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Explain Cascode stage, ssm, $r_o$

Subject: CMOS VLSI Design

Topic: Single Stage Amplifier

Difficulty: Medium

1 Answer
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Cascode Stage

  • cascading of CS stage and CG stage is called "cascode" topology.

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  • Small signal ac equivalent model:

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$gm_1\,V_1=gm_2\,V_2+gmb\,Vbs$

$\therefore V_{out}=- R_D(gm_1\,V_1)$
$\hspace{1.2cm}=-R_D(gm_1\,V_{in})$

$\therefore A_V=\frac{V_{out}}{V_{in}}=-R_D\,gm_1$


-> Expression for $R_O$
Important advantage of cascade is its high output impedance.
To calculate $R_O$, the cascade stage circuit will be modified as follows.

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The above circuit can be viewed as common - source stage with degeneration resistor equal to $r_{o1}$.
$\therefore \, R_O=[1+(gm_2+gmb_2)r_{o2}]r_{o1}+r_{o2}$

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