feedback of 2 stage fully differential amplifer

[edafisher]

hi guys,

In designing a 2 stage fully differential amplifier,

1) when we apply differential signal to input, the feedback is negative (designed to be so)
2) when we apply common mode signal to input, the feedback becomes positive.

Thus: a) since common mode gain is small (<1), there are no stability problem. b) if in any case the common mode gain exceed 1, the amplifier becomes unstable.

Am I correct? please comment. Thanks and happy weekend.
edafisher

 

[prcken]

i agree， you can do simuation to check the CM gain of your circuit

 

[edafisher]

thank prcken!

So the current tail is always necessary for the input differential pair. Otherwise the CM gain will equal DM gain. right?

 

[mfding]

i have not heard of similar situation, can you upload your schematic to elaborate what you are talking about?

 

[edafisher]

i have not heard of similar situation, can you upload your schematic to elaborate what you are talking about?

you can refer to this paper
**broken link removed**

 

[prcken]

thank prcken!

So the current tail is always necessary for the input differential pair. Otherwise the CM gain will equal DM gain. right?

the current tail acts as a virtual ground for differential gain, while a huge degenerated source resistance for common mode gain, you need it to make your CMRR as high as possible, aren't you?

 

[edafisher]

thank prcken!

So the current tail is always necessary for the input differential pair. Otherwise the CM gain will equal DM gain. right?

the current tail acts as a virtual ground for differential gain, while a huge degenerated source resistance for common mode gain, you need it to make your CMRR as high as possible, aren't you?

hi, in advanced technology and shrinked VDD, are you able to obtain "huge" source resistance?

 

[prcken]

cascode the current tail can increase the resistance but limited headroom issue in low operating voltage, you have to do trade-off, actually what i meant huge is relatively large.
and in high frequency range the common mode gain will be increased to gm of the input transistor due to the capacitance at that current tail high impedance node