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why folded cascode amplifier has lower voltage than cascode?

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Hi there,

I have done an amplifier design by using folded-cascode configuration. I found that the supply voltage for folded cascode is about 0.6V, however when the same amplifier by using cascode configuration need 1.8V supply voltage.

Can anyone talk about

1. why folded cascode need MUCH LOWER supply voltage that cascode configuration?

2. What is the minimum supply voltage for a folded cascode and cascode configuration? how to calculate the minimum voltage?

3. Are there any advantages of choosing NMOS-PMOS or PMOS-NMOS configuration of folded cascode?

4. Does PMOS contribute less noise or any else advantage to put at second stage?

Thank you
blue
 

Hi
1- because there is no tail
2- it depends on many thing but
it is not good that your transistors work in sub-threshold
3- yes but it depend onmany things
noise, cmfb, next stage
4- if the first stage has large gain, don't worry about noise.

regards
 

Re: why folded cascode amplifier has lower voltage than casc

if these amplifier work in saturation region, in theory, the minimum supply voltage of folded cascode amplifier is min{(4*Vds,sat),(Vgs+Vds,sat)}, and that of telescopic amplifier is min{(5*Vds,sat),(Vgs+Vds,sat)}. but considering the gain and output swing, the actual supply voltage is larger
 

pmos contribute less 1/f noise
 

Re: why folded cascode amplifier has lower voltage than casc

gingerjiang said:
if these amplifier work in saturation region, in theory, the minimum supply voltage of folded cascode amplifier is min{(4*Vds,sat),(Vgs+Vds,sat)}, and that of telescopic amplifier is min{(5*Vds,sat),(Vgs+Vds,sat)}. but considering the gain and output swing, the actual supply voltage is larger
Click to expand...

Hi gingerjiang,

I don't understand how you come out with the above equation, can you provide some explanation about how to get the formulas?

for my case, i have Vgs=0.54V, Vds,sat=0.04V. So do you mean that my max folded cascode supply voltage is 0.58V and min supply voltage is 0.20V?

thank you.
 

Re: why folded cascode amplifier has lower voltage than casc

oh, sorry, it should be max{(4*Vds,sat),(Vgs+Vds,sat)} and max{(5*Vds,sat),(Vgs+Vds,sat)}
i assume the transistors work in saturation region, and i also consider the bias circuit including a mirroring MOS and a diode-connected MOS, the supply voltage should make these circuits work well
 

Re: why folded cascode amplifier has lower voltage than casc

gingerjiang said:
oh, sorry, it should be max{(4*Vds,sat),(Vgs+Vds,sat)} and max{(5*Vds,sat),(Vgs+Vds,sat)}
i assume the transistors work in saturation region, and i also consider the bias circuit including a mirroring MOS and a diode-connected MOS, the supply voltage should make these circuits work well
Click to expand...

Hi,

I am just wondering what you mean by max{(4*Vds,sat),(Vgs+Vds,sat)}. Since you have two values, so are you refer the supply voltage is in the range from 4*Vds,sat to Vgs+Vds,sat? By the way, what is telescopic amplifier?

Thanks
 

Re: why folded cascode amplifier has lower voltage than casc

for max{(4*Vds,sat),(Vgs+Vds,sat)}, it refers to the larger value, for example, (Vgs+Vds,sat)=0.58V is larger than 4*Vds,sat=0.2V, so it's (Vgs+Vds,sat), 0.58V.
for telescopic amplifier, in your situation, the results is same
 
Hi Gingerjiang,

Can you provide some explanations about how you get the expression of {(4*Vds,sat),(Vgs+Vds,sat)}?
 

Re: why folded cascode amplifier has lower voltage than casc

in output stage of the folded cascode amplifier, four transistors stack together, the supply needed is 4*Vds,sat for them working normally. considering the bias circuit consisting of a mirroring MOS and a diode-connceted MOS, the supply needed is Vds,sat+Vgs. the two value may be not equal, so we take the maximum value for the whole circuit working well
 
FYI, my folded cascode is just a simple folded cascode comprise of one NMOS (input) and one pMOS (output) with a diod connected NMOS to supply biasing voltage to input NMOS. Therefore should i change the expression to MAX{(1*Vds,sat),(Vgs+Vds,sat)}?

From simulation for input NMOS, my Vgs=0.54V and Vds,sat=0.04V. So does my maximum supply voltage is equal to Vgs+Vds,sat=0.54+0.04 = 0.58V?

thanks again...
 

Re: why folded cascode amplifier has lower voltage than casc

yes, you'r right

maybe your circuit differs from that i think. can you upload your circuit?
 
Re: why folded cascode amplifier has lower voltage than casc

Hi i have attached my schematic. Please kindly advice how is the calculation to get the maximum supply voltage. Because i get the VDD = 0.6V, but can't figure it out the reason behind it.

Thank you.
 

Re: why folded cascode amplifier has lower voltage than casc

indeed, your circuit differs from that i think. it's a circuit related to RF? i'm not familiar with this type of circuit, sorry.
look forward to others' reply
 

It is a LNA schematic. However i think the way to calculate supply voltage should be same as other analog circuits.
 

Re: why folded cascode amplifier has lower voltage than casc

cascode has transistors stacked one over anoother,so need more voltage for operation..
 

Re: why folded cascode amplifier has lower voltage than casc

This is clearly explained in Razavi's book!
 

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