single ended Folded cascode design - starting point!

[jayanth.neo]

folded cascode design

Hi all,

I am new to analog design and I am currently designing an Nmos input folded cascode amplifier. I have the following specs to meet at Vdd 2.7V and 0.35u technology:


can use an ideal current source of 5uA which can be mirrored accordingly.
Aol >50dB
swing = 2 V
power dissipation = 0.8mW.
Load cap = 15pF
UGF - 25MHz
Phase margin = 60
HD3 of 50dB with 5Khz input signal.

I would like to know how to start with the sizing of the cascode stack consisting of 2 PMOS and 2 NMOS on each branch. I am designing for a single ended output ( and have diode connected an NMOS on one side of the branch.

What imp points should I consider while sizing the Diff pair and the cascode stack for a UGF > 25MHz and phase margin of 60?

What can be the length of the transistors? I am currently using 1um for all transistors at .35um technology.

Any suggestion on the design procedure and useful directions will be greatly helpful.

Thanks

 

[lwjbh]

cascode design

maybe you could find the answers from Allen's book

 

[jayanth.neo]

folded cascode amplifier design

Do you mean Allen hasting's book?

I just wanted to know how to design the bias circuit for a cascode stack? 2 stacks of PMOS and 2 stacks of NMOS.

I jus wanted to know the basic procedure for that.

 

[ahmad_ee]

single ended folded cascode

I think lwjbh means Allen ad Goldberg book for analog.
I think also Razavi's book is good " Design of Analog CMOS Integrated Ciruits"

 

[raduga_in]

cascode amplifier with w/l values

Jayanth,

You can start with a current requirement first
or with the slew rate spec. > 50db is easy to get
with lower current but speed would be a tradeoff
So, choose a current first and then see whether
you get the required bandwith or not .. Load cap
if fairly larage so choose 70% max current to
start with.. legths of the devices should be at lest 2 times
the min .. and use min lenth for input pair ..Increase
if you don't get requred gain

raduga

 

[malizevzek]

design of folded cascode

As Raduga suggested, start with biasing currents, 70% seems like a good idea. Then, think how much overdrive you want to allocate to specific transistors. Keep in mind that your output swing is directly determined with allocated overdrives. Then, you select transistor dimensions, so that these transistors can provide the necessary current for the chosen overdrive voltage. by choosing greater L you'll get more gain, but you may get problems with a low pole, as well. if you need single ended output and have a problem obtaining enough swing, try with high-swing cascode configuration.

 

[malizevzek]

single ended folded cascode power amp

One more comment regarding the transistor lengths.

If you perform back-of-the-envelope calculation, you'll be able to determine which gm and rds are crucial for your amplification. The formula is not to complicated.
In general, input transistors have smaller L, and transistors in cascode are larger, especially the highest ones and lowest ones in cascode stack. Bigger L means bigger W, so at some point you can get in trouble with low poles.

 

[shady205]

folded cascode single ended swing

Hello jayanth...!


Firstly see the image that i have up loaded

The Tail current Iss can be find out from the power dissipation or slewrate.

and above i considered about 1.2 to 1.5 times Iss bcoz considering the worstcase that all the current flow through the M1 transistor itself then the cascode pair has to be off, so inorder to make the current flow through cascode pair we consider around 1.2 to 1.5 times Iss.

From bandwidth u can find out gm1.
From gm1 u can get the W/L values of 1,2 transistors.
From the Output Swing( max) u will get the W/L values of M11,M4.(Coz we need to consider that the Vds of both transistors to be same from which u can get the values of W/L)
From the Output Swing (Min) u will get the W/L values of M6,M8.( Considering the same thing as in output swing max.)
From the current and Vds we get all the W/L values of the Transistors.
The tail W/L value can be found be considering the overdrive or from the ICMR,Min we get the W/L of the tail transistor.


I think this could give a better ideal to design a single ended Folded Cascode


Bhagath.

 

[yschuang]

cmos folded cascode amplifier design

I want to add one more point. Reduce the the length of cascode devices if you want to have good phase margin under high speed.

 

[udintbr]

choosing w/l for folded cascode

TQ for starting this topic. I wanna ask another question on how to start designing folded cascode diff amp with CMFB. Can I design the folded cascode independently from the CMFB, coz the diff amp circuit that I want to design has a single NMOS for the tail current. Looking at the topology of the diff amp with CMFB, it adds a pair of nmos transistors to the source of the tail current transistor. And the pair operates in triode region.

Correct me if my method is wrong. Can I assume Vds(pair) = 0.02V so that (Vgs - Vtn) >> Vds/2, which makes Vo - Vtn >> 0.01V, hence the transistor pair is always in triode region. My OCMR is between 1V & 2V. Vdd=2.5V. But then, how do I determine the size of the transistor pair?