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Why 1/(gm+gmb) connected to ground (vss) while ro is connected to delta V?

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tomph

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Hi All,
I read Razavi book and got stuck at the below figure
figure3_23_razavi.png
I do not know how Figure3.23(b) be simplified to Figure3.23(c).
Thing that bother me is that why 1/(gm+gmb) connected to ground (vss) while ro is connected to delta V.
For existing configuration, I think 1/(gm+gmb) // ro so, the other ends of both resistors 1/(gm+gmb) and ro
have to be connected to the same potential.
Thank you very much for your explanation.
Regards,
Tomph
 

Re: How to simply the circuit

In2Dusk said:
Figure C is small signal model of the circuit.
You can read more about it here: https://en.wikipedia.org/wiki/Hybrid-pi_model
Click to expand...

Hi In2Dusk,
Thanks for the reply. I still did not get why 1/gm+gmb connected to VSS and ro connected to delta V in the figure 3.23(c) ( previous post).
In the book "Design of Analog Of CMOS Integrated Circuits", page 65, Prof. Razavi mentioned he derived the figure 3.23(c) from the below
razavi_fig3.8.jpg

However, according to this figure , when looking from the source of transistor M1, 1/gm+gmb and ro are connected to the same potentials.
Please help me on this .

Thank you very much.
Tomph
 

The mosfet you have there has its substrate(bulk) connected to ground (as shown in the picture in your 2nd post)
and not to the source as is usual, so 1/(gm+gmb) is resistance of the subsrate.

Pages 15-18 of this document shows equations and small signal model of mosfet with its substrate not connected to its source:
**broken link removed**
 
Last edited:

go deeply through the small signal models of transistor in some renowned think u need more practise

they are really cool once you get the basics right
 

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