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Noise in Cascode opamp

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nxing

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Hello everybody,

Does anybody tell me why the cascode transistor is not important when we calculate the noise of the telescopic cascode opamp in the low frequency range?

Thanks!
 

The current trough cascode transitor is fixed by the current mirror. So, there is no effect of the noise, the current stays the same (assuming output resistance of the current mirror transistor large enough).
Try to change the bias voltage at the gate of the cascode transistors, is trhe same thing. Constant current => Vgs constant => the noise at gate appears at the source of cascode transistors with the same amplitude => the source is connected to a drain => high resistance => no variation in current.
 
because when the cascode noise converted to the input node, it shoudl divided by the input transistor gain.
 

Because resistance seen by the cascode transistor is to huge. That effect noise current is less(non dominant)
 

There is one situation where the cascode contributes with noise. This happends when the impedance of the cascode node is low taking this node to ground, when the frequency increses. This problem is not usualy known, but is importante for high frequency circuits. When this happends the cascode transistor contributs with high frequency noise. In DACs this is particular important.

Bastos
 

Because from its small signal equivalent circuit, one could easily find that the gate of the cascode transistor is connected to ground, so no noise signal will be introduced.

Rocky
 

Its gain from the input of cascode transistor to the output port is too small to contribution to the noise. Therefore , it can be negligible.
 

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