Opamp input bias current question

[Jon86]

Just a very quick question, should an opamp's input be regarded as a constant current load, or a resistance? Which one is more accurate?

[w2aew]

Just a very quick question, should an opamp's input be regarded as a constant current load, or a resistance? Which one is more accurate?

Depends a bit on the op amp in question, but in general they would be considered a constant current sink (or source) when the op amp is running closed loop. If saturated with a large input difference voltage, then the input bias picture will differ.

[Jon86]

So would it be true to say if a charged capacitor is connected to the input with the opamp running a non inverted configuration, the capacitor would discharge linearly?

[w2aew]

So would it be true to say if a charged capacitor is connected to the input with the opamp running a non inverted configuration, the capacitor would discharge linearly?

It will depend highly on the op amp.  But, in general, over a limited range it is likely.  But, there are a lot of "realities" that will affect this, some of which are:

1) The technology use for the op amp:  If it has a JFET or MOSFET input stage, the input bias current will be very, very low, and may even be smaller than the natural leakage current of the cap.

2) The structure of the op amp input: The input bias current for an op amp could be positive (flowing into the op amp), if the input transistors are NPN; or could be negative (flowing out of the input pins) if the input transistors are PNP. 

3) The bias point of the op amp: Some rail-rail op amps have complex input stages than whose input bias current magnitude and direction will depend on the common mode voltage.

4) Temperature, supply voltage, etc. will also affect input bias current...

Bottom line, you have to examine it carefully for a given op amp. In general, it's probably not a good idea to *rely* on the bias current to form an integral part of your design's functionality.

[tszaboo]

Just a very quick question, should an opamp's input be regarded as a constant current load, or a resistance? Which one is more accurate?

+ It changes with temperature by factors of ten sometimes. And if it is a copper opamp, than it is not even constant in time. And it can be either direction. And sometimes if you put resistance compensation on the opamp, it makes everything worse. So really, we might only help you if you can tell us the part number.

[KJDS]

Regardless of the op-amp configuration, it's a complex non-linear effect with a significant temperature coefficient that is also dependent on common mode voltage, supply voltage and it's also going to vary from batch to batch and from one manufacturer to another. About the only thing I'd rely on it for is to be unreliable.

[dannyf]

About the only thing I'd rely on it for is to be unreliable.

Reliably unreliable; predictably unpredictable....

[free_electron]

and that bias current is not a constant .... it depends on what the input voltage is...