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Body effect problems

BlackMelon

Aug 7, 2012
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Hello all,

On the pictures, one is the problem and another is a solution in a solution manual. I'm curious why Vsb = Vin = 0.2, since Vin is across the source and ground, but Vsb is across the source and bulk (substrate).

Thank You
BlackMelon
 

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Harald Kapp

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Sorry, I don't get it. The problem and the calculation are imho totally unrelated.
The problem states that you are to ignore the body effect and the expected result is a time (settling time).
The calculation does not ignore the body effect, ib fact it is about the body effect and the result is a voltage.
You're sure you posted the right images?
 

BlackMelon

Aug 7, 2012
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Sorry, I don't get it. The problem and the calculation are imho totally unrelated.
The problem states that you are to ignore the body effect and the expected result is a time (settling time).
The calculation does not ignore the body effect, ib fact it is about the body effect and the result is a voltage.
You're sure you posted the right images?

Please look at the problem image again. Yes, on 1.14, it doesn't count body effect. But on 1.15, it says that I have to repeat 1.14 with body effect.

thank you
 

Harald Kapp

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I'm sorry, an oversight on my side.
The figure P1.13 obviously implies an integrated MOSFET with the substrate being tied to 0V (usually the substrate of an NMOS in an integrated circuit is at the lowest potential). Therefore Vsubstrate = 0V and VS = Vsource-Vsubstrate = Vin.
As Vin changes, the body effect will change accordingly.
Is that what you were looking for?
 

BlackMelon

Aug 7, 2012
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I'm sorry, an oversight on my side.
The figure P1.13 obviously implies an integrated MOSFET with the substrate being tied to 0V (usually the substrate of an NMOS in an integrated circuit is at the lowest potential). Therrefore Vsubstrate = 0V and VS = Vsource-Vsubstrate = Vin.
As Vin changes, the body effect will change accordingly.
Is that what you were looking for?
Oh! Thank You! I did googling with your words (To tie the substrate to the lowest potential) and figured it out.



Just some more curious points about the red-underlined parts of the question, why does the problem have to say something about this?

Thank You
 

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Harald Kapp

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I'm not perfectly sure, but this may have been emphasized so you do not take into account any other charges (e.g. on Cgd, Cgs etc.) which may make the calculations unnecessarily complex (unnecessarily in the context of understanding the MOSFET per se).
 

BlackMelon

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About this current mirror, why there is no (gm)(Vgs) current source of the M2?
 

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Harald Kapp

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You should be able to find out by yourself.
Hint: (b) is supposed to be the small-signal model of (a), right?
What is the value of Vgs for M2 in a small signal model?
 

BlackMelon

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You should be able to find out by yourself.
Hint: (b) is supposed to be the small-signal model of (a), right?
What is the value of Vgs for M2 in a small signal model?
Vgs2 = 0 right :D Because I(in) is constant>>>Vgs2 in bias model is constant >> Vgs2 has no AC value >> Vgs2 =0 in the small signal model.

If that is the case, in the pdf file, should there be a short circuit from g2 to s2 in the Figure 3.3(a) too?


I still have a question left about a diode connected load. From the proof in the pdf file, I understand how Carusone derives the input resistance of the diode connected load. After that, he plugged this resistance into the model. My question is about the reason why the value of the input resistance alone can substitute the whole diode-connected load model (Q1)?


Thank You
 

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Harald Kapp

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If that is the case, in the pdf file, should there be a short circuit from g2 to s2 in the Figure 3.3(a) too?
Answer this for yourself: Gate to source is a capacitor. What is the impedance of a capacitor to DC?

My question is about the reason why the value of the input resistance alone can substitute the whole diode-connected load model (Q1)?
Remember what a small signal model is all about: linearization of the characteristics around the operating point. A linear V/I characteristic is...?
 
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