gain equation of a cascode amplifier

[siddharthtaunk]

Hello,

I have been working on cascode amplifier.

Can. Any one please tell me the gain equation for the cascode amplifier.

As we know. Cascode is a combination of CS-CG and the gain of alone CS ampli
Amplifier is -GM*rd.

I kindly request you to please give me the overall gain.equation of the cascode amplifier



Thanks in advance.

Siddharth

 

[goldsmith]

Hello,

I have been working on cascode amplifier.

Can. Any one please tell me the gain equation for the cascode amplifier.

As we know. Cascode is a combination of CS-CG and the gain of alone CS ampli
Amplifier is -GM*rd.

I kindly request you to please give me the overall gain.equation of the cascode amplifier



Thanks in advance.

Siddharth

Hi siddharthtaunk
Can you please attach the schematic which you're referring to please ? cascode amplifier can be achieved by a lot of configurations . i hope you know about it ?

Best Wishes
Goldsmith

 

[siddharthtaunk]

Hello goldsmith

Attaching herewith the circuit diagram
{\rtf1\ansi\ansicpg1252
{\fonttbl\f0\fnil\fcharset0 Verdana;}
{\colortbl;\red255\green255\blue255;\red0\green0\blue0;}
\deftab720
\pard\pardeftab720\sl380\partightenfactor0

\f0\fs26 \cf2 \expnd0\expndtw0\kerning0
\outl0\strokewidth0 \strokec2 }

- - - Updated - - -

Hello goldsmith

Attaching herewith the circuit diagram
{\rtf1\ansi\ansicpg1252
{\fonttbl\f0\fnil\fcharset0 Verdana;}
{\colortbl;\red255\green255\blue255;\red0\green0\blue0;}
\deftab720
\pard\pardeftab720\sl380\partightenfactor0

\f0\fs26 \cf2 \expnd0\expndtw0\kerning0
\outl0\strokewidth0 \strokec2 }

Please not that the JFETs used are N-CHannel (BFW10)

 

[FvM]

gain of alone CS Amplifier is -GM*rd.

Using the simplified transistor model assumed in your gain expression (infinite output impedance), the additional cascode stage doesn't change the gain of the basic CS stage.

Your circuit however implements additional source degeneration. With Rs = 200 ohm and Rd = 100 ohm, the voltage gain will be at least below 0.5.

In a first order estimation, G = - Rd/(Rs + 1/gm)

 

[diarmuid]

Assume M1 is your driving FET and M2 the cascode.

Voltage gain is simply: Av = gm1.gm2.rds1.rds2

Thats why a cascode is so effective at increasing gain ... it gains up the original output impedance (rds1) by a factor of gm2.rds2 i.e. the gain
of the cascode device. Reason why a cascode should be long and biased in saturation.

I recommend looking at Razavi or Willy Sansen. In fact most books on analog design will give you this info.

 

[goldsmith]

Hi again
As "FvM" mentioned you can simply use the AC equivalent circuit to analyze it .
Are you able to draw it ? then it will be like a simple game ! it is like the BJT analysis , ( pretty much like but of course simpler )

Best Wishes
Goldsmith