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the miller capacitance is important cause of the gain of the ordinay CS stage is large so the cap. that result is multiplied by this gain, while in cascode the miller cap. is multiplied by the gain between the gate of the Input pair and the source of the cascode this gain is nearly one (equivelant resistance is one over gm) so u get small cap.
From Miller effect , we can get the equavalent cap on the output terminal
is Z/(1-A) , Z=1/SC , A= -Gm*Ro , so , if the Ro is bigger, we get bigger
A, and bigger C .
In the cascode case , the Ro=1/Gm , so the A is small and so is the C .
basically, miller effect is the small g-d capacitance multiplied by the voltage gain at the drain. a high impedance at the drain gives a big miller multiplication. however, looking into a source is a very low impedance so the voltage gain, and consequently the miller effect, is smaller.
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