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how to improve swing of a fully differential op amp

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bhargava834

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how to improve on an op amp

i have designed a fully differential op amp. Op amp is designed with telescopic gain boosting topology.

It is designed for 1.8V single supply in 0.18um technology.

which is having ICMR of 0.48V (0.65V-1.13V), and UGB of 1.06GHz, DC gain of 116dB, PM of 61degrees.

When i adjusted the Input Common mode voltage to 0.8V, and given a sine input of 4uV P-P at 10Hz, I got output of 746mV. It shows that the gain is of 105.5dB. Where as according to ac characteristics the gain at 10 Hz is 116dB.
What could be the reason for this?? and how to improve the swing ??

Plz help me.

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Thanks & Regards
Bhargav Reddy
 

ravirajdv

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Hi,
Why don't you try the folded cascode arch with telescopic load, you can improve the gain as well get a good ICMR or the swing.
The answer for you first question, my guess is (which means I don't understand completely), the transient accounts large as well as the small signal parasitics, where as in the ac simulation the circuit is considered to be DC stabilized and only the small signal equations are solved.
Others please correct me if I am wrong.
Regards,
RDV
 

bhargava834

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Thanks for the reply raj.

But i do not have time to redesign with folded cascode. All the parameters were ok, except swing and the slewrate.
My design results are as mentioned above and apart from that Settling time of 4nsec to reach 1 % of final value,

Any suggestions to improve swing and slew rate.



Thanks & Regards
Bhargav
 

ravirajdv

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Hi,
In that case I think you will have to have another gain stage to meet the o/p swing constraints.
Regards,
RDV
 

bhargava834

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Hi..
But, insertion of one more stage leads to the need of compensation and the other parameters like UGB, PM may change...
If i insert a classAB stage, then will there be any problem???

waiting for reply.

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Bhargav
 

ravirajdv

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Hi,
Insertion of one more stage will surely effect UGB, reduce PM, need for some sort of compensation, since there in one more stage the o/p will get inverted and all sorts.
But looking at your problem, that is the only solution that I can think off.

Regards,
RDV
 

bhargava834

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Thanks for replying..

That is ok...

raj can u explain me, exactly how to measure the swing in simulator(ELDO), What is the input frequency i have to consider?? and does the swing anywhere related to the Harmonic Distortion???
What is the typical THD does an op amp can have???



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Bhargav
 

ravirajdv

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Hi Bhargav,
Even I need to dig into the THD issues.
Can somebody chip-in and help us out.

Thanks in advance,
RDV
 

steadymind

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hi bhargava,

the way you are evaluating you gain by giving a small signal is wrong. plot the graph between input diffferential voltage and ouput differential voltage. this will gie you the gain and maximum and minimum input/output swing levels.

to improve the swing given you specs would be impossible and you will have to add another stage which will mean changing some values but you will be able to improve your phase margin by doing compenstation
 

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