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Electronics => Beginners => Topic started by: Nhan95 on May 22, 2015, 01:16:06 am

Title: Help me about opamp amplifier ?
Post by: Nhan95 on May 22, 2015, 01:16:06 am
In this inverting amplifier opamp, Why do they add the resistor R3 = R1||R2 to the non-inverting input although It doesn't affect to the result of Vout = R2/R1 * Vin. Thanks  :bullshit:
Title: Re: Help me about opamp amplifier ?
Post by: rs20 on May 22, 2015, 01:21:13 am
Explained here:

https://www.youtube.com/watch?v=TxBJb-Z0XFI (https://www.youtube.com/watch?v=TxBJb-Z0XFI)

TLDR: Because an actual, real-life op-amp has a small amount of current flowing at the inputs, which tends to be balanced between them. Having equal impedances at the input means these input currents cause an equal voltage drop, and so the effect on the output is nil.
Title: Re: Help me about opamp amplifier ?
Post by: w2aew on May 22, 2015, 02:58:34 pm
In this inverting amplifier opamp, Why do they add the resistor R3 = R1||R2 to the non-inverting input although It doesn't affect to the result of Vout = R2/R1 * Vin. Thanks  :bullshit:

To provide fundamental cancellation of the input offset caused by the op amp's small input bias currents.