Input bias current opamp

Aamir Ali

Other Parts Discussed in Thread: LM339

Do bias current of opamp flow into inverting & non-inverting terminal? how to determine in which direction current will flow.

over 12 years ago

0 Ron Michallick over 12 years ago

TI__Guru**** 155286 points

Aamir,

It depends. If the data sheet shows a negative IIB then conventional current flows out of the device. If it is positive then current flows into the device.
If the data sheet IIB value shows positive and negative values then the op-amp has bias current cancellation circuitry and the current will be small but the polarity may be either.

PNP input devices have a negative IIB (current flows out of inputs)
NPN input devices have a positive IIB (current flows in to inputs)

Regards,
Ron Michallick

0 Aamir Ali over 12 years ago in reply to Ron Michallick

Expert 2525 points

Hi,

Thanks for ur reply.

1. IN NPN, JFET devices current always flows out while in PNP devies always current flows out. Right. Now I think there no exception here. Right?

2. In below diagram, with dual supplies,direction of flow of current is shown, which gives input bias current & error accordingly right.

Now, in case of single supply, as shown below, what is direction of current, I don't understand, what is direction of input bias current here

0 Ron Michallick over 12 years ago in reply to Aamir Ali

TI__Guru**** 155286 points

The input current on a JFET (junction field effect) transistor is based on PN leakage current and is exponentially proportional to temperature.

The OP-AMP in the picture is a LM339 with a PNP (bipolar, not JFET) input and the current flows opposite of your drawn arrow.
In the second setup, the current still out of pins 6&7 (just like the first schematic).

Regards,
Ron

0 Aamir Ali over 12 years ago in reply to Ron Michallick

Expert 2525 points

Hi Ron,

Consider it any other general NPN input. Then how does current flows

0 Ron Michallick over 12 years ago in reply to Aamir Ali

TI__Guru**** 155286 points

Aamir,

For NPN input stage, current flows as you drew in first picture.
On second picture, there would be no current flow and the op-amp would not work. NPN input op-amps do not support input voltage equal to the negative supply rail.

Regards,
Ron Michallick

0 Aamir Ali over 12 years ago in reply to Ron Michallick

Expert 2525 points

Is that one of reason for NPN opamps don't work for inputs equal to -ve rails that they don't get properly biased

0 Ron Michallick over 12 years ago in reply to Aamir Ali

TI__Guru**** 155286 points

Yes, that is the reason.

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Input bias current opamp