Need help in 4th-Order Source Follower Lowpass Filter design

[JondorZ]

Hi there,

I have a project to design a 4th-order source follower lowpass filter for medical applications. Here are the specifications:

1. Supply Voltage: 2.5V
2. Filter order: 4th-Order
3. Cutoff Frequency: 3kHz, 5kHz, 8kHz, 10kHz programmable
4. Power Consumption: <2mW
5. Design Tech: 0.35um CMOS


I am using Cadence Virtuoso to design and simulate the circuit. I am designing the circuit using NMOS, PMOS transistors, resistors, capacitors, and other relevant components. So I use the biquad design and it doesn't seem to be working. I need help in whether the biquad design is suitable or there is other better circuit design. Thanks a lot!

 

[LvW]

Some clarification seems to be necessary:
* "Source follower" means lumped transistor design rather than usage of opamps? (later you mention opamps);
* "Biquad design" is not unequivocal - perhaps you are referring to KHN or Tow-Thomas topology? However, some authors denote single opamp filter stages sometimes also as "Biquads".
* I would not recommend a two- or three opamp design for each 2nd order stage (because of power restrictions)

As you can see: If you ask for help you should give us information as much as possible (in your own interest).

 

[FvM]

I understand "source follower" as a synonymous of positive SAB (single amplifier biquad) with a gain of 1. Using a single transistor or an OP would of course affect the gain precision and thus the filter characteristic. For a "low Q", e.g butterworth filter, a small gain change won't matter much.

The intended method to get a programmable filter should be mentioned at least.The question is dedicated to IC design, so I fear, that a kHz frequency integrated filter most likely doesn't make any sense without external capacitors. Most designers would go for a switched capacitor circuit.

 

[JondorZ]

Thank you for the quick reply. Sorry for the vague description of my project.

Clarifications:

• "Source follower" as in lumped transistor design, not opamp.
• I have attached the "biquad" design I used.
• The project only requires me to design the circuit schematic followed by layout design.
• It is fine to use any external capacitors, resistors, or other components to help achieve the specs.
• It is up to me to decide on how to design the programmable part, but i am not sure how to design it. Maybe you can give a suggestion? Thanks.

Please state if further clarifications are required. Thanks a lot.

 

[JondorZ]

Hi all, i need some help here. What is the difference between the opamp that is built on BJTs and opamp that is built on NMOS or PMOS? I have a schematic of BJTs inside the common 741 opamp from wikipedia (Operational amplifier - Wikipedia, the free encyclopedia) and i convert the BJTs to NMOS and PMOS. Can i do this? I am not sure how much width i should set for each of the CMOS transistor. Please help. thanks.

 

[JondorZ]

Hi, can somebody help me?? thanks in advance..