How to start desiging a Folded Cascode?

[dream_taker]

Folded cascode Design

Hi,

Can anyone tell how to start desiging a Folded Cascode. For a particluar speed, settling time how to calculate the output current. I f we knwo the Iout then I think we can use the formula and then find the W/L of each transistor.

But what are the consideration has to be taken to Find Iout ?

Thanks in advance

 

[khouly]

Re: Folded cascode Design

check the slew rate
and the gain of the cascode
this is affected by Iout

khouly

 

[ambreesh]

Re: Folded cascode Design

Your bandwidth requirmrnt would decide the gm for the input pair.
Also the slew rate requirment would decide the bias current through the tail current source.
Once we have the 2 we could start with the design.
You choose the architecture depending on what is your input range and PSR requirment.
Take care of the current you are going to need for active loads. Read holberg to get idea on that.
The active load architecture could be choosen on your output swing requirment.
Once u have all the W and L you would know the capacitance at non-dominat pole locations. U could get an idea on your pole locations and a phase margin.
Hope it helps

 

[wholx]

Re: Folded cascode Design

Your bandwidth requirmrnt would decide the gm for the input pair.
Also the slew rate requirment would decide the bias current through the tail current source.
Once we have the 2 we could start with the design.
You choose the architecture depending on what is your input range and PSR requirment.
Take care of the current you are going to need for active loads. Read holberg to get idea on that.
The active load architecture could be choosen on your output swing requirment.
Once u have all the W and L you would know the capacitance at non-dominat pole locations. U could get an idea on your pole locations and a phase margin.
Hope it helps

could you explain a little more on how the slew rate decides the tail current?

 

[dream_taker]

Re: Folded cascode Design

Slew Rate is given by the maximum rate at which the load cap is charged or discharged

SR = I/C
where I - Current flowing in the Output node
C - Load Cap.

Current in the output is determined by the tail current. So SR = Iss/Cload

 

[xz781122]

Re: Folded cascode Design

read allen's book 'cmos analog circuit design'