[sdiy] Using Opamps as INV or NON-INV amplifiers

[Gene Stopp]

Yup what Seb says... the INV and NON INV configurations are actually just as easy one way or the other, it's just a matter of what you connect to ground - the "+" input, or the end of the feedback divider. The NON INV configuration gives you the "+" input of the opamp, which by itself is an infinite impedance (ideally) which is good for passing a signal along without loading it down at all. Of course you can add any impedance with another resistor to ground. The INV configuration has a finite impedance to ground (actually to the virtual ground at the "-" input, which behaves like real ground for loading purposes) but you get a true summing node for other potential inputs you may want to mix in. This is why most control voltage scaling and summing circuits use two INV's in a row, like a keyboard output circuit. It's a good way to sum pitch bend, tune, keyboard, and modulation sources. In an audio chain, the INV configuration is used often since the polarity of an audio signal!
  may not matter, and you get free mixing at the summing node.

So since the INV is so flexible, you usually see it more often. The NON INV is used when buffering is needed, or other impedance-sensitive applications.


- Gene



-----Original Message-----
From: owner-synth-diy at dropmix.xs4all.nl
[mailto:owner-synth-diy at dropmix.xs4all.nl]On Behalf Of Seb Francis
Sent: Wednesday, June 21, 2006 11:17 AM
To: Dave Kendall
Cc: synth-diy at dropmix.xs4all.nl
Subject: Re: [sdiy] Using Opamps as INV or NON-INV amplifiers


If you want to sum multiple signals together you need to use inverting 
configuration.  That's quite often why you see an inverting followed by 
another re-inverting stage.

Seb


Dave Kendall wrote:
> Hi Guys.
>
> Apart from the gain being minimum of 1 for a non-inverting amp, are there
> any other advantages or gotchas about using one form or another? There's
> lots of stuff about op-amps out there, but I'm still a bit unsure about the
> practical differences in use. It would seem that a single NON INV amp is
> easier to implement, and yet often the INV configuration is used, with a
> second re-inverting stage ...
>
> Any thoughts welcome.
>
> cheers,
>
> Dave (still coming to terms with the impedance monster ;-)
>
>
>
>