Is cascode different from cascade ?

[Lord Loh.]

Is cascode different from cascade ?

I understand that cascade is connecting amplifiers/countrs etc in a series...

What is cascode ?

Thank you.

 

[Borber]

cascode cascade

It is connection of two amplifier stages where first is grounded emitter and secong grounded base. Stages are connected as totem, second above first. Such configuration is usual in RF amplifiers where stability and noise figure plays role.

 

[wholx]

cascade vs cascode

for mosfet case, the cascode is the common source followed by the common gate.

 

[Apollo13]

cascade and cascode

What I have read says that noise performance of cascode worsens at high frequency.
A cascode is always a combination of common source driving a common gate amplifier. The topology is fixed.

 

[Kral]

what is cascode

m'Lord,

An historical note:
Cascode comes from the tube days of yore. It's a contraction of "cascade-triode".
Back to the present:
I'll use a FET circuit as an example, but the same principle applies to junction transistors (or vacuum tubes, for that matter).
One of the problems with a standard grounded source circuit is that as the gate voltage increases, the drain voltage decreases. The decreasing drain voltage is fed back to the gate via the reverse transfer capacitance (Crss), thus providing negative feedback, and reducing the gain as frequency increases. This is the "Miller effect". By coupling the output of a grounded source stage to a grounded gate stage, the drain voltage of the 1st (grounded source) stage remains relatively constant, thus reducing the capacitive feedback to the gate, and extending the high frequency response. The 2nd (grounded gate) stage is immune to capacitive feedback, since its gate is grounded. The 1st stage essentially acts as a current source to the 2nd (grounded gate) stage, effectively passing its drain current throught to the load resistor of the 2nd stage.

I hope this helps,
Regards,
Kral

 

[strabush]

cascode vs. cascade

What everyone described is a cascode circuit. Cascade connection is a similar circuit but with a signal decoupling at the connection of the two stages, so that it is connected as in cascode for the DC current only (to get higher efficiency by using the same DC current for both stages), but still get two independent stages for the AC signal.