CMFB circuit explaination and why not CMFB used for single ended

[simplsoft]

Hello

I want to understand CMFB circuit which I have attached what happens when the common mode output is increased compare to thew reference voltage.

Why CMFB is not used in single ended output. Although single ended output amplifier also has one common mode output?

 

[frankrose]

Differential output voltage (e.g. Vo+ increasing and Vo- is decreasing) can't change the CMFB voltage.
But if the output common mode voltage is increasing (Vo+ and Vo- are increasing), than the current through the Mm6 will increase. It will increase the CMFB voltage too, which you can use to control a tail or head current of a differential amplifier.
CMFB sets the transistors into the correct operation region, and single-ended amplifiers don't need this, because the used one negative feedback around the amplifier is enough to set the transistors.
Single-ended amplifiers doesn't have got common mode output.

 

[simplsoft]

Thanks for your reply. I have attached another CMFB circuit. From the attached schematic when the output common mode voltage of the amplifier increases then the gate voltage of M7 transistors will also increase and current through M6 will increase as compare too the transistor M7' whose current will be less because vref will be less compare to gate volatge of M7 so less current will be flowing through M6'. Now I cant understand the diode connected transistor M6 and M6'. Why Vfb is taken from the drain of M6' not from drain of M6? where M8 and M8' are the tail current sources. I want to understand the diode connected transistor M6 and M6'?

Thanks

 

[FvM]

Did you noticed that the #3 circuit is simply a complementary copy of #1?

 

[simplsoft]

yes, its a complementary copy. I again attached the second ckt because I am using the complimentary one in my design.

 

[FvM]

Now I cant understand the diode connected transistor M6 and M6'. Why Vfb is taken from the drain of M6' not from drain of M6?

Vfb will be connected so that you get negative CMFB. Without seeing the connected differential amplifier circuit, we can't know which output is right. But I suppose you can figure out the feedback sign quite easily.

 

[simplsoft]

Thanks for your reply. but in some CMFB circuits available in literature don't have a diode connected MOS (like diode connected mos M6 and M6') simple PMOS common source amplifier is used without diode connection. Any particular reason for diode connection?

 

[frankrose]

M6 and M6' are diode connected, because you need diode connected device to mirror a head or a tail current for the differential amplifier. Also the mismatch caused offset will be smaller if you use diodes on both sides. To create the Vfb voltage M6' would be enough, but it will be better if you put M6.

 

[tenso]

Isn't the concept of common mode only applicable to differential signals? and as a result there is no need for CMFB for single ended outputs?

 

[simplsoft]

yes, this is my question why CMFB is not required for single ended structures?.Single ended also has a DC output.

 

[FvM]

The question has been already answered in post #2.