Need help with designing a tappered CML buffers

husseinadel said:

hello

i am designing a tappered CML buffers,,,,,, and i have a question ,,,,,i want to set the output common mode to a certain value so as the swing not from vdd to ,, vdd-I*R,,,but somewhat lower than vdd,so, how can i set it,,is that possible? or not????,,,,i think that as the switching of current completely from one branch to the other while swinging makes the output reach vdd.....so,,can anyone share me in thinking?????/ thanks

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tsb_nph said:

husseinadel said:

hello

i am designing a tappered CML buffers,,,,,, and i have a question ,,,,,i want to set the output common mode to a certain value so as the swing not from vdd to ,, vdd-I*R,,,but somewhat lower than vdd,so, how can i set it,,is that possible? or not????,,,,i think that as the switching of current completely from one branch to the other while swinging makes the output reach vdd.....so,,can anyone share me in thinking?????/ thanks

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Hi,
If i understand your question correctly, then yes, it is possible.
In an usual arrangement, the two load resistors are not connected to Vdd.
Instead, connect those two nodes together and then add an extra common mode resistor (Rcm) form that node to Vdd. Now the swing will start from Vdd - Itail*Rcm instead of from Vdd as there is a constant Itail*Rcm voltage drop across the common mode resistor.

I have attached a simple schematic to illustrate this.

Bharath

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