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Transconductance GM

The document discusses MOS transistor models, focusing on the transconductance (gm) in both weak and strong inversion regions. It highlights the relationship between transconductance and current, emphasizing the importance of the transconductance-current ratio (gm/iDS) as a measure of efficiency. Additionally, it includes a figure illustrating the transconductance behavior in saturation.

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38 views1 page

Transconductance GM

The document discusses MOS transistor models, focusing on the transconductance (gm) in both weak and strong inversion regions. It highlights the relationship between transconductance and current, emphasizing the importance of the transconductance-current ratio (gm/iDS) as a measure of efficiency. Additionally, it includes a figure illustrating the transconductance behavior in saturation.

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unknown
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CHAPTER 1: MOS TRANSISTOR MODELS 29

In order to provide the full expression in the saturation region, Eq. (l-25a) must
be multiplied by the same factor (1 + Xvds) as in Eq. (1-20). The same parameter À
can be used in strong and weak inversion.
For the weak inversion region, the transconductance gm is again obtained by taking
the derivative of ids versus Vgs in Eq. (l-25a), which yields

W Ido VGSQ \ iDSQwi


SmWl = , rr, / eXp (1-256)
L nkT/q nkT/q J nkT/q

The transconductance is now directly proportional to the current. The transconduc­


tance is sketched on a bilogarithmic scale versus the current in Fig. l-10a. The change
in slope from 1 to 1/2, going from weak to strong inversion, is clearly seen.

1-6-2 Transconductance-Current Ratio


The transconductance is the most important parameter of the MOST because it re­
flects the transfer efficiency from input to output. A better criterion, however, is its
transconductance to current ratio gm/iDS- This ratio shows how efficiently the current
is used to generate transconductance.

FIGURE 1-10 (a) gm and, (b) gm/lDS of MOST in saturation.

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