I have stumbled today with a very simple and basic question.
I used to put a resistor in series with the non-inverting input of the Opamp in order to "cancel" the input bias current effect (I learned that some time ago and use it till today as a rule of thumb: Rn-i = Rfb || Ri. Rn-i - resistor at the non-inverting input. Rfb - resistor at the feedback loop. Ri - resistor at the inverting input).
But today, building a basic power amplifier with an Opamp pre-amp stage, I first thought about the noise that the resistor will add to the circuit.
If anyone is familiar with noise analysis: to quantify the noise in the circuit, generally what's done is to compare the total noise present in the circuit with the input signal, "at the input". As this resistor IS at the input, ALL its noise will be clearly amplified too.
My question is: in terms of noise, is it worth putting this resistor at the input? Is the "bias current cancelation" so important?
I couldn't HEAR any difference in the noise with and without that resistor in the circuit I've built. But in really noise critical applications, what's worse, the noise or the input current?
Castilho
I used to put a resistor in series with the non-inverting input of the Opamp in order to "cancel" the input bias current effect (I learned that some time ago and use it till today as a rule of thumb: Rn-i = Rfb || Ri. Rn-i - resistor at the non-inverting input. Rfb - resistor at the feedback loop. Ri - resistor at the inverting input).
But today, building a basic power amplifier with an Opamp pre-amp stage, I first thought about the noise that the resistor will add to the circuit.
If anyone is familiar with noise analysis: to quantify the noise in the circuit, generally what's done is to compare the total noise present in the circuit with the input signal, "at the input". As this resistor IS at the input, ALL its noise will be clearly amplified too.
My question is: in terms of noise, is it worth putting this resistor at the input? Is the "bias current cancelation" so important?
I couldn't HEAR any difference in the noise with and without that resistor in the circuit I've built. But in really noise critical applications, what's worse, the noise or the input current?
Castilho
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