# First order low pass filter

Would connecting a resistor to the positive input of an amplifier make any difference in its output and if not why in some schematics there is always a resistor attached to the opamp positive input? simulate this circuit – Schematic created using CircuitLab

The resistor you see in some schematics is when the op-amp is a bipolar device. It will have significant input bias currents compared to a FET type op-amp. Those bias currents form an error voltage due to the external resistors. If the same net value of resistance is placed in both inputs those error voltages largely cancel out.

So, in your 2nd example, the net resistance "seen" at -Vin is 10k || 1k and this approximately equals 900 ohms.

By the way a TL081 is a FET type op-amp so putting a 900 ohm resistor in the +Vin pin is pointless because the bias current is only 50 pA. Compare this with the old bipolar LM324 workhorse - it has a bias current that is ~1000 times higher.

• Could you elaborate more. What does it mean exactly when you say opamp is bipolar device? That means that there is a current at negative and positive inputs?
– Jack
Apr 22 '16 at 11:40
• Another question: I don't get it how 10K|| 1 k?
– Jack
Apr 22 '16 at 11:46
• It's the technology of how the op-amp is designed. Bipolar op-amps use BJT technology. FET op-amps tend to mainly use JFET technology etc.. Apr 22 '16 at 11:46
• 10k || 1k means 10k in parallel with 1k = 909.09 ohms. Apr 22 '16 at 11:47
• It makes no sense to verify/calculate the need to use the parallel connection 1k||10k with 100% accuracy because both input bias currents are not identical (but unknown!). But - because we don`t know better - we assume that both are equal.
– LvW
Apr 22 '16 at 13:45