[SOLVED] bias current of op-amp

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Hi,
i am trying to understand the basics of operational amplifier.
I`m a little confused with the concept of bias current.Is it the current need to active the inverting and the non-inverting terminals of the op-amp ????

or the current which is present when op-amp has zero biasing (currents which are flowing in and out of op-amp and very small and can be negligible,we keep resistors to drive that currents to the ground)
 

Thank you, so to remove the effects of bias currents we need to keep resistors in series to the input terminals of op-amp. for example: what value of resistor do we keep for LM741 OP-AMP ? the supply voltage is given as +/- 15v
 

You don't generally use input series resistors. In some situations, you add a series resistor to balance the bias current induced voltage drop across the external circuit. The method mainly applies to OPs with BJT input stage and without input current compensation.

Resistors increase circuit noise and are only reasonable, if small DC offsets matters in circuit operation.
 
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so what can we do about 80nA of bias current of LM741 ? is it ok to neglect it ?
 

Lets pretend that this amp has the same 80nA bias to both + and - inputs. If the + input has a 10K to ground then offset voltage = 10k X 80nA = 800 micro volts. if the - input has 100K to ground then its offset voltage = 8mV. Now if the gain of the amp is > 100, then the output voltage will be (-8mV + .8 mV) X 100 = -.72V. So if it is important for the output to be at 0 DC in the absence of a signal, or the gain is higher then 100 or so, the input bias currents must be catered for and that must be checked right across the working temperature range of the device.
Frank
 
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