Fully differential opamps and common mode voltage

rtarbell · 15th August 2006, 05:33 AM

Okay, let's say I have a discrete fully differential opamp, but I do NOT have any kind of common mode voltage adjust.

So that I understand the concept better ==> what could happen at the outputs with no consideration to the common mode output voltage (worst-case)?

Tim__x · 15th August 2006, 07:13 AM

The worst thing that I can think of happening is for both outputs to slide to one rail and stay there. This would mostly depend on the various impedances within the amplifier, any particularly high impedance nodes, like a current source loaded Vas, will be very upredictable without common mode feedback.

A common mode amplifier (and feedback path) can be very simple, just a couple transistors, you'll save a lot of trouble by adding one

15th August 2006, 05:33 AM	#1
rtarbell diyAudio Member Join Date: Sep 2005	Fully differential opamps and common mode voltage Okay, let's say I have a discrete fully differential opamp, but I do NOT have any kind of common mode voltage adjust. So that I understand the concept better ==> what could happen at the outputs with no consideration to the common mode output voltage (worst-case)?

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15th August 2006, 07:13 AM	#2
Tim__x diyAudio Member Join Date: Jun 2004 Location: Edmonton area, Alberta	The worst thing that I can think of happening is for both outputs to slide to one rail and stay there. This would mostly depend on the various impedances within the amplifier, any particularly high impedance nodes, like a current source loaded Vas, will be very upredictable without common mode feedback. A common mode amplifier (and feedback path) can be very simple, just a couple transistors, you'll save a lot of trouble by adding one