What is noise gain? I've heard of signal gain, but I'm not familiar with the term noise gain, can you explain?
For noise gain, first consider the basic op-amp gain circuit in the diagram. If it is being used as an inverting amplifier (B), the non-inverting input will be grounded, the signal will be applied to the free end of Ri and the gain will be -Rf/Ri. On the other hand, in a non-inverting amplifier (A) the signal is applied to the non-inverting input and the free end of Ri is grounded; the gain is (1 + Rf/Ri)
The amplifier’s own voltage noise is always amplified in the non-inverting mode; thus when an op-amp is used as an inverting amplifier at a gain of G, its voltage noise will be amplified by the noise gain of (G+ 1). For the precision attenuation cases, where G<1, this may present problems. (A common example of this is an active filter circuit where stopband gain may be very small but stop-band noise gain is at least unity.)
Only the amplifier voltage noise-and any noise developed by the noninverting-input current noise flowing in any impedance present in that input (for example, a bias-current compensation resistor)-is amplified by the noise gain. Noise in Ri, either Johnson noise or arising from inverting input noise current, is amplified by G in the same way as the input signal, and Johnson noise voltage in the feedback resistor is not amplified but is buffered to the output at unity gain.
Thanks for you excellent explation of "Noise Gain". It is really a great help for me. After reading your words, I come across some future problems. What does the amplifier's owm voltage noise specifically include? Knowing this and the noise gain, we can accurately compute the voltage noise on the output pin. Why is it always amplified in the non-inverting mode? Is there any thing we can do to reduce the effect of voltage noise which will be amplied by noise gain on the whole precision of the amplifier?
The Noise gain is the same for both noninverting or inverting amplifier configurations so whether you use an inverting or noninverting configuration the Noise gain will be the same for both. As for what the amplifier noise sources contain, its all of the internal circuitry and the noise associated with the amplifier circuitry. You can limit noise a few different ways.
1. Use low noise op amps
2. Use low vale resistors
3. Limit the bandwidth to reduce the integrated noise.
I hope this helps.
That is excately what I want to know. I will try those ways that you suggested in my design.
Thanks a lot.
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