The Designer's Guide Community Forum

The Designer's Guide Community Forum
https://designers-guide.org/forum/YaBB.pl
Design >> Analog Design >> output common-mode of Op-Amp
https://designers-guide.org/forum/YaBB.pl?num=1328636207

Message started by gido on Feb 7^th, 2012, 9:36am

Title: output common-mode of Op-Amp
Post by gido on Feb 7^th, 2012, 9:36am

Hello,
does the output common-mode voltage of an op-Amp depend on the input signal type and also the magnitude of the input signal. For example, i got a voltage of -5.137mV using a Sin wave input whilst it was -397.7mV using a square wave for my transient simulations. Thanks

Title: Re: output common-mode of Op-Amp
Post by RobG on Feb 7^th, 2012, 8:46pm

Ideally the common mode is perfect, but it is only going to be as good as your common mode feedback circuit.

Given that it fails with a square wave I'd look at the outputs individually to make sure the slew rate is the same for positive and negative slewing, but that is just a guess.

rg

Title: Re: output common-mode of Op-Amp
Post by raja.cedt on Feb 8^th, 2012, 1:26am

are you using opamp in open loop, and one more thing when you have sin wave input common mode is -ve is it correct, it does n't make sense for me. One thing just check are you using correct common mode at the input or not. Any how to use CMFB or at least closed loop

Title: Re: output common-mode of Op-Amp
Post by gido on Feb 8^th, 2012, 4:58am

is the output common-mode voltage of a single -ended op-Amp the same as the offset? if so , then that's what i mean......i got an output offset voltage of -5 mV without any input signal. With a sin wave input, and running the transient, i still get a voltage of -5mV at the output but with a square wave input,i get an output voltage of -397mV.. thanks...

Title: Re: output common-mode of Op-Amp
Post by raja.cedt on Feb 8^th, 2012, 7:00am

no wrong...i guess what you are talking about is offset (which is the difference between o/p and input common mode, only your case). But how you are quantifying a number by the way, because with signal type it is changing may be your operating point calculation is not correct..any how try to read basic micro-electronics books by razaavi

Title: Re: output common-mode of Op-Amp
Post by RobG on Feb 8^th, 2012, 2:52pm

Common mode really only makes sense with a fully differential circuit: VO_UT = V⁺ - V^-
V_CM = (V⁺ + V^-)/2

Offset is the difference between the two terminals of the opamp - and it is measured at DC so offset when outputting a square wave makes no sense.

I assume you are talking about the average value of the output. To answer your question (again), ideally it shouldn't matter if it is a sine wave or a square wave, but real amps distort the signal so you will see some shift in the average value. I'd guess there is a different slew rate when the opamp is slewing positive vs what it is doing when slewing negative.

rg