# Need help in understanding input offset curent of Op-Amp

1. Nov 30, 2013

### null void

This picture is from http://www.ecircuitcenter.com/Circuits/op_ibias/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?

2. Nov 30, 2013

### Jony130

Try watch this video

Also how can you add current into resistor ?
Vout = IB+R3( 1 + R2/R1)

Last edited by a moderator: Sep 25, 2014
3. Nov 30, 2013

### null void

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.

4. Nov 30, 2013

### Jony130

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

Last edited: Nov 30, 2013
5. Nov 30, 2013

### Jony130

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

6. Nov 30, 2013

### null void

get it, thanks you very much !!

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