Electrical – Current gain in MOSFET CS Source degeneration


There is always some formulas for calculating "Au" or "Gu". But there are never given any equations that reffer to current gain in MOSFETs. I know current gain is almost infinite in normal CS amplifier, but can that value be shown as an equation?

And what happens when "Rs" is applied to the circuit? Since voltage gain is explained I know that "Rs" decreases voltage gain. But what happens with current gain in Common Source with source degeneration circuit? Is it also decreased? Can be that value calculated through equation that defines current gain ("Ai" & "Gi")?

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Best Answer

Current gain for MOSFETs is never referred to because it is meaningless. The drain current is not controlled by the gate current. (It isn't in BJTs either). When used in a circuit with other components the current gain can be relevant but it depends almost completely upon those other components.

With your example adding source degeneration (Rs) the voltage gain will decrease and so will the current gain because more current will be required to drive that voltage into R1 and R2.

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