OTA and CMFB design...

[Mostafa El-Meehi]

I was requested in my class to design a high-swing Telescopic-cascode fully-differential OTA to work in a voltage buffer (shown below) in an ADC. The expected load is shown below.

There is additional loading of 1 pF at each of the OTA output nodes (not shown).
The amplifier is required to accommodate a maximum swing of 3.4 V differential-peak-to-peak running at 30 MHz and applied to the input of the amplifier (closed loop). OTA’s open- loop gain must equal or exceed 60 dB. The circuit operates from a 2.7 V supply and can use no more than 0.5 mA (including CMFB)
how to start?? and does it mean to desin alone the OTA (open loop) then design a CMFB ? and how to DESIGN such?
Please help as fast as possible!
I'm tooo late and i didn't even start!! so i am very afraid...
Thank you very much
Meehy,

 

[ahmad_abdulghany]

Assalamo alaykom ya Meehy
look, in my opinion your problem can be divided into three parts mainly:
1- How to design the telescopic-cascode OTA (assuming you know what is an OTA as well as telescopic cascoding already).
2- How to design the CMFB amplifier.
3- How to connect the CMFB amplifier to bias certain gate of one of the MOSFETs used in your OTA to achieve certain CM level at the output without oscillation (which may need some sort of compansation as you will have too many poles that can make the circuit easily oscillate they said : "bad amplifier is a good oscillator).

I suggest you to follow suitable steps to finally fullfill these results or solve these three problems.

I may setup a sample circuit (without numirical values) to show you OTA and CMFB connected together.

Best Regards
Ahmad,

Added after 1 hours 5 minutes:

Here is a sample circuit you can use to start ..

 

[flushrat]

First decide OTA artechiture.
Then do some hand calculation according to OTA artechiture, such as DC-gain UGB S/R pole-position etc, and decide the parameters of each element in OTA according to spec.
Then run simulation and do adjustment.

 

[crouch]

hi ahmad_abdulghany

I have two questions, one about the CMFB, and one about bias network

How can I know whether my CMFB works?
It's strange that my main amp (one stage telescopic) is not sensitive to the CM output point. Maybe my amp's open-loop gain is not very large. I change the W/L of the PMOS load(4 together) by 10%, it still works well.

And how can I make a stable network?
I only know the simplist way: using a nmos and a diode-connected pmos to bia noms, i think you are smart enough to understand me
but this is very bad when i simulate with ss/ff tech-corners.

there is a ideal current source i can use as bandgap


thanks.