A question about opamp buffers...

[cpnyc23]

Hi,

I have a question about Jack Orman's opamp buffer design.

These circuits cause the output to be 180 degrees out of phase with the input.  How can I reinvert the signal so that I have one coming out that is in phase with the one that went in?

Or am I understanding the circuit incorrectly?

thanks
-chris

[PerroGrande]

Jack has several op-amp buffers on his page -- three under the basic buffer page.  Two of the three are non-inverting configurations, while the third inverts.  You can determine whether a stage inverts (generally) by whether the input signal is fed to the non-inverting (+) input of the op amp or the inverting (-) input.

If you go with the inverting configuration, phase really only matters if you plan to mix signals in some way, as an out-of-phase signal will introduce cancellation with an in-phase signal.  However, if you're not doing that, it doesn't really matter.   If it becomes a concern, you can either go with one of the non-inverting buffers or you can string two inverting stages back to back and restore the original phase.

[cpnyc23]

Thanks for the info.  I was thinking that I had to use the inverting because I don't have a voltage reference somewhere else (it is a circuit on its own, not in front of or behind another circuit).   If my understanding of the Vr is wrong, please let me know as I am new to much of this.

So, if I put another inverting stage behind the original inverting buffer, will there be any signal degradation?


Thanks again
-chris

[PerroGrande]

Chris,

The Vref voltage is going to be needed in either configuration.  Op Amps require this when running using a single supply voltage.  Most designers create it using a simple voltage divider and a bypass capacitor.  So -- in short -- you'll need it for your buffer, regardless of whether it is inverting or not. 

There are advantages and disadvantages to the inverting configuration.  In some ways, the circuit creates less demand on the op-amps internal circuitry.  However, inverting configurations have the issue of input impedance being determined by the value of the resistance in series with the input.  This isn't as much of an issue for a buffer, as you will not be adding gain (thus forcing high values for the feedback resistor).  For buffers, I typically use a non-inverting configuration.

But back to the original point of things -- inverting the signal is perfectly okay to do as long as you're not going to mix it with the original signal.  Adding another stage will probably not produce audible degradation of the signal. I should add, however, that from a purely technical perspective, adding stages without feedback probably will cause signal degradation, but probably not any that is audible.

[cpnyc23]

Gotcha - thanks!!  The more I think through the signal flow later on in my setup, I will not be mixing the post buffer signal with the original signal so the phase reversal won't be an issue.

Thanks again for the info - this is all much clearer now!