Noise voltage from t-network feedback in opamp circuit

In summary, the conversation discusses the computation of noise voltage on the resistors of an op-amp configured as a t-network. The formula for noise voltage is given for a single feedback resistor and the question of how to combine multiple resistors is raised. The possibility of using Thevenin equivalent is mentioned and the calculation for the noise voltage of the resistor network is discussed. The role of R1 in the circuit is also questioned.
  • #1
jrive
58
1
I'm struggling to compute the noise voltage due to the thermal noise on the resistors of an op-amp circuit, configured as a t-network.

If it were a single feedback resistor, the noise voltage referenced to the output (RTO)would be given by sqrt(4KTRB).

I'm not sure how to combine them. I mean, do I combine the resistors into an equivalent one using Thevenin equivalent, for ex, and use that as the R value for the johnson noise?
If I'm referencing the noise to the output, I assume I'm looking from the inverting terminal out toward the output of the op-amp, true? Assuming R1 is connected between inverting input and center node of T, R2 is connected from center node to gnd, and R3 is connected from center node to output of opamp, the thevening equivalent R is R1||R2+R3. Can I use that in the calculation for the noise voltage of the resistor network?

Thanks.
 
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  • #2
You are correct. Rth = R1 + (R2 // R3)
The noise due to Rth appears to be applied to the inverting input, referenced to ground. The noise will be amplified by the circuit in the same way as the non-inverting input signal.

Is the op-amp input bias current really so high that R1 is needed in the circuit?
 

1. What is noise voltage from t-network feedback in opamp circuit?

Noise voltage from t-network feedback in opamp circuit refers to the undesired electrical noise that is introduced into the opamp circuit due to the use of a t-network as the feedback network. This noise can negatively affect the performance of the opamp circuit, resulting in inaccurate outputs.

2. How does t-network feedback introduce noise voltage in an opamp circuit?

T-network feedback introduces noise voltage in an opamp circuit due to the high impedance and capacitance of the network, which can act as an antenna and pick up external electrical noise. This noise is then amplified by the opamp, resulting in a higher noise voltage in the output signal.

3. What are the effects of noise voltage from t-network feedback in opamp circuit?

The effects of noise voltage from t-network feedback in opamp circuit include reduced signal-to-noise ratio, increased distortion, and reduced accuracy of the output signal. This can lead to errors in the measurement or processing of the signal and can affect the overall performance of the circuit.

4. How can noise voltage from t-network feedback in opamp circuit be reduced?

Noise voltage from t-network feedback in opamp circuit can be reduced by using low-noise components, minimizing the length of the feedback network, and shielding the circuit from external sources of electrical noise. Additionally, using a different type of feedback network, such as a resistive or capacitive network, can also help reduce noise voltage.

5. Are there any advantages to using a t-network as the feedback network in an opamp circuit?

Yes, there are some advantages to using a t-network as the feedback network in an opamp circuit. It can provide high gain and stability, and is commonly used in high-frequency applications. However, the introduction of noise voltage should be carefully considered and mitigated to ensure optimal circuit performance.

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