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Example: The Input
Bias Current
Q: How do input bias currents I B 1 and IB 2 affect amplifier operation?
A: Consider both inverting and non-inverting configurations.
Inverting Configuration
R2
i2
vin
R1
i1
vIB 1
v+
IB 2
Jim Stiles
vout
+
The Univ. of Kansas
Dept. of EECS
5/11/2017
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KCL is now a bit more tricky!
In this case, we apply KCL and we find:
i1  i2  IB 1
However, we still find v  v   0 (neglecting the input offset voltage) by virtue
of the virtual short.
Therefore, from KVL and Ohm’s Law:
i1 
vin v  vin

R1
R1
and
i2 
v  vout vout

R2
R2
R2
Combining these results:
vin vout

 IB 1
R1
R2
The output voltage is thus:
vout
Jim Stiles
i2
vin
R1
i1
vIB 1
v+
R 
   2 vin  R1 IB 1
 R1 
IB 2
The Univ. of Kansas
vout
+
Dept. of EECS
5/11/2017
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Should we make R1 really small?
Note again that if IB 1  0 , the result reduces to the expected inverting
amplifier equation:
R 
vout    2 vin
 R1 
The second term in the above expression ( IB 1 R1 ) therefore represents another
output offset voltage!
It appears that we should keep the value of R1 small to minimize the output
offset voltage.
R2
Q: What would a small value of R1 do
to the amplifier input resistance?
A:
vin
i2
R1
i1
vIB 1
v+
IB 2
Jim Stiles
The Univ. of Kansas
vout
+
Dept. of EECS
5/11/2017
840998426
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Please welcome the non-inverting config.
Non-Inverting Configuration
R1
R2
i1
i2
IB 1
vin
vIB 2
vout
v+
+
Neglecting the input offset voltage, we can use the virtual short to determine
that:
v  vin
and KCL provides the same result as that of the inverting amplifier:
i1  i2  IB 1
Jim Stiles
The Univ. of Kansas
Dept. of EECS
5/11/2017
840998426
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Again, a DC output offset
From KVL and Ohm’s Law:
i1 
and likewise:
i2 
Combining, we find:
vin vin vout

 IB 1
R1
R2
0 v  vin

R1
R1
v  vout vin vout

R2
R2
R1
R2
i1
i2
IB 1
or rearranging:
 R 
vout   1  2 vin  IB 1R2
 R1 
Jim Stiles
vin
vIB 2
The Univ. of Kansas
vout
v+
+
Dept. of EECS
5/11/2017
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We have another trick or two up our sleeve
Again, we find that this result is simply the ideal non-inverting expression:
 R 
vout   1  2 vin
 R1 
with an added output offset voltage term:
IB 1 R2
In this case, we find that this offset voltage is minimized by making feedback
resistor R2 small.
In general, we find that the effects of the input bias currents can be minimized
by using small resistor values.
However, we will find that there is an additional
strategy for minimizing the effects of input bias
currents!
Jim Stiles
The Univ. of Kansas
Dept. of EECS