The non- ideal characteristics of op-amp which are DC and AC are:

DC error components are:

- input bias current
- input offset current
- input offset voltage
- thermal drift

__Input Bias Current:__

__Input Bias Current:__

The input bias current of an operational amplifier is the average of the two currents flowing at the inverting and non-inverting terminal respectively. The mathematical equation is given by:

* I _{B} = (I_{B}^{+}+I_{B}^{–})/2*

**For a Basic inverting op-amp :**

If V_{1 }= 0, then output voltage is offset given by,

* V _{0} = I_{B} – R_{f1}*

For 741 op-amp, R_{f } = 1mΩ and input bias current is 500mA or less.

Therefore, output voltage is driven to 500mV.

In applications where signal levels are measure in milli-volts it is totally unacceptable. Hence, a compensated resistor is added between non-inverting input terminal and ground.

Current I** _{B}^{+} **going through the compensator resistor has a voltage V

_{1}across it

From KVL,

*-V _{1} + 0 + V_{2} – V_{0} = 0*

By selecting proper value of R_{comp} V_{2} and be cancelled with V_{1} to make V_{0 }

Therefore,* V _{1} =I_{B}^{+} R_{comp }(or)*

* I _{B}^{+} = V_{1}/ R_{comp _}(A)*

At mode ‘a’ voltage (-V_{1}) exists because voltage at non-inverting terminal is (-V_{1})

Therefore, when V_{i }= 0

*I _{1 }= V_{1}/R_{1 }and I_{2} = V_{c }/ R_{f} *

For compensation V_{0} = 0 for, V_{i} =0

Therefore, From V_{2 }= V_{1 }= 0 => V_{1 }= V_{2 }

I_{2 }= V_{1 }/ R_{f }

KCL at node ‘a’

I_{B}^{– } = I_{2 }+ I_{1 }

I_{B}^{– }_{ }= V_{1}/R_{1} + V_{1 }/ R_{f }

= V_{1 }[ 1/ R_{1 }+ 1 / R_{f }]

= V_{1 }[ (R_{1 }+ R_{f })/ R_{1}R_{f }] __(B)

By equating _{ } (A) and (B)

** ** V

_{1}/ R

_{comp }= V

_{1 }[ (R

_{1 }+ R

_{f })/ R

_{1}R

_{f }]

__ V _{1}/ R_{comp }= (R_{1 }+ R_{f })/ R_{1}R_{f}__