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While I was studying the DC characteristics of op-amps I came across input bias current.

I understood the concept that it draws some current when input is applied but in proof they put both the voltages at ground and they were still considering input bias current. I'm a little confused that even when there is no voltage at input from where the op-amp is drawing current?

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  • \$\begingroup\$ I hope you've connected the two power rails of your opamp to DC supply or to DC supplies. Those connections are often not shown for convenience. That's the source where the opamp draws its main current. \$\endgroup\$
    – glen_geek
    Commented Jan 10, 2020 at 15:37

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but in proof they put both the voltages at ground and they were still considering input bias current.I'm a little confused that even when there is no voltage at input from where the op-amp is drawing current?

Consider the LM324's equivalent circuit: -

enter image description here

If either input is grounded or taken to the negative rail then current will flow from the base of Q1 and Q4 to the tune of about 45 nA.

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  • \$\begingroup\$ possible in LM741 too? \$\endgroup\$
    – well...
    Commented Jan 10, 2020 at 12:17
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    \$\begingroup\$ @user5841 yes, in every electronic component you've got leakage. and honestly, the 741 is pretty bad in that respect (the 741 is essentially the worst opamp that you can still buy, don't use it in new designs, unless you know exactly that you don't need a good opamp AND are looking to save a few € on a device of which you'll make more than 1000). \$\endgroup\$ Commented Jan 10, 2020 at 12:26
  • \$\begingroup\$ In a 741 the input transistors are NPN and so when the inputs are taken to within 2 volts of the most negative rail, the bias currents will decrease but, we only have "equivalent circuits" provided by manufacturers so we can't be absolutely sure that there isn't some silicon inside the 741 that acts as a high but very light pull-up and this will produce a bias current when the input is taken exactly to the most negative power rail. \$\endgroup\$
    – Andy aka
    Commented Jan 10, 2020 at 12:29
  • \$\begingroup\$ righto.com/2015/10/inside-ubiquitous-741-op-amp-circuits.html \$\endgroup\$ Commented Jan 10, 2020 at 12:42

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