Electronic – Resistor Between Voltage Reference and + Input of Op-Amp? Why

inverting-amplifiernon-invertingoperational-amplifiervoltagevoltage-reference

something I commonly see in schematics which include op-amps set up as inverting amplifiers is the inclusion of a resistor between the non-inverting "(+)" input of an op-amp and a strong voltage reference such as ground, power, or a voltage reference set up using an op-amp buffer.

What is the purpose for this? The (+) input of op-amps are high impedance, and the references mentioned above are strong sources of current, which aren't vulnerable to crosstalk in the way a weak reference would be.

Best Answer

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Figure 1. Both op-amp inputs have a bias current. If R3 is zero ohms then there will be a difference in the offsets between the two inputs. Source: ECircuitCenter.

From the same article:

One of the golden rules of op amp analysis says this: no current flows into either input terminal. This concept is key for analyzing an amplifier's signal gain. However, in reality, a small current flows into both inputs to bias the input transistors. Unfortunately, this bias current gets converted into a voltage by the circuit's local resistors and amplified right along with the signal. The result is an output error in your circuit. What can you do about it? A clever choice of resistor values can help you cancel most of the output error. The remaining error can be adjusted to zero if necessary.

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