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Cascoded amplifier can be defined as:

  • "The cascade of a CS stage and a CG stage" (Razavi)

Attach:cascode.png | General Cascode

Principles

  • An amplifier's gain is G_m R_{out} (Where Gm is current gain)
    • The cascode increases output impedance by (g_{m2} + g_{mb2}) r_{o2}
    • This in turn increases the gain.
    • We can not have an ideal current source for biasing, so the output impedance of the current source ends up in parallel with the output impedance of the gain stage. Because of, this the output impedance of the mirror stage must be as high as that of the gain stage or Rout will drop below the the mirror stage impedance.

Biasing

  • Vb4 sets the bias current
  • Vb3
    • V_{sg2} = V_{dd} - V_{sd4} - V_{b3} > V_{th} - Must keep the transistor on
    • V_{dg2} = V_{out} - V_{b3} < V_{th} - Keep the transistor in saturation
    • V_{out,max} - V_{th} < V_{b3} < V_{dd} - V_{sd4} - V_{th} - Put it all together
  • Vb2
    • TODO: Write these rules
  • See Also: Biasing Cascoded and Regions Of Operation

See Also:

Backrefs:

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Last edited by DrLock. Originally by DrLock.