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I am currently studying about small signal models of Common Drain & Common Gate configuration, where the body transconductance has significant effect on voltage gain and output resistance (when body effect is significant). I also know the principle of body effect - body effect occurs when Vbs < 0; for NMOS device.

Common-Gate T-model

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Common-Drain model

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  • I am curious about how a designer of electronic circuits deals with the body transconductance values, since its value isn't given by data sheets. I doubt its can be neglected when performing hand calculations.

  • Can body effect occurs, although it is connected to most negative potential of NMOSFET? (Perhaps oscillations can make Vbs ≠ 0)

  • Can someone gives me an example or two, where body transconductance (it can be also referred as "body current generator" or "back gate") has a significant affect on a circuit as a whole?

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  • \$\begingroup\$ If you are IC designer then you must worry about body transconductance. In discrete design, you do not have to worry about it. \$\endgroup\$ – G36 Jul 15 '17 at 15:05
  • \$\begingroup\$ @G36: Usually when transistor operates in the active region, the finite resistance between source and drain is neglected. If I am designing the transistor to work in that region, I would also neglect body transconductance. Neglecting two parameters can lead to failure of the circuit as a whole, don't you think that? \$\endgroup\$ – Keno Jul 15 '17 at 15:42
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    \$\begingroup\$ No, I disagree with you. Only if you are IC designer then you can care about gmb. All discrete MOSFET will have a "body/bulk" connected to the source. So, we do not care about gmb. \$\endgroup\$ – G36 Jul 15 '17 at 15:49
  • \$\begingroup\$ Well CMOS's bulk is not connected to source. \$\endgroup\$ – Keno Jul 15 '17 at 17:49
  • \$\begingroup\$ But can you buy a CMOS transistor? \$\endgroup\$ – G36 Jul 15 '17 at 17:57
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For integrated circuit design the backgate-transconductance can be found using design documents or simulation. For discrete circuit design the source and bulk are commonly connected so that it can be ignored.

Backgate effect in particular occurs because the backgate is connected to fixed potential while the source potential can change. If source and backgate are at the same potential (e.g. both at ground) then it can be disregarded.

NMOS Source-followers with the substrate connected to ground show a non-linear behavior because of the backgate effect. When the source and the backgate are connected and the current-source is a high output impedance, source followers work much better.

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In very low voltage IC processes, the BODY can be used as OpAmp input signal. Two FETS are used, in differential-pair config, and both bodies are used as the Vin+ and Vin- nodes.

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