dear friends
i have bias circuit see the figure below, it is feed my ota design (folded cascode) until the current become of ota reach 450 ua, for settling requirement i need 700 ua, could some one help me for this problem(VDD=1.8v_
regards
thanx, in fact the transistors already very big for example the input of the ota(PMOS transistors) is 270/0.22 um, so it is big value, and for the bias circuit i reduce the size to increase the current of the ota, but i reach the minimum.
regards
that right, reducing the resistance is good but has limit, and the transistor conected to res is usualy equale to 4 times other n mos, so i cant play much here
regards
Firstly see KenMartin Book, which could help u in someway..
For my circuit i need a 100ua for tht circuit it self the W/L ratios are very large and u need to consider the length to be 8 to 10 times of the min length inorder to ignore the Lambda Effect.
So i think i will he hard to design that.. The complete chip area would be the bias it self....
thank u for ur answer shady, i mean the input transistor of the amplifier by 270/0.27u. so it is not the bias circuit.
it should be big because high gain, high phase margin withe low settling time
regards
Even we should consider the Lengths of all the transistors to be 8 to 10 times of the minimun length if it's analog, we can go for the minimum length for digital..
what is your input bias coming from you bandgap ,feeding your bias block. i believe its too low, because of which you are having to use a huge size and stil not able to get 700ua.
U can generate high currents using current amplification (via mirroring), But it is advisable to do so in 2-3 stages rather than a single stage with large multiplication factor.
I had seen better biasing than that of Kenmartins book. It's the modified bias ciruit of kenmartin's .It's not in book it's in some IEEE paper...
When iam designing for 100ua there is a mismatch of about 40-50na and also the sizes of the transistors are large it's about 30. When we need a current of 3mA the sizes should be reflected to the corresponding transistors which would be ( 30 * 30 = 900 ).