About design full differential amplifier with CMFB?

[Patrick Yang]

Hi,
I am a freshman in designing a FDA with CMFB. I am now reading the Sansen's book and occurs many questions, it seems a tough thing for me to really understand this book.
There is some question about CMFB,
1. What's the function of CMFB, how it works?
2. what's the relationship between GBWcm and GBWdm? how to determine the CMFB circuit speed?
3. what's the ads and dis-ads of linear MOST as CMFB, and the resistive CMFB? and others?
there is a example of FDA with CMFB, who can help me to understand and analysis the circuit, from aspect of the pole zero splitting,CMFB, its speed, power etc thank you so much!!!
P.S. who can recommend some books or materials for me to have in-depth study in AMP, thanks again!!!

 

[Dominik Przyborowski]

ad1. CMFB provides negative feedback to setup correctly a DC output voltage. Without it your output potentials will goes up to positive and negative supply rails beacuse drain potentials of load current sources are undefined.
In single ended opamps this problem doesn't exist due to use a current mirror as load - a diode connected transistor well defining potentials on one branch.

ad3. The main advantage of resistive CMFB is a simplicity but if your opamp has high output impedance this kind of CMFB seriously decreasing your open loop gain. The mos-based CMFBs usually needs more elements but their doesn't load your opamp.
At some applications most often a switched capacitors CMFBs are used, especially at ADCs - their doesn't need a dc biasing current but only a clock and a few capacitors.

I think a good enough theory is shown in Grey, at al. "Analysis and design of analog ICs". Chapter 12 if I good remember.

About attached amp.
Its a single stage OTA. The Rf and Cf elements provides high frequency feedforward compensation described by Sansen in "Feedforward compensation techniques for high-frequency CMOS amplifiers" journal of solid-state circuits from 1990. The CMFB are made on nfets degenerating current sinks - this nfets works in linear region and three of them has gate connected to reference common-mode voltage, while two of them in output branches sensing output dc potential. The symmetry of circuit provides that dc level of outputs follows a Vcm reference voltage.