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Design >> Analog Design >> 1.2V Rail to Rail Input Op Amp Architectures
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Message started by Bean Nakamura on Dec 26^th, 2017, 1:12am

Title: 1.2V Rail to Rail Input Op Amp Architectures
Post by Bean Nakamura on Dec 26^th, 2017, 1:12am

Hello all,
I am currently working on a school project which requires me to use a wide input swing op amp. I have attached the schematic in the link below. The circuit I am using is basically a combined input of PMOS and NMOS to get a wide input range (rail to rail) and also in folded cascode configuration.

With a limited supply of 1.2V and a Vth of 0.4V, I really couldn't get the transistors to saturate and I'm not allowed to use native and twin well devices. I've tried to find a non-cascoded version of this circuit on the internet but with no luck.

My question is:-
1)Is it even possible to saturate all the transistors with such a high Vth at 1.2V Vdd?

2)Is there a version of this circuit which isn't cascoded? If so, can anyone point me to a link or good paper on this?

3)If I were to use the same circuit but remove the cascode will it still work as intended?

4)If anyone has experience designing this, would they mind sharing the architecture they are using?

The architecture I'm using is almost similar to this but the conventional version.

Thanks in Advance!

Title: Re: 1.2V Rail to Rail Input Op Amp Architectures
Post by Abdullah on Dec 31^st, 2017, 5:14am

Hi,

You forgot to attach the circuit.

Abdullah

Title: Re: 1.2V Rail to Rail Input Op Amp Architectures
Post by Tako on Jan 3^rd, 2018, 1:32pm

1. Should be possible, but it may be a difficult task. If they did it in the paper you provided, that should be a proof that is should be possible. Wide transistors should be used (big W/L ratio), so they would work a little bit over Vth, e.g., VGS = 0.5 V. That lead to Vov = 0.1 V (overvoltage) - your transistor needs only VDS = 0.1 V to work into saturation.

2. There should be opamps that are not folded cascoded and have both NMOS and PMOS input. However, I cannot point to such architectures as I do not have a time to search the web. Have you tried books?

3. Folded cascode is a folded cascode. The cascode cannot be removed. Look into books for opamps architectures, e.g. Baker and Razavi. For online, you may look at page 49 at the free preview here https://payhip.com/b/5Srt , where opamps are categorized a little bit (maybe not all categories, but should be useful, especially for beginners but not only).

Hope it will help a little. In case of more questions, ask. :)