Op-Amps, regarding Inverting vs Non-Inverting modes.

Was trying out a few different 'stereo' or left and right microphone amp circuits.
Have to amplify both left and right mics so I can use them in the 'Line-In' input of my computer.

Been reading up on Op-Amps as well as Inverting and Non-Inverting inputs or modes.
Tried the TDA2822 dual low volt. power amplifier (stereo) using the non-inverting input and works well.

I've seen some other mic amplifying circuits using the Inverting mode.

I guess my question is (maybe in line with what I'm doing here with audio and mics..) why you would want the polarity to be reversed in the Inverting mode? What does this accomplish and/or change?

thomas

It accomplishes an inversion of polarity - sometimes wanted.

Also the inverting configuration has some useful properties - firstly you can have a gain of less than one,
whereas non-inverting setup can only have gains >= 1.

Secondly it is the fundamental building block for differentiators and integrators since its transfer-function
is a simple ratio of the feedback component impedances - the algebra is simple and direct. You can also
build current-to-voltage converter, voltage adder and logarithmic converter (using a diode in the feedback
path).

Downside is the finite input impedance (although its essentially that of the input resistor you choose) and
the inversion (should that not be required - for instance in a single-rail application)

why you would want the polarity to be reversed in the Inverting mode?

Polarities rarely matters.

In cases where it does matter, you can simply swap the speaker terminals (you can achieve that through labeling on the amp's output terminals).

Most (audio) power amp's are configured as non-inverting (there are exceptions: gainclone for example is originally inverting because folks insisted it sounded better than non-inverting, until they insisted otherwise).

Applying an audio AC signal into an op-amps inverting or non-inverting terminals is a function of how the op-amp stage is wired up, gain desired, etc. As the signal is AC and usually AC coupled through a series capacitor, it rarely matters if the complete op-amp stage inverts the signal or not, as long as it processes both channels the same in a stereo system. Any good op-amp tutorial will show the proper method to wire up the op-amp for inverting or non-inverting configurations. In either configuration the op-amp (in a audio amplification application) is utilizing externally applied negative feedback to fix the gain to be a constant value.

Lefty

Yeah... When it comes to audio signals and polarity, "There are no rules". Except, if you invert one channel, invert both channes (or all channels in a surround system). With all of the unknown-random equipment (and software) that the signal passes-through between the microphone in the recording studio and the speaker in your home... Who knows what the original phase/polarity was?

I think most pro equipment does not invert. But the specs usually don't say one way or the other, since it doesn't affect the sound. With home equipment, it's probably 50/50.

Many microphone preamps will have a phase (polarity) switch, but this is to compensate/match different microphones or other equipment, or to (partially) compensate for phase differences that come from having two mics at different distances picking-up the same sound. (And of course, pro mics are differential so if the +input is not inverted, the -input will be inverted.)

There are various design considerations when choosing between an inverting or non-inverting circuit. Summing amplifiers (used in mixers) always invert. So if you don't want your mixer to invert, you need to add another inverting stage to flip it back. Or if you want a buffer amplifier (gain = 1), you can build a non-inverting amplifier with an op-amp and no resistors, whereas an inverting buffer requires two resistors. When I build audio circuits, it's whatever's easiest!

Summing amplifiers (used in mixers) always invert.

It doesn't have to be that case.

Appreciate all the replies.

Speaking of gain,
Gain (positive gain) would be an increase in signal power (Watts), voltage, or current by an amplifier?
And a negative gain would decrease the above (P, V, I) from a source to output?

Thanks again for the info and explanations.

thomas

thomas3120:
Appreciate all the replies.

Speaking of gain,
Gain (positive gain) would be an increase in signal power (Watts), voltage, or current by an amplifier?

It can be any of those depending on the purpose of the specific amplifier stage in question. In typical audio op-amp circuits they deal with just voltage gain as the signal after amplification by the op-amp is going on to further stages that will supply power gain such as to drive a loudspeaker to a desired power level.

And a negative gain would decrease the above (P, V, I) from a source to output?

No, you don't require a amplifier stage to decrease the gain of a signal, that would be a rather dumb thing to do. A simple two resistor voltage divider can provide a fixed 'negative gain' if that is what one needs to do. A volume control in a typical audio amplifier is performing the function of providing an adjustable 'negative gain' on the signal going to the next audio amplifier stage.

Thanks again for the info and explanations.

thomas

With opamps we're almost always concerned with voltage, so voltage gain is what is meant (unless quoted in dB, in which case
"power" is often assumed - especially in audio).

Gains of >1.0 or < -1.0 will 'increase' voltage (we normally mean signal amplitude, not instantaneous voltage, the sign
of the gain is irrelevant for considering signal amplitudes, its counted as a phase-change).

Gains between -1 and +1 reduce voltage.

Actually the "power" isn't usually actual power, its notional power assuming a given output impedance (which may be different from
the actual output impedance of that stage in the circuit - for power we tend only to care about the final load impedance - such
as a 8ohm speaker.

To clear about whether voltage or power is meant, we often say "dBV" or "dbW" or some such.

Notional power gain is simply voltage gain squared.

Speaking of gain,
Gain (positive gain) would be an increase in signal power (Watts), voltage, or current by an amplifier?
And a negative gain would decrease the above (P, V, I) from a source to output?

Well... for negative gain it depends if "gain" is measured in dB, or as a multiple. :wink: Basically, amplifiers are analog multipliers... The input is multiplied by the gain factor. If I say, "This amp has a gain of -2", that would mean that +1V in gives -2V out. However, if I say, "This amp has a gain of -6dB", +1V in would give an output of +0.5V (or a numerical gain of 1/2). But since there is no polarity associated with dB, it might put-out -0.5V.

Typically, amplifiers have a specific voltage gain so output voltage is a functon of that gain but the power and current depend on the load impedance/resistance (within limits of the amp).

With audio power amplifiers, you often get twice the power with 4 Ohm speakers as you get with 8 Ohm speakers, because the voltage stays (about) the same, but current doubles. If the impedance is too low, you can "pull" too much power/current and burn-up the amp. :wink: