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How to design an OTA with gain of 8000?

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airboss

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Hello,

We want to design an OTA with a gain of 8000. The first stage is a telescopic cascode and the 2nd stage is simply a CS. We tried to size our transistor but it seems like a mission impossible for us to get such a high gain. Can anybody help us? Thank you very much!
 

airboss said:
Hello,

We want to design an OTA with a gain of 8000. The first stage is a telescopic cascode and the 2nd stage is simply a CS. We tried to size our transistor but it seems like a mission impossible for us to get such a high gain. Can anybody help us? Thank you very much!
Click to expand...

Just use the gain-boost to increase the gain.

Bastos
 

airboss said:
Hello,

We want to design an OTA with a gain of 8000. The first stage is a telescopic cascode and the 2nd stage is simply a CS. We tried to size our transistor but it seems like a mission impossible for us to get such a high gain. Can anybody help us? Thank you very much!
Click to expand...

there is no problem i think
 

I think that an 80 dB gain is achievable with a cascode as first stage and a CS amp as a second stage. Cascode configuration by itself for reasonable speeds would give a gain of about 60 ~ 70 dB (1000 ~ 4000 V/V). Asimple CS stage as a second stage would give the rest of the gain.

Only thing to worry is the load capacitance the amp is trying to drive. Stability can be achieved using Cascode compensation.
 

Hi,

we are designing a differential i/p & o/p OTA. for the first stage (the telescopic cascode) i mentioned before, we used two nmos & two pmos on each branch. we sized all of them 90u/1u but we still get a poor gain less than 20. we're using 180n technology. does anybody have any insights?


thank you....
 

Did you make sure if your transistors are in their correct region (saturation, triode or whatever you were designing for)?

I think it would be much easier for people to answer your question if you provided us with a schematic.
 

Hi,

this is my schematic. all of the mos are in saturation. the opamp is CMFB. vcc = 1.8v. thank you.
 

Can you print node voltages?
 

airboss said:
Hi,

this is my schematic. all of the mos are in saturation. the opamp is CMFB. vcc = 1.8v. thank you.
Click to expand...

Reduce the current from 200 uA to 50 uA, decrease the W/L's appropriately to increase the DC gain.
Use length of 1u in both NMOS and PMOS transistors in the cascode amplifier.
Increase the current in the second stage to push the second (non-dominant) pole to higher frequencies, since the dominant pole will be at the output of the cascode amplifier. This should help you to get 80 dB gain. 80 dB should be easy to get from cascade of telescopic cascode and CS amplifier configuration (which is the circuit configuration you have described in the first post)

Bharath
 

I also believe that there is a problem with using 1 Meg Ohm resistors as CMFB sensing elements.

The output impedance of the telescopic cascode is probably much higher than 1 Meg. Try increasing them to 10 Geg or so and see if it makes a difference.


Another thing: I think the CMFB forces the output to be Vdd. Are you sure all the transistors are in saturation?

Also: I see you have connected the source to bulk in both NMOS and PMOS transistors. You must check if this is possible for the technology u are using ( for PMOS it is usually OK but for NMOS it is usually not )
 

Check whether there are transistors in linear region
 

The 1M resister will reduce your gain.
Try big resister or use capacitor instead.
 

can anybody suggest transistor sizes for the same telescopic ota for gain >80db and ubg >300MHz . I decided mu Iss be 320uA according to my power Specs etc.I did calculations for the same but my all transistors are not working in saturations for those values of W and Ls. I wasted a lot of time to tweak the design in a logical manner but could not get success. My technology is UMC 0,18um in Cadence spectre and Vdd=1.8v. please help......
 

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