Op-amps

[moonlite]

How would I calculate offset on the output of an inverting amplifier due to the bias current specification of the circuit, for a particular amplification factor?

Thanks for the help.

 

[Skogsgurra]

If you ignore voltage offset, which isn't always possible, you just multiply bias current out of (or into) the inverting and non-inverting inputs by the resulting resistance between each input and your reference (zero) voltage. The difference between the two voltages is your bias current dependent offset voltage.

Example.

An inverting amplifier with 100 kohms to inverting input and 1000 kohs feedback. 100 kohms from noninverting to ground. Bias current out of both inputs is 1 microampere (high value chosen for clarity).

Non-inverting will be at 1x100//1000 = 91 millivolts and non-inverting will be at 1x100 = 100 mV. The difference is 9 mV and that is also your offset. You may also need to add the voltage offset.

Gunnar Englund

http://www.gke.org

 

[itsmoked]

Well stated skogs, and I'll add that is why you would mess with the above mentioned 100k from the non-inverting pin to get its value to be the same as the equivalent resistance on the inverting input, which then reduces that 9mV to 0 if possible.

Keith Cress
Flamin Systems, Inc.-

http://www.flaminsystems.com

- kcress@<solve this puzzle>

 

[Skogsgurra]

Yes, I left that as an excersise for the reader.

Gunnar Englund

http://www.gke.org

 

[logbook]

Sure provided the input offset current is zero, which it isn't. Modern opamps often have input offset currents nearly equal to input bias current (bias current is compensated).

Note that these "virtual input offset voltages" are not measurable directly. The bias current created offset voltage due to the non-inverting input will appear on the non-inverting input to the opamp. You will be able to measure it with a DVM. The inverting input will not show a voltage error due to the bias current in the inverting input.