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student_of_analog New Member Offline Posts: 6	Bias voltages in terms of threshold voltage and overdrive voltage for folded cas Sep 11^th, 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.
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yvkrishna

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Re: Bias voltages in terms of threshold voltage and overdrive voltage for folded cas
Reply #1 - Sep 11^th, 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

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student_of_analog

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Re: Bias voltages in terms of threshold voltage and overdrive voltage for folded cas
Reply #2 - Sep 11^th, 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.

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yvkrishna

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Re: Bias voltages in terms of threshold voltage and overdrive voltage for folded cas
Reply #3 - Sep 11^th, 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

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student_of_analog

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Re: Bias voltages in terms of threshold voltage and overdrive voltage for folded cas
Reply #4 - Sep 11^th, 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 ?

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harpoon

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Re: Bias voltages in terms of threshold voltage and overdrive voltage for folded cas
Reply #5 - Sep 12^th, 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.

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yvkrishna Senior Member Offline Posts: 117	Re: Bias voltages in terms of threshold voltage and overdrive voltage for folded cas Reply #6 - Sep 12^th, 2011, 1:13am Yes , we need to make sure vth difference is not too high.
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sharezhao

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Beijing RPC

Re: Bias voltages in terms of threshold voltage and overdrive voltage for folded cas
Reply #7 - Sep 15^th, 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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