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I have read this sentence below in a paper which describes a low noise amplifier. I can't understand the reason why cascode transistors contribute little to overall noise in an OTA?. Can someone explain me the reason?

Also, I search in the group and I have found a similar question which has posted one year before but the answers don't help me.

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vl123
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Noise contribution of a device at the output will be multiplied by the gain from the noise source input to the output. Voltage gain for a transistor can be approximated as $$A_v = \frac{Z_d}{Z_s},$$ where \$Z_d\$ and \$Z_s\$ are impedances seen at drain and source respectively.
For a cascode device \$Z_s\$ is high since it sees the output impedance of a transistor. So its gain is very small, consequently its noise contribution will be negligible compared to the other devices which will have high gain multiplied by the noise sources.

sarthak
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Cascodes are normally used when you want to include a source degeneration resistance. So the value of R is going to be high anyway.

Udit Kotnis
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