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Design >> Analog Design >> Bias voltages in terms of threshold voltage and overdrive voltage for folded cas
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Message started by student_of_analog on Sep 11th, 2011, 12:15am

Title: Bias voltages in terms of threshold voltage and overdrive voltage for folded cas
Post by student_of_analog on Sep 11th, 2011, 12:15am

Hello,

I am trying to understand what the bias voltages Vb1, Vb2 etc of the attached opamp should be in terms of Vov and Vth.
Can anybody help me how to proceed with this ?

I could figure out that:
Vb2 = Vov2 + Vth2
Vb4 = Vov4 + Vth4.

But I dont know about the others.

Title: Re: Bias voltages in terms of threshold voltage and overdrive voltage for folded cas
Post by yvkrishna on Sep 11th, 2011, 4:29am

Hi,

This is a famous classAB  opamp topology, It uses translinear principle to bias the transistors Q10,Q11.

Vb1,vb3  are the voltages to bias the  cascode transistors
Vb3 = vov_Q1c + vth_Q1c + vov_Q5

vb1 = Vdd - ( vov_Q3c + vth_Q3c + vov_Q3)


Vb5,vb6 will be clear to understand  if you draw the complete bias circuit.

you can refer chapter12 of Analog Design Essentials by Willy Sansen


Regards,
yvkrishna

Title: Re: Bias voltages in terms of threshold voltage and overdrive voltage for folded cas
Post by student_of_analog on Sep 11th, 2011, 8:59am

Thank you Yvkrishna.
I have also attached the bias block, but it is confusing, because
for example if I look at Vb3 in the opamp I see that
Vb3 = Vov(Q1c) + Vth(Q1c) + Vov(Q5) = 2Vov + Vth---- OPAMP
but if I look at the bias block, I see that
Vb3 = Vov(Q25) + Vth(Q25) = Vov + Vth ----Bias block.

So how can Vb3 be different, shouldnt it be the same ?
I have same type of confusion for Vb5 and Vb6 too.

Title: Re: Bias voltages in terms of threshold voltage and overdrive voltage for folded cas
Post by yvkrishna on Sep 11th, 2011, 9:42am

you are correct in writing the expr for vb3, but the Vov of the transistors  Q25, Q1c,Q1  are not same owing to different sizes,currents of them.

hope you already know this  Vov = sqrt(2*Ids/(k*w/l))

a simple math can show you that both your expr for Vb3 are close enough.

Vb5,Vb6 also follow the same.


Regards,
yvkrishna

Title: Re: Bias voltages in terms of threshold voltage and overdrive voltage for folded cas
Post by student_of_analog on Sep 11th, 2011, 10:50am

Thank you Yvkrishna, that was very helpful.

So, now I understand that:
As long as the bias voltages match numerically in the opamp and bias block, it is good. 2 things

1)So for example if I need a bias voltage of
2Vov + Vth for a nmos in the opamp = 2 (200mV) + 0.7V = 1.1V.

And I have a single diode connected nmos in the bias block whose
drain voltage is Vov + Vth= Vov + 0.7V.
Then I need to ensure by sizing that Vov is around 400mV so that total bias voltage in the bias block is 1.1V right ?

2)Also this means that I am free to choose whichevr type of biasing scheme I want for the bias block as long as the current mirroring happens and bias voltages are aprroximately the same ?

Title: Re: Bias voltages in terms of threshold voltage and overdrive voltage for folded cas
Post by harpoon on Sep 12th, 2011, 12:37am

I just wish to add a few points ...
a) sometimes your bias block may not be well matched over process, voltage and temp. This is because you are not matching the body effect on Vth, hence your translinear biasing can fall apart slightly. This may be ok in some cases.
b) Once you understand what effect the different changes can have on your output stage, you can certainly use different biasing schemes. Just be careful with process variation.

Title: Re: Bias voltages in terms of threshold voltage and overdrive voltage for folded cas
Post by yvkrishna on Sep 12th, 2011, 1:13am

Yes , we need to make sure vth difference is not too high.


Title: Re: Bias voltages in terms of threshold voltage and overdrive voltage for folded cas
Post by sharezhao on Sep 15th, 2011, 3:00am

Bias voltage is not as mysterious as you think. It function is to make the circuit work on a properly DC operating point.So you just adjust the Bias voltage to make sure the transistor work in the right region and you can also consider some other factor like output swing etc.

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