Author Topic: Help me about opamp amplifier ?  (Read 1178 times)

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Offline Nhan95

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Help me about opamp amplifier ?
« 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:
 

Offline rs20

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Re: Help me about opamp amplifier ?
« Reply #1 on: May 22, 2015, 01:21:13 am »
Explained here:



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.
 

Offline w2aew

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Re: Help me about opamp amplifier ?
« Reply #2 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.
« Last Edit: May 22, 2015, 03:02:53 pm by w2aew »
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