Op amp gain multiple problem

傅里叶的猫

Op amp gain multiple problem [Copy link]

This is a Gaussian noise generating circuit. The first stage generates thermal noise from the resistor, the second stage is an inverting amplifier circuit, and the third stage is a non-inverting amplifier circuit. What I don't understand is whether the white noise of the first stage comes from the 10K resistor or the 1K resistor. The first stage can be regarded as a non-inverting amplifier or an inverting amplifier. If it is a non-inverting amplifier, then the overall gain will have a negative sign. Please solve it.

cruelfox

In your circuit, each resistor contributes to the output noise.

Let's look at the first stage. The three resistors 1k, 10k, and 310k each have thermal noise, which can be analyzed as ideal resistors connected in series with a noise voltage source. The thermal noise of the 1k resistor is amplified in phase (32x), the thermal noise of the 10k resistor is amplified in reverse phase (31x), and the thermal noise gain of the 310k resistor is doubled. Comparing the thermal noise of these three resistors, the noise of the 10k resistor has the greatest impact. However, considering the typical value of the input voltage noise of the op amp LF411 of 18nV/sqHz, I am afraid that the noise contribution of the first stage op amp is still the greatest.

chunyang

In addition to the thermal noise of the resistors, the output of the circuit also includes the ground noise of the system, and the entire circuit is a high-gain AC amplifier with a grounded input.

傅里叶的猫

cruelfox posted on 2019-10-20 15:54 In your circuit, each resistor contributes to the output noise. Just look at the first level, 1k, 10k, 310k three resistors each have thermal noise, ...

I calculated that the thermal noise of 10K is 0.92V and the thermal noise of 1k is 0.3V. Thank you for your guidance.

傅里叶的猫

chunyang posted on 2019-10-20 16:54 In addition to the thermal noise of the resistor, the output of the circuit also includes the ground noise of the system. The entire circuit is a high-gain AC amplifier with a grounded input.

Yes, ground noise is not negligible.