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OTA Design : DC gain 120dB, Sensor Applicaiton ( low speed and small BW)

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han6716914

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Since I am such a beginner about designing amplifier, so I will gladly get some suggestion for my design.

I want to design a high DC gain ( 120dB ~ 140dB ), and for sensor application amplifier OTA, with the supply is 3.3V. It is for humidity sensor, and sampling frequency is 1MS/s, so not high speed, that means, mostly the input is DC for the integrator/the amplifier, so the bandwidth is quite small, around 500~1kHz, that is ok for me. Right now I am quite struggling about the choice which topology I should choose ( telescopic, folded-cascode, or two stage, and power consumption consideration ) and how can I get that high DC gain.....

Anyone has designed some high gain amplifier for biomedical circuit or sensor circuit ? Would appreciate much if I can get some resources how to design it. Any paper recomendation is also welcome.

Thanks.
 

140dB (!!) is a voltage gain of 100 million. Then an OTA with an input offset voltage of 5mV will produce an output of 500,000V when its input is zero! What about low frequency noise being amplified 100 million times? Don't be silly.
 

It depend to technology, but assuming 40dB/stage (process>0.25µm, or thick oxide devices in modern ones) You will need regulated cascode single stage or two stage both cascodes.
 

han6716914 said:
Since I am such a beginner about designing amplifier, so I will gladly get some suggestion for my design.

I want to design a high DC gain ( 120dB ~ 140dB ), and for sensor application amplifier OTA, with the supply is 3.3V. It is for humidity sensor, and sampling frequency is 1MS/s, so not high speed, that means, mostly the input is DC for the integrator/the amplifier, so the bandwidth is quite small, around 500~1kHz, that is ok for me. Right now I am quite struggling about the choice which topology I should choose ( telescopic, folded-cascode, or two stage, and power consumption consideration ) and how can I get that high DC gain.....

Anyone has designed some high gain amplifier for biomedical circuit or sensor circuit ? Would appreciate much if I can get some resources how to design it. Any paper recomendation is also welcome.

Thanks.
Click to expand...

For similar applications I would recommend to use a chopping operational amplifier. Several models are available from Linear, Intersil, Texas and others. Those amplifiers virtually exclude any DC drift while keeping a high gain. Check their specifications and as you wrote you are a beginner in the field, purchase evaluation boards to test with your source and load.
 

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