Help on designing cmfb circuit

[abdj_project]

Hi all,

Wanted to design a CMFB circuit for an open loop continuous time voltage amplifier... Could anyone help us by a commonly used CMFB circuit.. We are using a 180nm library with supply tied to 5V. So the VCM for the CMFB is 2.5V. Please help us with a suitable circuit and its design....

Thanks.

Regards,
abdj

 

[erikl]

Something like that?

 

[abdj_project]

Thanks erikl,

How about the sizing of transistors and the value of resistors and capacitors? Could you help us with that too..

Regards,
abdj

 

[erikl]

Here's a suggestion you could start with: It's for a PMOS (pair) control.

For I1 use double the current which cmfb should control.

For R1, R2 use values which don't short circuit your outputs too much, perhaps 1 .. 10MΩ .

If your diffOpAmp needs tail current control, just inverse the circuit.

 

[abdj_project]

Thanks.

But somehow this is not working for us. Dont know why.. When connecting it back to the Differential circuit, output is driven to close to 5V or 0V. Supply of the circuit is 5V and so VCM is set to 0V. What do we do.. What can be tried to get the output. Please help.

Regards,
abdj

 

[erikl]

Supply of the circuit is 5V and so VCM is set to 0V. What do we do..

VCM has to be set to VDD/2 , i.e. to 2.5V , of course, as you mentioned in your first post.

If you need more help, show your circuit pls!

 

[abdj_project]

Hi,

VCM is set to VDD/2 itself, which is 2.5V.... Attached is the complete circuit...



It is an open loop continuous time amplifier... We have a cascode stage at the output for high gain. Is there any constraint on gain for CMFB. If we design circuit for high gain, then CMFB is not working... donno if our understanding is correct or not.. Please help.

Thanks a lot for your help...

Regards,
abdj

 

[erikl]

Hi,

VCM is set to VDD/2 itself, which is 2.5V.... Attached is the complete circuit...

View attachment 75795

You didn't connect the VCM input of the CMFB cell. Generate VCM by a 1:1 resistive voltage divider from VDD .

 

[abdj_project]

We have connect VCM as a supply of 2.5V. It is just mentioned as VCM in circuit. Thats not issue. Could you help us more on this.

 

[erikl]

How could somebody help you without any info about:

• W/L ratios
• operation point voltages
• stage currents
• internal circuit of your CMFB module

???

 

[abdj_project]

[Hi all,

One more doubt.Whether there is any constraint for gain in circuits using CMFB?... We wanted to design an amplifier for a gain of 66dB.Is it possible?

Thanks.

Regards,
abdj[/QUOTE]

 

[erikl]

Whether there is any constraint for gain in circuits using CMFB?

No.

We wanted to design an amplifier for a gain of 66dB.Is it possible?

Sure!