How to design this OTA for a maximum GBW?

[Mike28]

So I am trying to design an OTA to meet the following specs:



Plus the following constraints:
 The maximum allowed transistor width is W=100um.
 The maximum allowed transistor length is L=20um.
 The maximum allowed capacitance (if you use Miller compensation) is Cc=2pF.
 The supplies are Vss=0 & Vdd=1.8V.

______________________________________________________________________________

I decided to go with the typical 2 stage miller OTA:



and the best result I got was (while meeting the specs) GBW = 20 MHZ.

______________________________________________________________________________

• Is the miller OTA suitable for this design problem?
• Is there any rule of thumb on choosing the transistor's W and L?
• What is the maximum GBW that I can achieve while meeting the specs? (this will help decide if my design was okay, at least.)

NOTE: I checked few posts in the forum, and I saw a lot of designers suggest using the RFC OTA. I checked it and its design seems to be 10 times harder than the Miller OTA (I am still new to the analog design :grin

 

[Dominik Przyborowski]

Try with classical telescopic and/or folded cascode designs and check what are You can get for different classic topologies but it is possible that for given load and power consumption a GBW around 25 MHz is a maximum what You are able to obtain.

RFC provides higher GBW due to higher transconductance achieved through positive feedback

 

[deepsetan]

So I am trying to design an OTA to meet the following specs:

View attachment 116641

Plus the following constraints:
 The maximum allowed transistor width is W=100um.
 The maximum allowed transistor length is L=20um.
 The maximum allowed capacitance (if you use Miller compensation) is Cc=2pF.
 The supplies are Vss=0 & Vdd=1.8V.

______________________________________________________________________________

I decided to go with the typical 2 stage miller OTA:

View attachment 116642

and the best result I got was (while meeting the specs) GBW = 20 MHZ.

______________________________________________________________________________

• Is the miller OTA suitable for this design problem?
• Is there any rule of thumb on choosing the transistor's W and L?
• What is the maximum GBW that I can achieve while meeting the specs? (this will help decide if my design was okay, at least.)

NOTE: I checked few posts in the forum, and I saw a lot of designers suggest using the RFC OTA. I checked it and its design seems to be 10 times harder than the Miller OTA (I am still new to the analog design :grin

Hi,

1. It normally depends on how you manipulate the design.
2. Yes. Your sizing and biasing current will determine you amplifier specs. Like from your diagram, if you increase W for Q1 and Q2, you will found that your gain will increase. The basic rule is by understand the square law concepts.
3. It's all depend on how you manipulate the design especially your transistor sizing and biasing current.