Inverted or non-inverted... that's the problem

[Faber]

Hi guys!
maybe my question is silly, but wich are the difference between those configurations?

I tried to search on forums, website and application notes, but I didn't found anything about this topic.

Why all the gainclones use inverted input and not the non-inverted input that has higher impedance and so on?

Thank you all.
Bye, Fabrizio

[millwood]

Originally posted by Faber
Why all the gainclones use inverted input and not the non-inverted input that has higher impedance and so on?

Thank you all.
Bye, Fabrizio

if it makes you feel better, pretty much all solid state amps are non-inverted. NSC's application note on those chips are non-inverted, as BB's for the opa54x family.

I have asked the same question here before and other than "inverted is better", I have yet to receive a convincing answer.

I have tried inverting and non-inverting (on the lm3875) and for the life of me I couldn't tell them apart.

so I would be also interested in what you can find this time.

[dkemppai]

I've never built a gain clone...

...But, the gainclone is based on an op-amp, and op-amps usually have lower distortion when operated in an inverting configuration. That is due to the fact that the voltage on the input stage stays at a constant value, when operated inverted. This means that the differential pair (the input section of the op-amp) operates at constant voltage levels, causing less distortion. (Remember, Op-Amps in theory, have 0 volts between the + and - inputs when operating)

Do some research on input stages to op-amps (Or ask for further info here), if you're interested in more theory on this.

Hope this helps!
-Dan

[peranders]

Most opamp alike amps has up to 10 times lower distortion in inverting mode. Maybe one reason?

[dkemppai]

Originally posted by peranders
Most opamp alike amps has up to 10 times lower distortion in inverting mode.

I knew it lowered the disto, but didn't have any idea the numbers were that big!

-Dan

[millwood]

Originally posted by peranders
Most opamp alike amps has up to 10 times lower distortion in inverting mode. Maybe one reason?

here is a link with some data, and you can see the magntitude on the 2134.

http://www.dself.dsl.pipex.com/ampins/webbop/2134.htm

[Richard C]

The main reason why op-amps perform better in inverting mode is that the input and feedback signal are summed at the input of a single device (one half of the differential input stage).

In non-inverting mode the input and feedback signal are fed into different inputs and different devices, the effectiveness of distortion cancellation is entirely dependent on how closely matched the two halves of the differential input stage are matched which although usually very good it can never be perfect, hence higher distortion.

[Faber]

Originally posted by peranders
Most opamp alike amps has up to 10 times lower distortion in inverting mode. Maybe one reason?

WOW!
That's great, but why this distinction is not mentioned in the datasheet nor application note?

I'm using an LM3886 and I didn't find any distinction about inverting and non-inverting THD+N graphs.
In addition, normally I see that opamps (also discrete designs) are used in a non-inverting config.

Can you tell me were to find some doc about this lower distortion theory?
TIA
Ciao, Fabrizio

[Richard C]

For discrete differential input stages I prefer inverting (shunt feedback) since precise matching of input devices is not easy (or even possible).

Common-Mode-Rejection-Ratio is the measure of how closely matched the inputs are and for op-amps (where the devices are fabricated on the same piece of silicon) CMRR is very high and so distortion is very low whichever configuration is used.

Distortion in op-amps may be 10 times worse in non-inverting mode but ten times worse than what? does 0.01% distortion sound worse than 0.001%? maybe not but given the choice...

[peranders]

Check the datasheet of LT1115, as an example.

The reason for lower distortion is that the inputs (both + and -) moves with the signal in the non-inverting amp and stand completely still in inverting mode. This has to do with common mode rejection (or gain for that matter) och the amplifier.