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

 

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

It is not as simple as that (it never is).

Yes the bias current flows into/out of both terminals.

It is not possible to determine which way as it could be either - it depends upon the individual op amp (not the type the actual op amp in question).

Bias current is modelled as two current sources configured as in the sketch.

Manufacturers provide two parameters.

The Average Input Bias Current usually called the input bias current which is simply add up the modulus of each source and divide by 2.

The input offest current, which is the difference between the two moduli.

 

The type of input transistors (NPN or PNP) determines which direction the input bias current flows. Most opamps use NPN input transistors but a few like the LM358 and LM324 use PNP.
Some opamps use Jafet or Mosfet input transistors that have no input bias current.

 

I'm not quite sure why you would want to be calculating the input current at the inverting and non-inverting inputs. Remember that an ideal op amp has infinite input resistance...read more

 

Disagreeing with studiot, agreeing with Audioguru,

By looking at the internal construction of a bipolar input op-amp, you can determine the direction of current flow. Often enough, it is significantly large to require you to account for it in your design. This type of op-amp is not, "Ideal".

I would guess that no real op-amp is ideal. If that one was ever invented, all others would cease being produced. Choosing the best op-amp for each circuit is a major part of good design.

 

Disagreeing with studiot, agreeing with Audioguru,

By looking at the internal construction of a bipolar input op-amp, you can determine the direction of current flow. Often enough, it is significantly large to require you to account for it in your design. This type of op-amp is not, "Ideal"..............

And that's not entirely true either. Some bipolar op amps have compensation bias circuits at the input (not always shown on the data sheet schematic) in an attempt to cancel the input bias current. This compensation is never perfect and sometimes the left-over uncompensated current goes out of the input terminal and sometimes it goes into the terminal. There's no way to predict.

 

The only certain method of determining direction is to measure it.

In addition to the input offset voltage the parameters I mentioned can also be practically determined.

Perhaps that is why they are quoted by manufacturers?

Since all three effects will be present it is normal to measure all three as a set.

 

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

If the opamp uses PNP devices, the bias current flows out of the inputs. If it is NPN devices, flows in. If it's FET input, current doesn't flow in or out.

 

If the opamp uses PNP devices, the bias current flows out of the inputs. If it is NPN devices, flows in. If it's FET input, current doesn't flow in or out.

For Jfet, the bias current is actually leakage current and if measurable is in the order of 1nA or less
For Cmos, leakage is probably 100pA or less
For specialty devices, leakage current is in the order of 10pA or so

 

Has anyone suggested reading the specification sheet of the particular device being used, seeing the direction and magnitude of offset current it defines, and using those numbers as the prediction?