When I was little my grandfather would ask me “what’s heavier, 5 pounds of lead or 5 pounds of feathers?” and I would immediately answer “lead of course!”
Although the volume is not the same, the weight is the same. When we talk about amplifier noise what do we assume? Signal gain or noise gain? Well it depends, and it matters. It depends on who you talk to because generally people say gain, referring to signal gain, and it matters because your bandwidth is affected by the noise gain. The closed loop bandwidth, or usable bandwidth, is written as GBW/NG regardless of the configuration you’re using (inverting or non-inverting). For an inverting amplifier, the signal gain is –Rf/Rg whereas for a non-inverting configuration the signal gain is equal to the noise gain, 1+Rf/Rg.
Let’s look at the OPA348, a general purpose 1MHz op amp. Let’s assume the feedback and gain resistors are both equal to 10k. The usable bandwidth in this case is 500kHz or -1MHz/(1+(10k/10k)). The term in the denominator is the noise gain and is the same for inverting and non-inverting configurations. But notice the signal gain in the inverting configuration is –R2/R1 or 1 (magnitude) in this case. The non-inverting DC gain is 1+(R2/R1) and is equal to the noise gain, 2 in our case.
So now your DC gains don’t match and you want to double R2 in the inverting configuration -2(R2/R1) which gives you 2, the same as your non inverting circuit signal gain.
However, the noise gain is now different. It’s 1+2*(R2/R1), or 3, which means the closed loop bandwidth is 333.33kHz
So the question becomes why would you consider using an amplifier in an inverting configuration? One reason is the benefit of the common mode. The inverting input is at ground (virtual) and you don’t see the modulation you normally would in a non-inverting configuration and you usually get better distortion this way.
You can read more about the importance of CMRR in “Will the real Vos please stand up?” on TI Precisions Design Hub and “What you need to know about CMRR - the operational amplifier (Part 1)” right here on Analog Wire.
Figure 1
Figure 2
DC gain of figure 1 and 2
Increasing the feedback resistor in figure 2 to 20k to match the DC gain of figure 1
