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opamp design at 4V supply with gate oxide break down 1V

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viperpaki007

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Hi,

I am designing a simple operational amplifier, which should work at 4V supply and have an output voltage swing from 3.7 to 0.5 V. I made the following design. The design is working fine at supply voltage of 1.2 volts. Now i have to make it working for supply voltage to 4V. Considering the breakdown voltage of each transistor is 1V. How can i increase the supply voltage? Do i have to use cascode transistors now? if yes then in which way?


 

Yes, You cannot exceed 1V on Vgd and Vgs voltages on any transistor, so to use cascodes and some diodes especially at output stage.
 
correct me if i am wrong, M7, M9 and all the transistors in output stages need to be cascoded
 

In ouput source follower You need to add diode connected transistors to avoid vgd and vgs exceed 1V for large signals. Each current source/sink should be cascoded.
Check this paper: https://ieeexplore.ieee.org/xpls/abs_all.jsp?arnumber=5642503

BTW. I assume You are using n-well process so nmos bulks should be connected to ground unless You are using triple-well nfets. And of course M10 should be diode connected ;-)
 
Thank you for the help.. I have modified the opamp structure (diagram attached) using cascoded transistors in first stage of opamp. However, the 2nd stage consist of CS amplifier. How can i maintain the gate oxide breakdown limitations for CS M10 transistor. Will cascoding over M10 not cause the circuit response to change?.. By the way, now i am using 2.5V gate oxide devices because with 1V devices, the number of cascoded stages were quite many.

 

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