the average input bias current.
IB = (IB+ + IB-)/2
and offset current
Ios = IB+ - IB-

so

IB+ = IB + Ios/2

IB- = IB - Ios/2
null void
#6
Nov30-13, 09:34 AM
P: 92
get it, thanks you very much !!


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