folded-cascode opamp [Allen's book]

[davyzhu]

learning to find folded cascode w to l ratio

Hello all,

I am learning Analog COMS IC (for two month) and using Allen's book, there he talked about the folded-cascode opamp.

I calculated one of his Ex, and simulated the result, but found the two current mirror transistrors and two output transistors are working in the cutoff region, but I do not know how to make the transistors working in the saturatied region.

Allen said in his book that analog designer should have a intuitive approach in design. But I always found it is hard for me to gain that intuition. Can you give me some suggestions and references?

Regards,

Davy Zhu

 

[extraord]

Most op amps will not have the output voltage so that both
the upper and lower transistors are saturation when simulated or
measured because they are being operated open loop. You could use
the technique of Fig. 6.6-15 to find the offset necessary at the
input to make M7 saturated or you could put the op amp in the unity
gain mode (a buffer) and the output would be forced to have the dc
value of the input.

The problem in this homework is not expected to provide a
circuit that under simulation or measurement will have the output
transistors in saturation because the op amp is never used in the
open-loop mode.

Phil Allen

 

[terryssw]

If you use fully-differential folded-cascode, the use of Common-mode Feedback circuits can also solve the problems

 

[guamak_menanak]

The main reason why transistor in operates in ohmic is when Vds<Vgs-Vt!

Therefore, look at Vgs and Vds on the ohmic transistor and try to adjust their W/L to make sure they have moderate Vds to ensure transistor in saturation.

1 more thing, choose a good node voltage assumption to make sure your transistors are saturate!

 

[playdc2002]

Firstly, connect input ports to common mode input voltage and outputs to two capacitors with common mode output voltages as initial value, then check out the transitors' mode, if not in Sat, change W/L ratio.

 

[davyzhu]

Hello playdc2002,

Thank you for your help. There are one things I cannot understand, why 'outputs to two capacitors with common mode output voltages', and may you give me a more intuitive description, maybe a drawing will be help, or where can I get the informations about adjusting the circuit?

Regards,

Davy Zhu

 

[playdc2002]

two capacitors with common mode output voltages emulate the common mode feedback circuit. It will keep outputs around the common mode output voltages, otherwise, one side of op amp will have no current.

 

[jennisjose]

The design of my Masters project operational amplifier is uploaded here. I have a gain of 71 db, PM of 45, and bandwidth of 700Mhz.

https://jennisjose.webs.com/operationalamplifiers.htm