Subject: CMOS VLSI Design

Topic: Single Stage Amplifier

Difficulty: Medium

Cascode Stage

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

• Small signal ac equivalent model:

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

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

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

-> 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.

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