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Inverting vs non-inverting - Pros and Cons
Wimberleytech:
--- Quote from: Zero999 on February 14, 2020, 04:08:08 pm ---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.
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Absolutely
ß=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
Zero999:
--- Quote from: TimFox on February 14, 2020, 06:05:58 pm ---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.
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Yes 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.
TimFox:
--- Quote from: Zero999 on February 14, 2020, 07:19:46 pm ---
--- Quote from: TimFox on February 14, 2020, 06:05:58 pm ---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.
--- End quote ---
Yes 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.
--- End quote ---
When 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.
Zero999:
--- Quote from: TimFox on February 14, 2020, 08:33:27 pm ---
--- Quote from: Zero999 on February 14, 2020, 07:19:46 pm ---
--- Quote from: TimFox on February 14, 2020, 06:05:58 pm ---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.
--- End quote ---
Yes 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.
--- End quote ---
When 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.
--- End quote ---
Sorry, 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.
TimFox:
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.
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