Inverting vs non-inverting - Pros and Cons - Page 1
Forum section: electronics / projects

author: ricko_uk
author_rank: Super Contributor
timestamp: February 12, 2020, 03:29:19 pm
content: Hi,often using inverting or non-inverting does not really make any difference. But in some instances it does.Does anybody have a summary of pros and cons of the two configurations? From simple things like gain (R2/R2 vs 1+R2/R1) to input impedance to noise levels to the fact that inverting can be used as a mixer etc. etc.?We could compile a list here then I can summarise all points at the end into a downloadable PDF for anybody to download.Many thanks

author: Wimberleytech
author_rank: Super Contributor
timestamp: February 12, 2020, 03:34:06 pm
content: Quote from: ricko_uk on February 12, 2020, 03:29:19 pm1)A non-inverting configuration suffers from CMRR limitations of the opamp because it naturally has a common-mode signal, whereas an inverting configuration has zero CM input.2)An inverting configuration has a lower BW than the non-inverting for the same absolute gain.3)Non-inverting configuration has a higher input resistance than inverting.

author: ricko_uk
author_rank: Super Contributor
timestamp: February 13, 2020, 12:15:53 am
content: Any other suggetions/contributions from anybody else?And thank you Wimberleytech!!

author: moffy
author_rank: Super Contributor
timestamp: February 13, 2020, 12:16:11 am
content: Inverting, according to Jim Williams, has lower input distortion because the input circuit is kept close to a constant value. Therefore less overall distortion.

author: Wimberleytech
author_rank: Super Contributor
timestamp: February 13, 2020, 12:23:50 am
content: Inverting configuration offers the ability to do summing in a straightforward manner.

author: David Hess
author_rank: Super Contributor
timestamp: February 13, 2020, 02:28:32 am
content: Wimberleytech covered everything I might have said.Bootstrapping the supply voltages to follow the non-inverting input suppresses common mode errors in non-inverting amplifiers but it is a lot of extra complexity.

author: Circlotron
author_rank: Super Contributor
timestamp: February 13, 2020, 04:50:39 am
content: Quote from: moffy on February 13, 2020, 12:16:11 amAnd presumably also because the feedback signal subtracts directly from the same node that the input signal is applied to. With non-inverting, the feedback signal goes to the opposite side of a long-tailed pair and so when the feedback signal finally reaches the summing junction it does not truly represent the output signal. Very close of course, but not exactly.

author: Bud
author_rank: Super Contributor
timestamp: February 13, 2020, 07:11:47 am
content: Inverting inverts and non-inverting does notAs trivial as it may sound but may be important in many cases.

author: Circlotron
author_rank: Super Contributor
timestamp: February 13, 2020, 07:44:02 am
content: An inverting amp can have a gain of less than 1 and consequently an unlimited input signal voltage, provided the output doesn’t clip.A non inverting amp input voltage is limited to, at most, the supply rails, often somewhat less.

author: Wolfram
author_rank: Frequent Contributor
timestamp: February 13, 2020, 11:31:22 am
content: For non-inverting, the input common mode range can limit your signal amplitude. Inverting has a higher noise gain for the same signal gain, reducing bandwidth (as mentioned) and increasing output-referred voltage noise.

author: Rerouter
author_rank: Super Contributor
timestamp: February 13, 2020, 12:00:07 pm
content: Been there done that, Op amp noise is calculated using non-inverting math, you don't get a free lunch no matter the architecture,https://www.analog.com/en/analog-dialogue/articles/op-amp-issues-noise-gain.htmlInstead you have to get creative with what you can alter,

author: Wimberleytech
author_rank: Super Contributor
timestamp: February 13, 2020, 02:47:03 pm
content: Quote from: Rerouter on February 13, 2020, 12:00:07 pmSo at first glance the S/N for the inverting configuration would seem to be better (in terms of spectral density). However the BW of the inverting configuration is less (thus less RMS noise after integrating over the BW). In the end, it may be a wash. I would need to do some math to verify this...and it is still kinda early in Texas.

author: pwlps
author_rank: Frequent Contributor
timestamp: February 14, 2020, 10:33:57 am
content: Another point to add to the list: stability issues and the role of parasitic capacitances at high frequencies, as discussed here:https://www.researchgate.net/post/Why_are_inverting_op_amp_amplifiers_used_in_most_cases_instead_of_non_inverting_amplifiers

author: Zero999
author_rank: Super Contributor
timestamp: February 14, 2020, 04:08:08 pm
content: The inverting configuration is more stable, than non-inverting, because it has a higher noise gain. An op-amp which is only stable down to a gain of 2, in non-inverting configuration, will be unity gain stable, in the inverting configuration.

author: TimFox
author_rank: Super Contributor
timestamp: February 14, 2020, 06:05:58 pm
content: The main downside to the inverting configuration is noise. For a reasonable input impedance, there is a resistance of that value in series with the input signal, with associated thermal noise. The non-inverting configuration can present lower resistance to the op-amp inputs. One interesting possibility to avoid common-mode problems is to connect a floating source (such as a phono cartridge) between the feedback network and the inverting input, with the non-inverting input grounded. This is rarely done, since it requires a truly floating source.

author: Wimberleytech
author_rank: Super Contributor
timestamp: February 14, 2020, 06:22:15 pm
content: Quote from: Zero999 on February 14, 2020, 04:08:08 pmAbsolutelyß=1/2 for both cases you describe (NI gain of 2, IN gain of -1), and if that is the maximum ß for stability. The same ß for the NI and IN configurations yield closed-loop gains of 2 and 1 respectively

author: Zero999
author_rank: Super Contributor
timestamp: February 14, 2020, 07:19:46 pm
content: Quote from: TimFox on February 14, 2020, 06:05:58 pmYes that's true, although the current noise from the op-amp is often more of a factor, than the thermal noise from the resistor, especially with bipolar input op-amps.

author: TimFox
author_rank: Super Contributor
timestamp: February 14, 2020, 08:33:27 pm
content: Quote from: Zero999 on February 14, 2020, 07:19:46 pmWhen current noise is important, it will flow through a larger resistance in the inverting case than in the usual non-inverting case, where the source resistance from the feedback network is lower than the inverting-case input resistance, for a reasonable input resistance. This will still make more noise voltage than in the non-inverting case. The big problem with the non-inverting case is still common-mode distortion.

author: Zero999
author_rank: Super Contributor
timestamp: February 14, 2020, 09:43:19 pm
content: Quote from: TimFox on February 14, 2020, 08:33:27 pmSorry, I can't make any sense of what you've written.Are you saying that generally the inverting configuration has more noise, due to the op-amp bias current noise? If so, I agree and in many cases it's the current noise which dominates the thermal noise. Non-inverting amplifiers can use lower value resistors, this lower, noise, without compromising on input impedance.

author: TimFox
author_rank: Super Contributor
timestamp: February 15, 2020, 04:38:37 am
content: I believe that that is exactly what I said. The input series resistor in the inverting case is a thermal noise source, and the noise current from the op amp flowing through that resistor is also a source of noise voltage.For example, a x10 amplifier could use a 10k input resistor and a 100k feedback resistor hin order to have a reasonable input impedance), but the non-inverting case could use a 9k feedback resistor and a 1k resistor to ground for a much lower resistance seen by Theo pamper input. Which part of my answer did you not understand?An important rule for noise-critical applications is that adding a resistor in series with the signal (or one in parallel with the input) always makes the noise worse. To reduce that effect, series resistors should be small and parallel resistors should be large.