Input Bias Current Cancelation Resistors—do you really need them?

RE: Input Bias Current Cancelation Resistors—do you really need them?

Steve Judge — Wed, 24 Feb 2021 19:03:03 GMT

Depending on your test strategy and the surrounding circuit, Rb's presence may be also (or entirely otherwise) supporting an independent in-circuit test of the amplifier.

RE: Input Bias Current Cancelation Resistors—do you really need them?

Pete%20Semig — Tue, 07 Apr 2020 19:37:28 GMT

Hello Kalpesh,

Great question! The use of R2||R1 yields the effects of Ib at the input of the device, which is also known as referred-to-input, or RTI. The use of just R2 (or Rf as used in the TI Precision Labs presentation), yields the effects of Ib at the output of the device, which is known as referred-to-output, or RTO. Another way to look at it is if you take the effect at the input, which uses R2||R1 and multiply that by the non-inverting gain of the amplifier (which refers the effect to the output), you're left with just the feedback resistor: (R2*R1)/(R2+R1))*((R2+R1)/R1)=R2. Of course R2 is the same as Rf in the TI Precision Labs presentation.

RE: Input Bias Current Cancelation Resistors—do you really need them?

KALPESH SHELKE — Mon, 30 Mar 2020 15:05:05 GMT

Hi Bruce,

I have a query.

Referring to page 31 of another TI document (training.ti.com/ti-precision-labs-op-amps-vos-and-ib, it was derived that in a non-inverting amplifier configuration the input bias current only flows through the feedback resistor (R2) and not through the resistor R1 (10K).

So in such a case, why should Rb be (R1||R2) and not R2.

Requesting the clarification.